Custom PCB Fuse Box Fix - need input

E28 technical advice asked and given! Troubleshooting, modifications and more.
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nebulous1
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Joined: Oct 22, 2019 12:47 AM
Location: OR

Custom PCB Fuse Box Fix - need input

Post by nebulous1 »

Hello all,

If you've had trouble with your e28 fuse box and would love a realistic replacement please read on...

I've had a 1986 E28 535i for about a year and a half now. Like many, I've had trouble with the fuse box. I'm not an electrical engineer but have tinkered with custom PCB design in the past. I've done successful projects that involved mechanical and electrical design along with PCB layout and assembly. Thru my job I also have access to a lot of electrical engineering prototyping tools. With my latest electrical issues in my car and searching the forums for upgrades, fixes and conversions, it seems like there would be a decent amount of interest for a legitimate fix that's relatively easy to implement, doesn't cost an arm and a leg and converts the whole mess of bullet fuses to blade fuses. As such, I'm considering designing a custom PCA to do just that. I've put some thought into the best way to do this, but I'm guessing there are a lot of people on this forum who have a lot more experience with these cars than I do and would have some great input, suggestions and technical advice. I'd love to get that input before going to far only to realize there isn't much interest or I missed something really important.

Off the bat here are a few questions that come to mind:
1. How much interested is there in such a solution?
2. How much would you pay, $100? Not trying to turn this into a retirement plan but also not giving stuff away. My initial guess is in $100-$125 range, at prototype quantities, to convert all 17 fuses in the box.
3. Sounds like folks think the overheat/melting problem comes almost entirely from poor contact right at the fuse-tab interface? I tend to agree but would love to hear what folks think on this matter.
4. I'd love to keep this simple with access completely from the top. Is that a waste or time? Tonight I tested several laser cut piece of plastic with slots for the existing fuse holder tabs. I removed the bullet fuses and popped the plastic down over the tabs. To complete the install with real boards you'd need to solder each tab. If all the OEM fuse holder tabs were solder to a PCB with a new blade fuse holder for each circuit soldered to a real PCB, would that solve the major problem or is it a waste of time without replacing stuff on the underside that you can't see? (I'm nervous to pull my box and want this to be easy.)
5. The approach in #4 makes me a little nervous because disassembly (if you ever needed to) would be a challenge with all that unsoldering. For that reason I thought about somehow using the small holes in the existing fuse holder tabs to run a screw thru. The custom PCB would need a corresponding custom right-angle tab to screw to. Seem crowded and would certainly be a challenge to fit it all in but easier replacement if that was ever needed. It would undoubtedly make the cost higher as well.
6. My fuses are 8, 16 ad 25 amp. The 25 is no problem but standard blade fuses offer 7.5 and 15. I'm OK with that but wondering if it would concern others.
7. Blade fuse type - Leaning towards a smaller form factor than the standard size because there isn't much room. If you go here https://en.wikipedia.org/wiki/Fuse_(automotive) I'm looking at Micro2, Mini or LP Mini. Looks like fuses for all sizes are readily available on Amazon.

I hope all of my descriptions make sense. I'm really looking forward to what people have to say. All input is appreciated.

Thanks!
Bonsaibacker
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Re: Custom PCB Fuse Box Fix - need input

Post by Bonsaibacker »

Following this as I just had the AC fuse melt the box.

Would like to keep original fuse box cover.
My electrical experience is primitive at best but with success- make it relatively easy to implement.

That doesn't solve your challenges but integrate those bullets and I am game.
benstoked
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Re: Custom PCB Fuse Box Fix - need input

Post by benstoked »

For whatever reason, the wikipedia link doesn't have the closed parenthesis ")" included. It's there in the text, but not link.

1) Definitely interested in a direct-swap fuse box.
2) As for what's fair, I'd say I'd pay about that ($100-$125USD).
3) That's my speculation, as well.
4) They do make through hole 1/4" blade connectors (see this as an example). would basically make for a direct stock type replacement.
5) would be alleviated by the above, or at least, as difficult as stock.
6) not an electrical engineer, but I doubt 0.5 to 1.0a would be a severe change, at least not enough to worry about a car with an otherwise good electrical system and components.
7) I'd prefer AT0 fuses. Much more ubiquitous, and fairly period correct. Found on all sorts of vehicles of the era.

final thoughts: Building a board, versus the factory style of wires, extra consideration will likely be needed. There's a ton of potential current flow on some of the circuits, at a given time. A single-sided copper clad PCB, as it's regularly made might not have enough capacity on the traces, and will require extra work to make suitable.
jayjaya29
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Re: Custom PCB Fuse Box Fix - need input

Post by jayjaya29 »

Very interested in this project.

Only thing I can think of to add is that once the new PCB is soldered to the existing fuse holders, you would want a secondary plastic enclosure on top so prevent accidental shorts when tinkering/replacing fuses. This would give more support to the new fuses too.
Mike W.
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Re: Custom PCB Fuse Box Fix - need input

Post by Mike W. »

nebulous1 wrote: 2. How much would you pay, $100? Not trying to turn this into a retirement plan but also not giving stuff away. My initial guess is in $100-$125 range, at prototype quantities, to convert all 17 fuses in the box.
Sounds reasonable.
3. Sounds like folks think the overheat/melting problem comes almost entirely from poor contact right at the fuse-tab interface? I tend to agree but would love to hear what folks think on this matter.
I emphatically agree. All of them are prone to poor connections especially as they age, it's just a poor design dating from the 50s or earlier. But the 16 and 25A are the only ones that overheat and melt the base.
4. I'd love to keep this simple with access completely from the top. Is that a waste or time? Tonight I tested several laser cut piece of plastic with slots for the existing fuse holder tabs. I removed the bullet fuses and popped the plastic down over the tabs. To complete the install with real boards you'd need to solder each tab. If all the OEM fuse holder tabs were solder to a PCB with a new blade fuse holder for each circuit soldered to a real PCB, would that solve the major problem or is it a waste of time without replacing stuff on the underside that you can't see? (I'm nervous to pull my box and want this to be easy.)
5. The approach in #4 makes me a little nervous because disassembly (if you ever needed to) would be a challenge with all that unsoldering. For that reason I thought about somehow using the small holes in the existing fuse holder tabs to run a screw thru. The custom PCB would need a corresponding custom right-angle tab to screw to. Seem crowded and would certainly be a challenge to fit it all in but easier replacement if that was ever needed. It would undoubtedly make the cost higher as well.
6. My fuses are 8, 16 ad 25 amp. The 25 is no problem but standard blade fuses offer 7.5 and 15. I'm OK with that but wondering if it would concern others.
Some might get worked up about the size, but few I suspect. It's a ~7% difference which is almost certainly within the range of tolerance on a 30 year old car.

Attachment will be the difficult part, although keep in mind most PC boards aren't designed for 25A circuits either. It wouldn't be as neat looking, but you might try to just focus on the high current ones with a clean, easy to install replacement for them rather than the entire board.
WVe28
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Re: Custom PCB Fuse Box Fix - need input

Post by WVe28 »

If there were a good, attractive, pnp fuse box replacement available at that price point, I'd buy it. My main concern would be that it seems like pretty high current for PCB traces. I'm no electrical engineer by any stretch, though. I'd assume you'd have to oversize the traces quite a bit to ensure the [25] amp fuse blew before the trace burned.
epearce
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Re: Custom PCB Fuse Box Fix - need input

Post by epearce »

Definitely interested in this.
nebulous1
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Re: Custom PCB Fuse Box Fix - need input

Post by nebulous1 »

Thanks for all the interest and feedback! Looks like this is worth moving forward. I have not official timeline but will try to remember to post some updates from time to time.

Several concerns about the current loading. Copy that. Was one of my concerns too. I’ll be chatting with an actual EE about that. I have a few ideas.

@bonsaibacker: Definitely a must to keep the original fuse box cover.

@benstoked: Great feedback.
4. Thought about the ¼” blade connectors too. With so many I’m concerned about getting them all to line up and fit thru the slots in the plastic fuse box base. Remember I'm trying to make this all work from the top and not remove the fuse box at all. That might not pan out but that’s my default at this point. (fingers crossed)
7. Should be able to make the design flexible to fit 2 different sizes. Haven’t made any measurements yet but to my point above about keeping the original cover, not sure there’s enough head room to get standard fuses and the cover. If there is, not a problem.

@jayjaya29: Hadn’t thought about the secondary cover. Could probably figure out something simple. Could also do some simple masking and spray with plastidip. Just a thought.

Thanks!
Sagi
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Re: Custom PCB Fuse Box Fix - need input

Post by Sagi »

I've bought these holders going to put them when I'll have some time. https://www.jex-electronics.com/in-line ... -100a.html
ahab
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Re: Custom PCB Fuse Box Fix - need input

Post by ahab »

This sounds like a neat idea but it seems fraught with potential failures. Solid electrical connections can be difficult to maintain in an environment with lots of heat and vibration. Rather than overlay a separate PCB board, it might make more sense to purchase a blade-style fusebox from a late 635CSi which is currently $149.25 at ECS. The harness requirements are nearly identical to the E28 due to the shared foundation between the two chassis and should make for a relatively easy retrofit. Definitely more complicated that simply plugging something on top of the existing setup, but it should fit on top of the factory wiring box and achieve the desired result with much less risk of failure down the line. The other approach would be to use the entire box but the slope of the inner fender would have to be dealt with, certainly not impossible to overcome however. I think there's a write-up here where someone discusses it but I couldn't locate it with a quick search. I found a thread with someone doing an E30 box, it had a lot of good info but wasn't complete. I'm not trying to throw a wet towel onto this, DIY projects on these cars are part of what makes them so awesome and I encourage you to follow your heart. Personally I will draw the line at modding the electrical system with something of this caliber. I'd much rather invest the time to do a proper conversion up front than try to sort out gremlins later on, and I can't envision there being none.
nebulous1
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Re: Custom PCB Fuse Box Fix - need input

Post by nebulous1 »

No problem at all ahad. No wet towel at all. I appreciate the input. I want to get as much info as I can now to avoid trouble down the road.
ahab wrote:neat idea but it seems fraught with potential failures. Solid electrical connections can be difficult to maintain in an environment with lots of heat and vibration.
The fraught with potential failures part of your comment is what I'm trying to avoid with this solution. While I agree its a challenging environment, that's a level playing field in all cases - under the hood. Replacing the old/suspect barrel fuses with more robust blade fuses is a net benefit, so not sure how that would be worse than the standard E28 fuse box or any upgrade to blade fuses for that matter. Its the same number of connections in the end. If its the solder joints you're worried about, that will depend entirely on the skills of the installer. If done properly, those joints should be no more trouble than the other solder joints in the electrical system of the car.

Thanks for the link to the E24 fuse box on ECS. I didn't realize those could be purchased new for that low of a price. The pics of the underside really show what's needed to handle the current load for the 25A circuits! In poking around on this topic originally I thought I read several threads that made that conversion sound like quite a bit of work? It certainly requires some electrical surgery, which is what I'm trying to avoid. My hope is to make a solution that can be performed by someone with reasonable DIY soldering skills who only owns one car and will need it Monday AM to get to work. I'd like this person to be able to start 9am on Saturday and be done with the shake-down drive by 11, noon at the latest, same day of course. I am fully aware this is a lofty goal. :D
ahab wrote:than try to sort out gremlins later on, and I can't envision there being none.
- Amen to that. Its naive to expect zero gremlins. However, I'd bet a clean solution from the top will have fewer induced gremlins just due to the fact that you don't have to dislodge/bend/wiggle or otherwise tamper with the old connections underneath. Who knows. I've been wrong before. I just hope I figure it out so we all get an answer!
nebulous1
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Re: Custom PCB Fuse Box Fix - need input

Post by nebulous1 »

Update -
Sorry took me a while to get to this. Life...

Have first round of proto boards arriving early next week. I have 3 variants on the same theme. Should be fun.

Some mentioned wanting to keep things looking stock and period correct as possible. I went with mini size fuses, and those will be a tight fit to keep the OEM fuse box cover. Minis are a little shorter than what I think is more standard ATO. I won't know if minis fit until I have tried installing.

I'll keep folks posted. I'm kind of excited.

Thanks!
milarsky
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Re: Custom PCB Fuse Box Fix - need input

Post by milarsky »

Count me interested.
cek
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Re: Custom PCB Fuse Box Fix - need input

Post by cek »

Have y'all seen this:

viewtopic.php?p=1496236#p1496236

Curious how this solution compares. Got any pics?
nebulous1
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Re: Custom PCB Fuse Box Fix - need input

Post by nebulous1 »

Wow, that's pretty dang impressive! Mad skillz for sure. This @Galahad guy has a lot more free time and patience than I do. :D I really wish I had known about this before I got started, but I still think my solution has merit.

I actually thought about doing it just like that. Not with all the LEDs etc, but one complete PCB. I quickly realized you'd need to take all those steps in the design and installation. That solution has a lot of benefits, and is no doubt more robust in the end, but I think the pictures show that job is not for the timid tinkerer. I am capable, but wanted something easier. After thinking about it a bit I wanted something that could be done quickly, all from the top w/o removing the fuse box and disturbing any wiring. I also wanted it to be something you could do on one circuit or all of them and not be too intimidating to the average E28 owner. Doing this without adding problems does require soldering, but its all from the top. My board gets soldered to the existing fuse tabs.

Love the spare fuse spot on Galahad's board. I had the same plan.

My rev 1 proto turned out just about perfect. (yes, they are purple, sorry) I'm very pleased with it. I tried a few different blade fuse sockets, 2 surface mount and one thru-hole, but they're all bare. I did this because the ones @Galahad used are quite large (relatively), even when using the mini fuses, which I selected. Remember, my goal was to do all this from up top and there isn't much space for a soldering iron tip in there. I also wasn't sure what the final stack up would be and wanted to be sure the fuse box cover would still fit on. That's imperative.

Hopefully the link below works. Its a link to a google photos album. Seemed like the easiest approach.

https://photos.app.goo.gl/w4b81oyobNkcAnxNA

Its not soldered into the fuse box yet but it pretty obvious how that works. Also some will probably not like more exposed metal. I might look into a cheap plastic cover to lay over it. Honestly I'm not that worried about it. All those contacts should be at the same potential anyway. Plus, don't drop conductive shit into your fuse box, duh. :shock:

Please have a look and let me know what you think. Any an all input is appreciated!

Thanks!!!
Galahad
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Re: Custom PCB Fuse Box Fix - need input

Post by Galahad »

This is an interesting idea - not what I had pictured based on your description. I don't know how well the solder joints to the stock fuse sockets will hold up to vibration, but there's only one way to find out.
I'd suggest using the through-hole parts; I'd be worried about pulling the pads off when replacing fuses. Additionally I don't know how much current those vias can move from the bottom side copper.
I doubt your stuff will fit under the fuse box cover if you use the full size fuses. Mine clears but not by a lot and I think your setup is thicker.
I'd look into JLCPCB if you end up making more of these - they were cheaper than oshpark for my prototypes. Also, I did a write up a while ago for posting images from google photos directly.
Good job!
nebulous1
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Re: Custom PCB Fuse Box Fix - need input

Post by nebulous1 »

That's awesome. Really appreciate you chiming in with your input @Galahad. I'm an ME so not that much experience with this stuff. EEs are my favorite colleagues to nerd out with though.

Vibration - thought about this but not much. I should probably be more concerned than I am. Have a vibe table at work and could send a test thru that. Would certainly be fun but our test lab is usually booked and its probably overkill. I'll just test it on the road. What could possibly go wrong!?!

Hadn't thought about the SMD parts lifting pads. Another great point. I started with thru hole parts just for ease but have been wanting to try some reflow with paste/hot plate so gave that a go. They also add less to the overall stack up as compared to TH parts. Now that I have parts and have test fit one, clearance looks like it will be OK either way. The geometry of mine certainly eats into a good portion of the useable heightbut not much I can do about that given my approach.

I ran my protos in 1oz copper. It's a full copper pour on both sides. Added the vias because why not, and hoping it'll help share the current load some across the top and bottom copper. Who knows if its enough. I'll do some tests and measure voltage drop and see if I can gauge power dissipation. I talked this thru a bit with 2 very sharp EEs I work with. Given the poor connections of the stock fuses neither were concerned about using 1oz copper and the large soldering area on the tabs where I'm joining. We'll see though. Did you use 1oz or 2oz copper?

The purple is a dead giveaway for OSHPARK. They're local to me. I've met the guys who started it. One of them is good friend of one of my co-workers. They're always super fast. I happen to love the purple but I'm sure some will turn their nose up at that. It is certainly out of place for this application. I was planning to look at other options for building more and would definitely go with black next time. I'll definitely check out JLCPCB next round. Thanks for the tip!

Also I did find your writeup on using google photos. Ironically it looked like there might be screenshots in there I couldn't see. I was on my phone. Maybe that was it.

Thanks again. I really do appreciate it. Hopefully I'll have time to do some actual installation this weekend.

Ben
Mike W.
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Re: Custom PCB Fuse Box Fix - need input

Post by Mike W. »

Vibration and weight are related. If it's light, vibration isn't a problem. If it's heavy, like SI board batteries, the weight/momentum becomes an issue, which is why they glued them to the board to support the weight. Largish capacitors (by PC board standards) often get glued. Diodes/resistors, even transistors, not much. How much I don't know, but if you're using the mini blade fuses, they're pretty light, both the holder and the fuse, so you might be ok.
Galahad
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Re: Custom PCB Fuse Box Fix - need input

Post by Galahad »

1oz should be fine, that's what I'm using and your trace length is significantly shorter. I don't think you'll have problems if you only run single sided but may as well use both if you can.
I have found that the google photos method doesn't show up properly in safari (desktop and mobile use the same render engine). Not entirely sure why.
nebulous1
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Re: Custom PCB Fuse Box Fix - need input

Post by nebulous1 »

Life got in the way for a while but finally back at it. Hopefully the link below works.

I've had 8 installed for about 6 months. I've had the heat and functioning AC on full blast (at different times) parked with the hood closed running for 15-20 minutes with no issues. I've also had a hot day trip with the AC on the whole time and to trouble. I did learn I had a bad clutch slave though. In total I've put probably 3-4k miles on the car and no sign of any trouble.

This past weekend I installed a full complement of my latest revision as seen in the pictures. The color was a mistake on my part but I actually kinda like it. I took the car to MotorFest 12 here in the PNW. There was a lot of interest, questions and concerns. It was great to get it in front of folks that have more experience with these cars than I do. There was some great feedback.

Installation is not easy. I've done a lot of soldering in my life and this was a little tricky. I have a Hakko FX-888 soldering iron which is a great entry level "real" soldering iron with replaceable tips and temp control. There's a ton of copper on the board and the tabs suck a lot of heat too. It works but requires proper prep, cleaning and some patience. Also disconnect the battery first, duh. Forgot to do that for the first few... oops.

Have a look and LMK what you think! I appreciate any and all feedback. I still love the ability to do one circuit at a time and no tear-out of the fuse box. I need to put a bit more thought into the installation though.

https://drive.google.com/drive/folders/ ... sp=sharing

Thanks!
Galahad
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Re: Custom PCB Fuse Box Fix - need input

Post by Galahad »

Those look pretty good!
Comments:
  • Why are the fuses at an angle? Just because of clearance issues on the bottom?
  • Maybe you could offer multiple colors? I don't know what other people think but I wouldn't really prefer beige. You could color-code the boards to the fuse value as well.
  • That soldering iron is very underpowered for what you are asking of it, you're going to want a soldering gun (like a Weller D550PK) because of how much thermal mass there is. It'll go much faster and you've got less of a chance of melting the fuse box. Use flux-core leaded solder, the lead-free stuff is crap, and you'll want a fairly large diameter solder since you're using so much.
  • Based on the photos you should probably increase the pad length at the two ends - the solder joints look cramped and a few of them are overflowing onto the solder mask (Take a look at the top side of fuses 6, 8, and 16).
nebulous1
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Re: Custom PCB Fuse Box Fix - need input

Post by nebulous1 »

Thanks galahad! I'm very pleased with how it all turned out but the install is tricky.

Typed this up a while ago and got interrupted. Didn't realize I never posted it...

Fuse angle - this was done for better access during installation. It still a bit tight with the soldering tip in there but the angle does open it up a bit and helps for sure.

Color - this was purely a mistake on my part. I think the color looks great but can understand if its not most people's first choice. I do like the idea of multiple colors for different fuse values. That would be slick.

Soldering iron - I'm not a huge fan of soldering guns as they seem like such a hammer. I suspect you're right in this case, its probably the right tool for the job. On my first rev I learned that you should not attempt assembly in an un-heated shop when its 29F outside. I know better but was excited to give it a try to couldn't help myself. I abandoned it after almost freezing the tip to a tab several times. This needs to be done in much warmer conditions and more power would help. It took a lot of patience with my 70W setup.

Pad size - I hadn't thought about this but looking at it you're right. There is some space to grow the pads on the ends which would make a more robust connection.

Ultimately the approach is solid as is the finished installation. I'm excited the old junk was removed before I melted anything down. I have functioning heat and R134a AC (paid for by the PO!) and I'd like to keep it that way. I have a few little things to take care of like recent power lock gremlins I understand are pretty typical. The lock problem got worse after the installation, which was probably self-inflicted. Pro tip - Disconnect the battery before going after the fuse terminals with a wire bush! Oops! :oops:

Anyway, it does work. I have enough to do 10 full fuse boxes if anyone wants the upgrade and has some soldering skillz. I'm not going to give them away but haven't decided how much to sell them for either. This is not the project to learn on either. I can give you some pointers but this really is not the time to decide to learn the skill.

If interested shoot me a PM.

Thanks!
Ben
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