Speedometer not working after diff swap

E28 technical advice asked and given! Troubleshooting, modifications and more.
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Dana in Reno
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Speedometer not working after diff swap

Post by Dana in Reno »

I just replaced the differential in my 88 535is with a 3.46 LS unit. It's all good, except that now the speedometer doesn't work. It worked fine before, so I assume it's not a problem with the cluster. I swapped the sensor from the original diff into the new one, and still no speedo. What could it be--wiring? Is there anything to go wrong with the sender inside the diff?
joegreen
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Post by joegreen »

the sensor is the sendor, there is only a toothed wheel where the sensor get the pulse inside the diff, you should leave the sensor from the diff in the diff and check wiring....
Brad D.
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Post by Brad D. »

As long as the tone ring (the toothed/bladed wheel that is attached to the differential) is intact, then it sounds like its a wiring or sender issue.
Dana in Reno
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Post by Dana in Reno »

How can I tell if the tone ring inside the differential is ok? With the sender revmoved, I can see the edge of it through the hole, but that's about all.
joegreen
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Post by joegreen »

you can try looking at it while spinning the diff, and see if a blade is gone or bent maybe. but if the sender is good its likely a wiring problem

try taking a good look at the connector in the light. see if a pin is bent or pushed back.
tsmall07
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Post by tsmall07 »

Dana in Reno wrote:How can I tell if the tone ring inside the differential is ok? With the sender revmoved, I can see the edge of it through the hole, but that's about all.
A missing blade on the tone ring would only cause the speedo to jump, not stop completely. It would only stop completely if all the fins on the tone ring were gone, which is very unlikely (not to mention that fact that the diff would be completely ruined with that much metal floating around in there).

It sounds like a sender/wiring issue. put a multimeter on the contacts and spin the d-shaft. The circuit should alternate between open and closed (I think) as the diff spins.

Edit:
mooseheadm5 wrote:THe sensor is actually a magnetic reed switch that pulses the ground open and closed as the blades of the tone ring in the diff pass through it. If you hook a multimeter up to it set to ohms you should get alternating open and closed circuit readings as you turn the wheels (off the ground, car in neutral.) If the sensor passes, you need to check the wiring harness from front to rear. You can use a spool of copper wire to make really long leads for your multimeter.
ref: http://www.mye28.com/viewtopic.php?t=68 ... ght=speedo
Dana in Reno
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Post by Dana in Reno »

OK, with the ignition on and the wire harness unplugged from the differential, I get about 8 V across the 2 wires coming from the front of the car. This suggests it's not a wiring problem. With the plug connected back to the sensor the voltage drops to about 0.1 V. I don't understand why this would happen, but assuming it's normal, it sounds like the next test would involve putting an ohmmeter on the sensor contacts with the power lead disconnected and spinning the wheels. Is this right?
tsmall07
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Post by tsmall07 »

Dana in Reno wrote:it sounds like the next test would involve putting an ohmmeter on the sensor contacts with the power lead disconnected and spinning the wheels. Is this right?
Yes. Why didn't you do that while you were already under there?
Dana in Reno
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Post by Dana in Reno »

Wish I had, but I hadn't seen the previous post at the time.
Dana in Reno
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Post by Dana in Reno »

tsmall07 wrote:
Dana in Reno wrote:How can I tell if the tone ring inside the differential is ok? With the sender revmoved, I can see the edge of it through the hole, but that's about all.


[
A missing blade on the tone ring would only cause the speedo to jump, not stop completely. It would only stop completely if all the fins on the tone ring were gone, which is very unlikely (not to mention that fact that the diff would be completely ruined with that much metal floating around in there).
This is weird. I put an ohmmeter on the sensor and got continuity in all positions while turning the differential. With the sensor removed, I could look into the hole and see the blades on the chopper wheel passing by as the diff turned. Then I put the same sensor in a spare differential, spun that, and got alternating open and closed circuits, as expected. I noticed though, that the blades on this differential were easier to see. It turns out that the distance from the sender mounting surface to the tip of the blade is about 12 mm in the differential where the sensor works and 19 mm in the one where it doesn't! That explains the problem with the speedometer, but how can it be like that?
alpinewhite
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Post by alpinewhite »

Is the differential you sourced from a e28? And are you sure about the ratio?
Dana in Reno
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Post by Dana in Reno »

According the the seller, who I trust, this differential is from an 88 528e automatic. For what it's worth, I have now opened both differentials and confirmed that the diameter of the tone wheel (chopper wheel) that generates the pulse is about 5 mm less in the LS differential than in the original 3.25 open unit. None of the blades appear to be damaged or worn, so I have to assume it was built this way. The question is: how did it work in the original car?
a
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Post by a »

The pickup is a one size fits all part. They either work or they don't. If a known good un doesnt fix the trouble , it could be a loose wire or a bad speedo or board.
Dana in Reno
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Post by Dana in Reno »

After starting this thread I heard from one other member who was experiencing the same problem after transplanting an e32 differential into an e28. Apparently some of these differentials really do have a smaller chopper wheel inside.

For the record, here's how I solved the problem. By making a series of measurements and testing with a multimeter, I determined that the speedometer could be made to work by moving the sensor toward the center of the differential by at least 3 mm. I had a machine shop modify the differential cover and the sensor to achieve this. They machined half the amount from the cover and half from the thick plastic flange of the sensor. Problem solved. The speedometer now works as it should.
euro635gas
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Post by euro635gas »

Or you could have just used this. Later style pick-up sender. I swapped in 3.64LS from an E32 R&P into an E28 case for my 533i.

Image
LarryM
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No Speedo after 3.64S (from E34) swap into E28.

Post by LarryM »

Dredging up this thread because I have the exact same situation: inoperative speedo after 3.64S diff swap, and I have to identify the correct parts that I need.

(euro635gas, I also PM'd you)

In the previous post on this thread is a pic of the diff-mounted sensor I think I'll need to get my speedo working again

Can anyone tell me if this is a common part to all E32 and E34 diffs, or is there a specific one I should be looking for? Do I need one from a limited-slip diff or will any one work? Part #'s would be greatly appreciated.

I'll also need the plug that goes into the sensor but I don't want to buy an entire harness, so if anyone has a lead on a used one I'm interested. This is assuming it's just a matter of splicing the new plug into my car's existing harness.

Thanks!
euro635gas
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Post by euro635gas »

PM sent
But I can't remember if it came from E32, E34, or E36.

Does not matter if it is LS or not

It does work as I am still driving that diff in my 533 and have a working speedo.
LarryM
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Post by LarryM »

Update:

I did purchase and install the impulse sensor that euro635gas used in his diff and it worked for me. I'm sure there are others who will swap a diff from an E32 or E34 into an E28, and it'll be very useful for them to know that it's necessary to use the sensor from that diff in order to have a working speedo.

Part # for the sensor (officially called a "pulse generator" on RealOEM): 62-16-1-390-063

You'll also need an O-Ring, part # 33-11-1-206-166

Finally, you'll need to find the two-pin female connector that plugs into the sensor. Best is to find someone parting an E32 or E34, and have them clip the wires to the connector leaving plenty of wire length for splicing into your car's harness. After that, it's a plug 'n play. :banana:

On a side note, the E32/E34 sensor doesn't really extend into the diff any further than the sensor from an E28; BMW must have made some other changes in the way it picks up the signal from the tone wheel ("chopper" wheel).
kodachrome
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Re: Speedometer not working after diff swap

Post by kodachrome »

Just to bounce this, ran into same problem with a Z3 torsen 188mm diff install into my e28. Had to swap over the Z3 speedo sensor, which I just jammed the stock spade connector into. Works fine.
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