Cluster problems - no gauges, burnt traces after 2 main boards, wires seem to test OK

E28 technical advice asked and given! Troubleshooting, modifications and more.
Post Reply
Corellian Corvette
Posts: 15
Joined: Apr 21, 2021 12:37 PM
Location: Washington State

Cluster problems - no gauges, burnt traces after 2 main boards, wires seem to test OK

Post by Corellian Corvette »

Still working through the last of the electrical problems in my '84 533i. Previously had no cluster lights, no OBC, and no gauges. I replaced the mini fuse in the OBC, traced a ton of burned blubs, and now have cluster lighting and a functioning OBC. Yay!

What I don't have is Speedo, Tach, Temp, Fuel or Economy. The PRNDL lights work, blinkers work, and the lower light bar (bright, fog, oil press, BATT, Parking Brake) all work. So the cluster is getting power from the C1 (Blue) connector.

After time spent with the Bentley and the ETM, I'm totally at a loss on this one - it's above my pay grade at this point so can use some help.

When I pulled the cluster apart, it looks like the previous owner had already installed the Solid State SI board replacement at some point, so I wasn't going to be dealing with bad batteries. The cluster itself looked fine, and super clean but I noticed some burned traces on the board. I assumed (wrongly) this was an older issue and found an exact replacement board on ebay. I installed that board tonight, fired the car back up and still no gauges. When I pulled the cluster out again, the NEW board was burned in exactly the same place. Crap!

I did the cluster wire test for the Bentley manual which I believe has an error. All C1 (Blue) connector power tests were good, but the manual says to check Pin 16 on C2 (White) for main ground. There is no Pin 16 on the C2 connector, but there is a ground on Pin 16 of the C1 (Blue Connector) and C2 (yellow) which both tested fine, so I assume a typo.

This is a picture of the burned connections on the board(s). The far right row of 26 is the C1 (Blue) and the one next to that is C5 (yellow). The 15 pins on the left is the C6 connector which has a ribbon cable to the replacement SI board.

I painstakingly went through the ETM and created an Excel file that labeled each of these pins, to try to figure out what was going on. I determined that the pin numbering starts from the bottom and goes to the top, so you read the bottom Pins 1 / 14, and the top is 13/26. C6 are numbered the other way so the bottom is C6/Pin 15 and the top is C6/Pin1

But I'm still stumped. The main burned trace is C1/Pin 7 to C5/Pin 23. C1/Pin 7 is main power from Fuse 6. That's main board power. That's connected to C5/Pin 23 which appears to be power OUT to the OBC. You can see this diagram on page 149 of the ETM. So for some reason, there is a short between these two pins even though the OBC is functioning at this point. I have no idea how to diagnose this.

What's even more strange is the traces on C5/Pin 10 to C6/Pin 5, and C5/Pin 3 to C6/Pin 12. Those are all grounds as far as I can tell. C6/12 is speedo ground and that passes through both the C5 and C4 Pin 16 grounds.

C6/Pin 5 I think is power to the SI board for Speedo and Gauges.

I know this is a lot to take in, but I'm hoping some electrical guru can help make sense of this with me. The best I can figure is that there is a short at the OBC power output on C5/Pin 23 that's interrupting power to the gauges. That still doesn't explain to me why the ground traces are getting hot.

Image
Galahad
Posts: 495
Joined: Sep 06, 2016 9:08 PM
Location: Beverly/Worcester, MA
Contact:

Re: Cluster problems - no gauges, burnt traces after 2 main boards, wires seem to test OK

Post by Galahad »

Can you email me the excel sheet you made? I'm already working on a gauge cluster so this won't be a huge detour.
DJM1986-5
Posts: 1617
Joined: Sep 29, 2008 11:00 PM
Location: Greenville, SC

Re: Cluster problems - no gauges, burnt traces after 2 main boards, wires seem to test OK

Post by DJM1986-5 »

In my experience not all solid state sub boards work 100% correctly on the pre 11/85 e28s. A couple of those tracks look suspect but may be fine. Curious what is the date stamp on the main board?
Post Reply