Temp/tach gauges still not working. I don't know what else to try.

[thegoynextdoor]

Guys, help. I feel like I've tried everything to get my temp/tach gauges working on my '86 325e with no success. It has the late production design so I can't just bypass the SI board for the temp gauge. Here's everything I've tried thus far:

1. Cleaned off all the contacts on the SI board blade contacts and verified continuity
2. Cleaned off corrosion on the GND nuts on the temp/fuel gauges and verified continuity
3. Checked that all traces on the main gauge cluster board had good continuity and repaired the bad ones.
4. Repaired the original SI board which had the NiCd batteries leak everywhere (surprisingly, none of the traces were damaged): cleaned the board of battery jizz, measured all transistors and diodes out of circuit, replaced the bad ones, replaced the batteries with freshly charged NiMH ones. They're indeed wired correctly because I'm getting correct voltage at all the appropriate pins like I should. From what it looks like, it's the DIP parts that are permanently damaged which I can't get replacements of because they're not only discontinued (or custom?) but the microcontroller probably has firmware on it that I have no way of loading on a new chip. Yep, it's beyond saving. Oof.
5. Bought a working gauge cluster out of a 325i and swapped the SI boards. This one looked like brand new and the Li-Ion batteries measured 3.2V, as they should. Now at least my gauges twitch when I turn the key, but that's about all they do. That means at least they're getting power with the good SI board.
6. Tried swapping the 325e and 325i gauge cluster and swapping their engine code plugs. The only thing that read different was the fuel gauge, as expected, since they have different fuel level senders. This confirms that the connectors on the back of the gauge cluster are not the issue.
7. Tried swapping the DME with a refurbished one from eBay (I read that an SI board failure can sometimes partially damage the DME's ignition output signals going to the cluster). This did nothing.
8. Measured the brown plug output at the diagnostic plug and C101. I'm getting voltage that drops (rises? Idk it's been a fat minute since I measured this sensor but the behavior is what I'd expect) slowly as the car warms up, as I'd expect.
9. Measured the ignition pulses with a multimeter at the diagnostic connector. The frequency I'm measuring makes sense, though I haven't looked at the waveform with an oscilloscope.

Does anyone else have any ideas? With how deeply recessed the contacts are inside the gauge cluster harness connectors, I'm not sure how I'd probe the signals at the connectors. I don't know what to do at this point. I feel like I've tried everything I can and need help.

[jbontke]

You have done more than most people would so you may not get any replies. Greg at https://www.bavrest.com/ would be the one with answers.

[thegoynextdoor]

Update: This week, the temp gauge randomly decided to start working out of nowhere for the first time ever in 7 months since I bought the car and I have no idea why. The right side backlight also went out again. I guess the gremlins have decided to bless me for fixing my vacuum leaks.

Tach gauge still doesn't work. I suspect there's blades on the SI board that aren't making good connection because they have corrosion inside the female blade receptacles on the main board, which I have no idea how to clean inside those slots (if anyone has any ideas how to clean out these contacts I'm all ears). If anyone knows the part number of them on Digi-Key or something, I could solder in new ones. That could very well not be the problem though.

[ADEN]

Clean female connectors one by one till each pin pass the continuity test with the board then connect to cluster and also test continuity pin by pin from si board point to cluster mating point.

Sometimes you lose connection not because of corrosion on female pin but because the female pins to si board soldering ...desolder them and resolder them all and check.

[cory58]

Just send it to Greg at Bavarian Restoration and have him refurb it, which it obviously needs. I am very happy with the refurb he did on my cluster. Now I can focus on all the other E30 needs!