Tach / MPG Gauges dead - Solved! Water Logged ECU, WTF?

[Mwishlist]

So my S52 swap lives! Only issue I can tell is that the Tach/MPG is dead. 1990 325i convertible

In order to get the car to start post swap, I had to bypass the OBC immobilizer, as found here:

http://www.r3vlimited.com/board/showpost.php?p=2488562&postcount=83

and here:

http://www.r3vlimited.com/board/showpost.php?p=2489328&postcount=87

So far:

• All Fuses checked out
• Tach signal wired properly from DME; continuity all the way from ECU pin to back of cluster
• Replaced SI Batteries, overall board looks good
• Installed known working cluster into my car and same exact behavior

I can't seem to get any voltage from the tach signal when the car is running, maybe a little but very small. My multimeter is very basic and I have seen a few different mentions of how to test it the tach signal. Can someone share with me exactly what to do to test that the ECU is actually outputting a tach signal? I can't believe the ECU would suddenly stop sending this. I have to believe it's wiring but I get continuity from pin 36 on the ECU to Pin 7 on the Blue C1 plug on the back of the cluster. So the wiring would have to be correct right?

[328ijunkie]

If its an early model and the OBC bypass harness isnt hooked up correctly then MPG/Tach wont work.

[Mwishlist]

Thanks for the tip! Unfortunately it's a 1990. I traced the wire from the C101 to the correct pin on the ECU and it also goes to the Data link port pin 1. And I think it's getting voltage but it's very low, but I think that's normal.

I am hearing that if the car sits for like 3 weeks like mine, not powered up; it could kill the SI batteries on the board. (Never saw that in all my research) That's my only other thought right now. Luckily my wife drives an e30 so I guess I'll pull her cluster to test and further trace it down. Two of my green service lights are staying on now even with the engine running, so that's weird.

I just thought maybe the OBC being out of the loop may be an issue since I had to bypass the immobilizer for some reason?

[Mwishlist]

If its an early model and the OBC bypass harness isnt hooked up correctly then MPG/Tach wont work.

When you say OBC bypass harness isn't hooked up correctly, how should it be hooked up/bypssed?

I pulled my wife's working cluster and put it in mine and exact same behavior: everything works except the Tach and MPG; even the temp gauge works.

I need to double check but when I ran down the tach traces again at my C101 I had continuity between the tach wire at C101 all the way back to the ECU AND continuity between the C101 and the black wire in the blue (IIRC) plug on the back of the cluster.

Totally stumped?

[Mwishlist]

Bump - and edited first post for clarity and troubleshooting so far

[Mwishlist]

In case anyone else comes across this, the answer was a water damaged ECU.

The car ran fine but no Tach / MPG gauge. I pulled my hair out for a while, traced down the wires checked everything under the sun and finally noticed the ECU pin 36 had a resistance of 165 Ohms. A guy in my local group tested his ecu Pin 36 to ground and had a massively different reading. Opened up the ECU and saw this:
[IMG][/IMG]

So yeah, famous water logged e36 ECU. I cleaned it up with some 409, scrubbed it down with a toothbrush and rinsed with distilled water. When I checked resistance again it was a massive difference. After I dried it out in the oven and looked over the solder joints. Most of them looked good but pin 36 looked a little sketchy so since it's my main issue I reflowed the solder joint.


Plugged it in and success!!! Tach and MPG gauge fired right up. Hard to believe everything else worked but that one thing. I'm sure the ECU would have failed eventually from all the oxidation etc. Anyway, another thing buttoned up.