Tach/MPG Guages Erratic + Transmission limp - 95 325i e36

[bayardo]

I'm having a strange automatic trans problem with my 95 325i with 128k miles. It is coupled with a tachometer and mpg gauge glitch.

All works well, then randomly the tach/mpg guages become erratic.
The tach jumps quickly between normal and 0, and MPG indicator jumps quickly between normal and infinity. Then the transmission light (the gear with an exclamation point) comes on and the tranny goes into 'limp' mode (which skips 1st gear in my case). When I turn the car off then back on... all problems go away and trans works flawlessly... until the next erratic behavior of the tach/mpg guages. ALL of these problems first appeared (and continue to appear) at exactly the same time, and no work had been done to the car just prior. All symptoms have happened in the first few minutes of driving or just after the car was cranked and still in Park.

I've always changed the trans fluid/filter regularly, most recently about 3k miles ago. I just checked fluid level, and it is clean and full. I plan to change the SI board batteries, but I'm wondering if that will help the transmission issue. If anyone has any ideas on what would cause this or what I should try, it'd be a huge help! Thanks!

[bayardo]

Update:
I removed the instrument cluster and couldn't find any SI board batteries in the e36. I'm guessing that's just an e30 thing.

I also noticed a clicking noise coming from the passenger-side floorboard/firewall when I shift from P to R or to D/1/2/3 etc. which only happens when the tach/mpg needles are going crazy. No clicking noise when car is operating normally. So I'm beginning to suspect ECM/TCM/DME.

I found two similar ECU-looking boxes there, does one control the engine and the other the transmission? If so, does anyone know which is which?

[Zhivoin]

This just happened to my 318is 1993 with automatic transmission. ECU is on the bottom and transmission computer on the top. I am getting the book on e36 soon, but I am guessing that maybe a relay is acting up, hence the clicking... I will post more when I know more...

[bayardo]

Thanks Zhivoin. Just got another transmission computer, so we'll see if changing that helps.

[Acb218]

I just started having the same problem that you described above with my dad's 325ic
Did you fix yours? If so, HOW??
Hopefully you can help!!
He just bought a new car for me anyways, so it wont be driven until it is fixed...
i would just hate having to say goodbye to the convertible over something minor.

[Manolito]

The good news is that many e36 automatic transmission problems are corrected by replacing the battery or even just cleaning the battery terminals. The transmission computer is extremely sensitive to voltage variations. An old battery that will start the car easily may not keep the system voltage stable enough for the transmission computer to stay happy.

The bad news is that you mentioned that it is a convertible. The e36 convertible uses its battery as a weight for its vibration dampener. That means that it uses a battery that is designed to tolerate being shaken vigorously.

The proper battery is hard to find and is very expensive. You mentioned having just purchased this car. There is a high liklihood that the previous owner put in a conventional battery, which may be the source of your problem.

[Acb218]

Sorry for the confusion.
My dad is the original owner.
He bought another new car so he wouldn't have to deal with the problem, so I will deal with the problem.

The battery is a good point though. I'm pretty sure the battery is about 6 years old, as I think we got it when we bought our audi.
Anyways, I will put the battery on the charger tonight and pick up a new battery during the Christmas sales.
Thanks again!

[fast2002]

Did you solved the problem? Thanks