please help! Temp gauge & tach not working still

[AlpineE30ic]

I know this is a popular post but I cant seem to figure this out still even with all the write ups out there

heres my issue: 1992 318i Convertible - gauge cluster has 153k on it. when i bought the car a few months ago, the tach and temp were dead. first thing i did was replace the SI batteries. no luck... then i purchased another used but tested temp gauge.. no luck. Both my existing and newer temp gauges did the same thing which is: upon turning on the ignition, the needle would peg to full hot and when the motor turned over, it would go back to cold..then no response.

You may say my SI board is dead, but when the car is on & idling - the service indicator lights are on. which is a sign that the SI board is getting power right?

any suggestions?? I'm worried that by replacing the whole cluster wont do it, and ill be into this issue for hundreds of dollars with no solution...

HELP!

Thanks guy!

[AlpineE30ic]

not sure if its worth noting but the car has been sitting the whole time even after the SI battery replacement. its a project but not sure if even new SI batteries need to be charged by having the car running a lot?

[DesktopDave]

The newer style lithium batteries do not recharge, from what I've gathered.

You've tested your coolant temp sender, right? Given how cold it is, you should see very high resistance. I'd think 12K ohms against ground would be OK. Resistance should drop as the coolant temp rises.

[AlpineE30ic]

I actually replaced the engine temp sensor which is under the top end of the engine near the fuel injectors. still nothing

[DesktopDave]

Not sure which you're talking about - the DME CLT sensor, or the gauge sender. There are two mounted next to each other on the head, they look very similar. The sender only has one wire.

[AlpineE30ic]

I think it was the gauge sender..? It was the one with the brown plug, not the yellow which is connected to the radiator

i think there is a bigger issue here since the tach is dead as well..or two seperate issues.. i just dont knoww!

thanks

[DesktopDave]

Stole this pic from another member...

If you look closely, you can see both sensors. In the center is the one I'm talking about - the other one is visible in the background, off to the left.



Don't replace the whole cluster. It's probably a small problem, perhaps a burned trace on the driver board or a bad SI board. I have replaced SI batts in the past and had them fail...very frustrating! Working SI lights don't necessarily mean the batteries are functional.

If you need some cluster parts, I have a two incomplete VDO clusters. I'd think there are enough good parts left over to (almost) make one functional set. Faces are scratched, neither has a working odometer, they're both from Etas (thus 4500 RPM tach face), and I'm pretty sure if the fuel gauge is calibrated for the dual sender fuel tank. Despite all that, I can tell you for sure that one has a working driver board and coolant temp gauge. I pulled that cluster from my own car when I rebuilt it this year. PM me if you're interested.

[DesktopDave]

I learned something new today. According to the ETM, the SI board also controls the tach. The signal comes in from the DME on a black wire via cluster connector C1, pin 7. A 14Kohm resistor conditions the signal as it goes to SI pin 8. It's processed by the SI computer then sent out to four pins (?!?) SI 15-18. In a related circuit, pins 11-14 control the economy gauge.

Note anything in common? I'd bet you need to re-check those batteries. Hopefully you don't need a new SI board.

[AlpineE30ic]

Hmm okay very interesting stuff. I appreciate your research. Ill bring my SI board somewhere tomorrow to have it tested and see if those batteries are doing anything.

One more question for you.. I did install a brand new coolant temp sensor. This is the sensor which sends the signal to the gauge. My question is, if the other sensor next door to that one ( the brown one) which sends a signal to ecu is junk - will this effect the temp gauge at all?

[monty23psk]

The ecu temp sender will affect how the engine runs, it has nothing to do with temp guage.