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Temp gauge pegged when engine cold

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  • Temp gauge pegged when engine cold

    My instrument cluster is giving me problems. One problem is that the temperature gauge is pegged even with the engine is cold. There's been a lot of history leading up to this. Let me try to get y'all up to date semi quickly. Probably not, but I'll try to be brief.

    My tach and coolant gauge have been a little wonky for a long time. The tach would intermitenly drop to zero RPM. The coolant gauge mostly worked correctly except that it would be unresponsive for a bit after i started the car. If I waited long enough or hit the dash, it would spring to life. I sent the cluster to Bav Restoration for service. Unfortunately this behavior didn't change after I got the cluster back. I checked what I could outside of the cluster including grounds and splices and didn't find any problems. I tracked down most of the splices involved, but didn't find one or two. I took notes on which splices I found.

    The cluster was still under warranty and Bav Restoration agreed to check it over again. They didn't find a smoking gun but did fix a few things.

    No comes where I may have damaged something. After I got the cluster back the second time, I added a couple warning lights to the unused spots at the bottom of the cluster. I added one light at K spot at the far left hand side and another at the rear fog light spot just to the right of the front fog light spot. I supplied these new bulbs with power and ground using external wires. It is possible that I made damaged something when i did this. Maybe i supplied power or ground to the wrong terminal.

    Next I hooked up the cluster to make sure everything worked. It did for a bit. The tach worked for a minute or so and then died. The temp gauge worked for a little longer than the tach but then swung all the way to hot even though the engine was not over heated. The OBC would illuminate but not display anything. After some more trouble shooing, I suspected that the SI board was giving me problems. At first I hoped that the batteries were low from the cluster being out of the car for months. I let the car charge the batters for a while and nothing changed. Then I suspected that I may have damaged the SI board by adding the warning lights I mentioned above. Now I have a second, refurbished SI board. I installed it. I removed the warning lights. I installed the cluster. The coolant gauge still goes to full hot as soon as the cluster is installed even with the key off. It will drop back to full cool if I disconnect the blue connector, C1. It will also drop to full cool if I disconnect the brown coolant temp sensor. I have not even turned the key to on. At this point I'm afraid to power up the car or start it for fear of ruining another SI board.

    Any ideas on what to check next? It's possible that I need a new printed circuit board for the cluster. Maybe I damaged the coolant gauge. I've looked it over and don't see any problems. I've replace the brown coolant temp sensor. The new sensor was installed with the old SI board for a while. It's reading 850 ohms with the coolant at about 60 -70 degrees F.

    - Brad

  • #2
    Update - I found a burnt trace on the printed circuit board. I jumpered across it and restored electrical continuity. Now tach is working intermittently, and the coolant gauge is now readying full cold when the car is cold instead of erroneously reading full hot. The temp gauge didn't move off of full cold, but I did not run the engine long enough to put any real heat into the coolant. I wish the tach was working properly. Maybe the SI batteries are a little low. I'll have to do more trouble shooting.



    • #3
      It's likely your SI batteries are failing and causing the tach to operate intermittantly. On the Temp gauge, check for a cold solder joint at the resistor for the temp gauge. SI Batteries can affect temp gauge as well.
      Ron ///Man

      • '91 Gr-A Former CiBiEmme / Ravaglia - Sold
      • '90 M3 Faux EVOII Alpineweiss 36K Orig Owner - The Queen
      • '91 M3 Faux EVO III Brilliantrot Euro Driveline - The Rocket
      • '91 M3 Faux Gr-A Club Racer DM - The Alter EGO
      • '89 M3 M3T / ITR Club Racer
      • '94 Spec E36 - Eh....
      • '09 M3 - Tarmac Terrorist
      • '04 330Xi Sport 6 Speed - Snowmobile
      • '07 530 Xi - Highway Star


      • #4
        An intermittently working tach can also be caused by a cracked solder joint in the coding plug circuit board. Don't know if it would affect the temperature gauge as well. It didn't on mine.


        • #5
          My SI batteries should be good. I have two SI boards. One was serviced by Bavarian Restoration recently and the other is a refurbished board from Programa. The batteries in the board I'm using now read 3 volts. I haven't tested the SI batteries under load, so i can't be certain that they are good.

          My coolant gauge seems to have gone back to normal after I repaired the electrical trace on the cluster's printed circuit board. Repairing the trace also improved the tach from not working at all to working intermittently. After a little more trouble shooting I found a bad fuse in the fuse 10 spot. The tach seems to be working correctly after I replaced that fuse. I don't think the fuse blew. It was a new fuse I had bought recently. I think it broke/cracked because I was removing and replacing it so much during troubleshooting. It was hard to see the damage to the fuse. After a closer look, it seems the new 7.5 amp fuses I bought are poorly made. The electrical contacts can wiggle around pretty easily. As they wiggle, they stress the fused connection between the contacts.

          Ron, I still have the cluster out of the car, so I should check the solder joints at the temp gauge resistor like you suggest.

          Thanks for the tips guys!


          • #6
            One coolant temp gauge issue I had in years past was a loose nut on the back of the cluster, right where the needle pin would be. Give that a good tightening.
            "It is needless to say that self-propelling vehicles, like other machines, will never do as much for one who does not understand them as for one who does."


            • #7
              The cluster is back in the car and everything seems to be working correctly.

              Thanks again for all the help.