tach & clock broken, short circuit in harness

[JHZR2]

Hi,

Last week, I posted a thread regarding the fact that I am having toruble with the tach and clock not working. It all started after working on the wiring for ly left tweeter. But that was all that I touched.

Heres the situation:

I found that the PCB (motherboard) had a burned trace between pin 2 on the blue connector and 23 on the yellow. This is the wiring for the constant power to the clock. I repaired the trace, and continuity everywhere on the boards was fine.

Today I went to the car, connected the three harnesses to the cluster. Immediately the trace burned where I had repaired it. I fiddled with it a bit, and got continuity again. I verified voltage on pin 2, blue connector (12V in), and it was 12.25, same as the voltage at the fusebox. I reconnected everything, and it burned the trace at a second spot on the PCB.

Thinking it was the PCB that was bad, I took a larger wire and jumpered from the voltage input (pin 2, blue) to constant to the clock (pin 23, yellow). Immediately fuse 21 blew. Thinking that I may hav e bridged a connection causing a short, I replaced the fuse, and carefully tried again. Once again it blew fuse 21.

So let me get this straight... fuses blow when too much power is flowing through, indicating an abnormality or overload. Chances are that the PCB trace is a far finer wire than what is in a 7.5A fuse, so it blows first. When the PCB is jumpered, then the fuse is the next weakest wire, and it blows.

What would make it blow? I know it is somewhere between the pin 23, yellow connector and the clock. This happened when the clock was disconnected, so the clock cannot be considered at this time as the culprit. Where does the wire that goes from pin 23 on the yellow connector go to? Does it only supply continuous power to the clock, or does it have other functionality?

Judging from research regarding the yellow harness (from the multitude of writeups on OBC retrofit), it seems that the connection between the yellow instrument cluster connection goes only to the clock. I never messed around in that area when fixing my tweeter.

Any suggestions on what to do now? If I was to have the harness replaced, is there anything that I need to know?

Also, the tach still doesnt work, and the SI lights remain illuminated (change betwen 4 and 5 lights, and inspection illuminated). All connections were good, and I verified their traces to connectors on the mainboard/SI board. I did not see that the trace that I was burning had any relevance to the tach power, although A2 (pin 2, blue connector) does apparently feed power to the SI board. Since my SI board batteries are OK, and would you guess it is themainboard that is causing its problems as well?

Thanks very much,

JMH

cc: roadfly

[JHZR2]

I also checked that there was continuity between pin 23 on the yellow connector and pin 7 (constant power in) on the clock's wiring harness. It was continuous and totally fine.

I then jumpered pin 2 on the blue (12V in) connector to pin 23 (12V out continuous) on the yellow, thus pypassing the instrument cluster completely. The clock was not connected to the harness on the other end. Fuse 21 immediately blew, thus verifying that the cluster's problems are unrelated to the issue that Im seeing.

Could I have 0 ohm resistance betweel pin 23 yellow and pin 7 at the clock end, and still have a short circuit? Or what does this result indicate?

Thanks,

JMH

[arnoldgtr]

I just checked the wiring for the OBC. I don't have the diagram for the clock, but I think both have similar setups. The red/white wire also powers the chime module. Check and see that the module is not shorted.
HTH,

[JHZR2]

great... now I'm getting somewhere.

I didnt see that in the wiring charts that I have. But the chime that youre talking about is the one in the kick panel near the hood pull, right?

If so, I did pull that chime out when working on my speakers, which was when all apparently went bad in the first place.

Maybe my chime went bad, I wouldnt doubt it, although I think it still works...Ill explore that tomorrow.

Can you by chance point me to the URL where you get your charts from, or post aq pic of the wiring of the chime?

Thanks very much,

JMH

[arnoldgtr]

Yes, the chime module is mounted in the knee bolster.
These wiring diagrams are from the ETM (Electronic Troubleshooting Manual) for a 1987 M3. Although the ETM's are year and model specific, I have found that virtually all the wiring is the same for all E30's. Look on the right side of figure 6581-1. Ignore the OBC relay box and board computer horn...those items are only on cars with the OBC.

I also have the diagrams for the instrument cluster. There are 5 pages.

[JHZR2]

thanks very much!!!

JMH

[Zoncers]

Did you ever figure out what was causing this? I have the exact same problem with my 91.