1988 325is: Tach not working, (yes i searched!)

[swerved300]

ok... here's the deal:

my tach doesn't work, neither does the MPG gauge. the MPG gauge is stuck high at >40, while tach stays at 0.

SI board lights up 3 green at start up and goes out immediately afterward.

I replaced the tach with another I believed to be good and am still experiencing the same issue. On a side note, the OEM cluster is VDO, and the tach I replaced it with was from a motometer(out of a 90 vert). It was plug and play, fit with no issues or visible differences.

The speedo, fuel, temp, etc. all appear to be working fine.

on a longshot, I will mention that my headunit immediately pops every fuse I throw at it(even with the car off, keys removed)- I haven't taken the time to fix this yet and am unsure of the root cuase for the radio.

All electronics aside from the tach and MPG gauge work.

TIA for any help, comments, or snarky remarks.

-ken in honolulu

[Kershaw]

did you verify that the replacement cluster worked?

[swerved300]

did you verify that the replacement cluster worked?

I was told that it was working by my friend who provided it to me. Is there a procedure to check functionality with the gauge out?
-ken

[bimmerkarl]

are any of your dash lights inop?? as in the ones that illuminate the intruments, OBC, buttons etc... the head unit could be the root cause... the faactory head unit had fadable illumination that tied into the rest of the dash via the headlight switch... my 87 had stereo wiring issues when I got it that caused all kinds of problems with the instruments and lighting...

[swerved300]

are any of your dash lights inop?? as in the ones that illuminate the intruments, OBC, buttons etc... the head unit could be the root cause... the faactory head unit had fadable illumination that tied into the rest of the dash via the headlight switch... my 87 had stereo wiring issues when I got it that caused all kinds of problems with the instruments and lighting...

yo karl, sorry i didn't make it to your old lady's party the other day. naw, all the lights and everything is working fine except radio and tach.

-ken

[bimmerkarl]

No worries... looks like it's time to break out the multimeter and start digging... if it's popping the fuse and that radio is aftermarket, it's most likely a short somewhere behind the radio... there are lots of places for wires to get caught in there once you start hacking stuff up... all the cable and bellow sliders for the ventilation tend to chew on wires if not secured properly...

[jlevie]

What fuse blows? Fuses 10, 12, & 21 are associated with the cluster. If one of those is blowing it could affect the tachometer.

[swerved300]

What fuse blows? Fuses 10, 12, & 21 are associated with the cluster. If one of those is blowing it could affect the tachometer.

fuse #21 pops/sparks out as soon as a new one is installed. Like instantaneously. Guess I'll pull the radio and poke around.

[swerved300]

I found a short in the trunk light. The plastic housing was broken and allowed the trunk light to contact the trunk lid, causing the short. The radio, and all associated lights, OBC all work now that this short has been alleviated. However, the tach still doesn't work.

Is it possible that the short blew out the tach, and it will never work unless replaced with a new unit? Any input appreciated. Hopefully this thread helps others diagnose the phantom #21 fuse blowout.
-ken

[swerved300]

Just swapped the DME with a known good one, it is not the DME.

[swerved300]

one more thing i just now noticed: when I turn the car off after a second there's a small 'blip' on both the tach and MPG needles. So there is some kind of signal.

I also tried kind of wiggling the CPS(crank position sensor) cable and there continued to be no reading on the tach.

[jlevie]

That small blip says that the tach has power. The next likely suspect, since you've swapped DME's is the SI board.

[rares]

si board batteries... i had the same thing just happen to me... no tach, no mpg, and no temp gauge... si board batteries are most likely dead... quick and cheap fix... no worries...

google si board for e30 ....lots of good info on how to change them...

good luck

[alreadyRogue]

si board batteries... i had the same thing just happen to me... no tach, no mpg, and no temp gauge... si board batteries are most likely dead... quick and cheap fix... no worries...

google si board for e30 ....lots of good info on how to change them...

good luck

Would this cause the fuses to burn through immediatly like he said. The exact same thing is happening to me right now. I put in two new fuses and all of them burst when the car key was turned to the first position. I do have an aftermarket stereo so I wouldn't be surprised if that was to blame.

[swerved300]

Would this cause the fuses to burn through immediatly like he said. The exact same thing is happening to me right now. I put in two new fuses and all of them burst when the car key was turned to the first position. I do have an aftermarket stereo so I wouldn't be surprised if that was to blame.

my fuse was blowing because the trunk light was grounding itself on the trunk lid. Check the wires and light. The wires can get chafed or the plastic light housing can crack and allow it to contact the body; grounding itself and blowing the fuses out.

If the SI board was bad/ batteries dead wouldn't the SI misbehave somehow? it's working fine, there's no trouble codes, i get the green lights like i should, check panel and OBC working fine....
Is that consistent with bad SI board?