Tach not working (yes, I've searched and replace the SI batteries already)

[thrashandburnn]

The issue that I am having is intermittent. Sometimes it works, and sometimes it doesn't. I have replaced the SI batteries already although I don't think that had anything to do with it. I don't want to have to get another tach if my issue can be resolved elsewhere on the car. Every other forum that I have read says it's the SI batteries, but like I said, I don't believe that's the issue. I don't know where to go from here. Any help is appreciated.

UPDATE: Nothing else ever came of this. I have not had any further issues. If something else comes up, I will come back and give an update. Thanks.

[twright]

There are two other (and maybe more) reasons that your tach isn't working:

1.) The black wire on the blue plug of the cluster (pin 7) isn't getting a square wave signal from the ECU
2.) The wire from fuse 21 to the cluster blue plug isn't getting 12 volts. On my car this was a red/green wire

[packratbimmer]

Early or late cluster?

[82eye]

also possible you might have to re-flow some solder on the board.

[packratbimmer]

If you have an early cluster, you can do the SI board bypass for testing purposes. I have a 30X jewelers lens to look for solder flaws.

[thrashandburnn]

Early or late cluster?

'87 VDO cluster

[thrashandburnn]

There are two other (and maybe more) reasons that your tach isn't working:

1.) The black wire on the blue plug of the cluster (pin 7) isn't getting a square wave signal from the ECU
2.) The wire from fuse 21 to the cluster blue plug isn't getting 12 volts. On my car this was a red/green wire

Thanks. I'll look into that.

[thrashandburnn]

also possible you might have to re-flow some solder on the board.

I've already reflowed what i could find. NO luck

[E30Bastard]

Hello,

Is it just your tach? Is anything else on the cluster acting up? Intermittent tach suggests a wiring short,

Do you have a megasquirt? There's an option to enable tach there. IIRC I had intermittent issues until I enabled it.

There is also a coding plug which plugs into the bottom of your cluster, IIRC it is only accessible after pulling the cluster from the dash. If this plug is not the correct plug for your cluster or if it is faulty, it would cause issues with tach.


A little more detail from Twright's first reply (good info):

coolant temp -- br/vi; pin 26 on C1 and pin 4 on C101
fuel level -- br/gn; pin 4 on C2
engine speed -- bk; pin 7 on C1

This is cluster info.

More help will be testing from ECU to tach, and input wires to ECU, etc. You should have Motronic 1.3 (173 ECU, green label, verify that you have the correct ECU for your car). Do a continuity test from ecu pin 6 to tach pin 7 first, that is easiest. Use alligator clips for your voltmeter and jiggle wires by ecu and by cluster to see if you can find a short. If everything checks out you'll want to run continuity tests on pins 47 and 48 from ECU to engine. These go through connector C104, which is an infamous location for wiring faults due to all the dust, grime, etc. This is the circular connector under your fuel rail on the intake side. Pull that apart and clean the connectors (both male and female side) with DeOxit D5, then run continuity tests to your ECU.

Hope this helps solve your issue. I'll be happy to loan you a tachometer for testing if you run these tests and cancel out the electrical faults.

[E30Bastard]

[E30Bastard]

Any updates? Resolution is great for the forum community!