850 Tachometer Problem

Helodrv · Post by **Helodrv** » 31 Aug 2007, 18:16

The tachometer in my 1995 850 wagon has become very erratic. Sometimes when I start the engine, it never moves off 0. Sometimes it will act as though it is beginning to register, get up to what appears to be the current RPM, then drops back to 0. Other times it runs perfectly. I do not have any other electrical problems, no caution/warning lights, no system outages. The speedometer and all other instruments and gages are working as they should.

Does anyone have any experience with tachs in the 850's? Where do they get their signal? It acts almost as though it had a loose connection or a short. I have removed the instrument cluster, cleaned all the contacts with a fiberglass pen and contact cleaner, then reinstalled the cluster, all with the same results.

If it had just died, I would have suspected the tach had died in the cluster. But this on again/off again behavior doesn't make me think dying instrument. Any ideas or suggestions would be appreciated.

Post by **MadeInJapan** » 01 Sep 2007, 00:57

I would just find a good used cluster on eBay and replace yours. Sounds like the tach has just died in it.

CarVolvo · Post by **CarVolvo** » 01 Sep 2007, 12:27

i can tell you out of all the 850's ive worked on or had that i never had a problem with the tach (now if you want to talk about 240's thats another story)

I would suspect like MIJ stated that the circuit board on the unit is starting to die. If i had a spare tach i would be happy to send it to you. But i only have complete assemblies. NONE of them have issues with the tach.

The main issue is with the odometer gear. I do find these units reliable.

Post by **billofdurham** » 01 Sep 2007, 12:39

On the turbo model the pulse comes from the auto transmission control module in the left front of the engine compartment, pin B20 green/brown wire. This goes to the Motronic control module in the right front of the engine compartment, again pin B20. From the Motronic control module, pin B21 is a white/black wire which goes to the tach.

On the non-turbo model the pulse comes from the ignition system control module which is in the right front of the engine compartment, pin B29 white/black wire to the tach.

Bill.