Intermittent Tachometer readout?

Post by **rsavin** » 19 Jul 2012, 15:24

Hi All, this is a 1994 850 NA Base, 185K and most things required have been done. PCV, TB, WP, Cam sensor, Crank sensor, Exaust cam seal, wires, etc. It runs great! However, my tachometer seems to work intermittently. It operates most of the time but every now and then it will read 0 when I start and drive the car. If I race the engine up to about 4K RPM the needle will swing quickly to 4K and above. If I let off on the gas it will settle back to 0 Rpm. This will happen in Park or while driving. Then after a while I'll look down and it's working perfectly. When it is not working I can always put my foot down and rev the engine and get the needle to move. It has never just remained at 0. I have changed the Crank Sensor (brand new Bosh) and the Cam sensor (used ebay). Is there another "RPM sensor" besides these two? AND (I forgot to mention) it also will malfunction after a car wash or in the rain but only sometimes? Forgive me if this has been covered but I've serched many, many topics here and at Volvo Forums and can't find this problem discussed. Thanks very much to anybody that can help with a little info.
RS

Rocky · Post by **Rocky** » 19 Jul 2012, 16:22

If the rev counter is run off the alternator, then the alternator is about to die.

Post by **Ozark Lee** » 19 Jul 2012, 17:50

The tach fed from the RPM sensor which is the same as the Crank Position sensor via the ECU. Since you changed that sensor you either have a cluster going south or a wiring problem going to the cluster.

The tach signal wire is white with a black stripe and it goes to pin B-29 on the ignition module in the ECU. It might help to just remove the ECU modules and give them a shot of contact cleaner on the edge connectors.

...Lee

Post by **rsavin** » 20 Jul 2012, 11:20

Thanks a million, that's great advice and gets me to the right spot to start! Much appreciated!!!
Rick