General: erratic rev counter

[anchovy]

my 95 850 auto has developed a small problem. twice the rev counter hasnt worked when driving but came good after 10 or 15 minutes.
today it stopped working but the needle sat on 0 just giving little jumps up to 1500 or 200 revs. after about 10 minutes it came good again and is currently working.
what should i look at in the first instance ?
thanks.

[green van man]

How are the alternator bushes?
I had this problem on a 460, turned out the alternator was on its way out. New alternator solved the problem.
Rev counter was driven from it on the 460 and believe the same system on the 850 but may be wrong on that one.

Paul.

[anchovy]

thanks for the reply. cant see the obvious connection between alternator brushes and rev counter but, doubtless someone will confirm or deny your theorem.

[Bigfella666]

Anchovy

No direct help in response to your post but after 5 yrs with no odometer I am hoping to make the time to restore the genuine miles on my 850. The garage that serviced her for many years told me it was a cable fault and the unit could only be purchased complete at £700 ! Naturally I told him to poke it.

Anybody any ideas what he was talking about - my guess is it is the usual cog fault and he was talkin rubbish ?

[Luxobarge]

Originally Posted by Bigfella666

Anchovy

No direct help in response to your post but after 5 yrs with no odometer I am hoping to make the time to restore the genuine miles on my 850. The garage that serviced her for many years told me it was a cable fault and the unit could only be purchased complete at £700 ! Naturally I told him to poke it.

Anybody any ideas what he was talking about - my guess is it is the usual cog fault and he was talkin rubbish ?

OP's fault is rev counter, not odometer.....

[colwyn]

I cant help but I I had a funny problem with my v70, the speedo needle would have a mind of its own going from 0 to 140 bouncing a few times then go back to normal then shortly after the abs unit went but I promply fixed it and it turned out to be cracked solder on the power pins in the connector block

[Luxobarge]

Originally Posted by colwyn

I cant help but I I had a funny problem with my v70, the speedo needle would have a mind of its own going from 0 to 140 bouncing a few times then go back to normal then shortly after the abs unit went but I promply fixed it and it turned out to be cracked solder on the power pins in the connector block

That's because the speedo is driven from a signal from the ABS ECU. However, the rev counter gets its signal from the engine ECU, and ultimately from the crank position sensor (CPS) - I'd have thought that the fault would be some bad contacts in the wiring between the rev counter and the engine ECU, but exactly where to look is anyone's guess. If it were me I think I'd take the dash top off (easy on these) and have a fiddle with the cluster, trying to make sure all the plugs and contacts are good?

[anchovy]

just noticed that every time i drive the car the rev counter works for about 60 seconds then dies. very strange. will attempt to check the electrical connections on back of instruments tomorrow.

[Luxobarge]

There's a "how to" on how to take the dash off in the articles section in this forum here:

http://www.volvoforums.org.uk/forumdisplay.php?f=79

8th one down the list.

Hope that helps?

Cheers

[anchovy]

really only need to check connections on back of instruments.
tried to get at them from under dash but found it difficult.
is there an easier way to access them than take the whole top off ?