The rev counter on my 1985 Ford Cargo is very erratic. I have been told it works off the back of the alternator so I have checked the connections and all OK. What else could cause the problem. The truck is a 12v system not 24 and has automatic transmission.

Clive Tonge, September 2009

output for rev counter is from "w" connection on alt
it should be approx 6 volts usual fault is corroded terminal

lecky, September 2009

The alternator produces pulses which operate the tachometer. How old is the alternator? The alternator may need to be replaced.
You could also check the drive belt tension. If the belt is too slack it may not be driving the alternator at the correct speed which will affect output. You need 1/2" deflection at midpoint between the pulleys.
You have checked the connections - often corroded which doesn't help. I have a 1985 Cargo and replaced the alternator on it a few months ago and the tachometer only started to work properly when this was done. Also you may have battery problems - i.e. batteries don't seem to last due to not be charged properly or at all by a dodgy alternator. If you are getting a new alternator, make sure that the one they give you has the same connections -they may have to solder in a lead for the output to the tachometer.

JD, September 2009