Got this message from my brother this morning about his rev counter reading wrongly and jumping around - anyone got any ideas? His car's an Oct 91 Roadster, which I'm sure is after they stopped making the lightweight crank. He says the car runs fine and feel good but the rev counter is all over the place.

"....and my rev counter is playing up. Since I got the car, odd times I've noticed it jump about a bit, not much, just jump about 250-500 rpm then go back to normal. But recently it's been doing it more often and last night on the way home it was reading way out. About 4250 rpm for 65 mph! It should have been about 3500rpm. I then accelerated up to 85 and it never moved. When I got home, it was reading 1200rpm at idle, but when I turned the fan off it dropped to 800rpm without a change in engine note."

We recently checked the timing and reset the idle on the car, but otherwise it hasn't been touched except for playing with the exhaust. I'll try the diagnostic connector codes tonight on it. Where does the rev counter get its readings - ECU. Could it be the needle is sticking and needs releasing a little with the two teaspoon trick? (4/01)
The rev counter is the most sensitive thing to a drop in battery voltage. Get him to charge the battery and check the terminals are on tight. See if that fixes it. (4/01)
Mine used to be tempermental, seemed to stick sometimes, I took it out cleaned it up and put it back in and it's been fine since (4/01
This is usually caused by bad grounds. The one that (for LHD cars) sits below the brake master cylinder would be the prime candidate for checking/cleaning. (4/01)
Second alternative to dodgy earth .. if it has aftermarket faces the needle is sticking mechanically not electrically. (4/01

I'd check fan belt tightness first.

As well as slow wipers, "rev-counter madness" (in retrospect!) turned out to be signs that we needed a new (&/or correctly tensioned) fan belt. Well, actually Malcolm had pointed out it needed a new belt last year when he did the timing but I'd only got as far as buying one - wasn't too much help lying on a counter at home!

See "KB's Roadster" at http://yee_har.tripod.com/index.html for the gory detail Thinking about it the exact symptoms were slow response to the engine speed as opposed to over-reading (IIRC it was slow to respond to easing of engine speed as well as pick-up) (4/01)

I can be driving along and all of a sudden the rev counter climbs from say 3200rpm at sixty to nearly 4000rpm. I dip my clutch and leave go of the throttle and as the rev counter comes down there seems to be a little resistance around 3000rpm then rests on 0! There also seems to be a chirping noise like the belts make but I've had them changed. Battery and charging system are all aok. (6/02)

It's:
a) faulty rev counter, or
b) faulty igniter (if 1.6), or
c) faulty igniter/coil (if 1.8), or
d) faulty electrical connection somewhere between the igniter and rev counter. The wire which connects them is probably Yellow with Blue tracer if 1.6 - I don't know what colour it's likely to be on a 1.8.

Of these, d) is most likely, unless the noise comes from the rev counter in which case it's likely to be the rev counter itself. To start with, you could clean the igniter plug/socket terminals and the terminals in the plugs/sockets on the back of the instrument panel, and next run a new (additional) wire between the igniter and rev counter - that might cure it.

(The igniter is mounted on the offside inner wing in front of the fuse box on 1.6, and is integrated with the coils in 1.8) Hope that provides enough clues! (6/02)