Bouncing Rev Counter

[ukkid35]

My rev counter has started bouncing around a lot. I'm going to swap out the crank sensor, but is there anything else I need to look at?

[Gazzab]

Big fuse...?

[fatjon]

Big Fuse. Change it before you get bitten.

[ukkid35]

Gazzab said:

Big fuse...?

Thanks - Checked yesterday, all good.

[fatjon]

And not the crank sensor, the sensor feeds a VR signal to the ECU and the ECU feeds a square wave to the rev counter. If the crank sensor was failing it would run like crap as it would miss sparks and injections randomly and also mistime them causing backfires, spitbacks and all kinds of nasty stuff.. What's happening is the voltage supplying the rev counter is up and down like a wes nickers as the fuse arcs and sparks, very soon it will do neither and you will stop.

[ukkid35]

fatjon said:

Big Fuse. Change it before you get bitten.

In that case I will definitely change it. I'm using a 125A Midi fuse, I guess it could be breaking down but still showing OK with my multimeter. It's been like that for 600 miles plus.

[Gazzab]

Could just need to clean up the contacts where your midi fuse is seated?

[Jhonno]

fatjon said:

And not the crank sensor, the sensor feeds a VR signal to the ECU and the ECU feeds a square wave to the rev counter. If the crank sensor was failing it would run like crap as it would miss sparks and injections randomly and also mistime them causing backfires, spitbacks and all kinds of nasty stuff.. What's happening is the voltage supplying the rev counter is up and down like a wes nickers as the fuse arcs and sparks, very soon it will do neither and you will stop.

I have a new fuse, and cleaned up and my rev counter still bounces.. I seem to have traced some of it to a dodgy wire behind the cluster.

[TheRainMaker]

Mine started to do this, changed the fuse etc etc, turns out the rev counter was actually broken

Sent it off to not so speedy cables, and it came back fixed

At least I now have a midi fuse fitted I guess.

[Byker28i]

Surprised no-ones mentioned the big fuse

Another good indicator of this is the wipers go really slowly. Pull them away from the windscreen to check. If you're still using the blade type then it's worth spending 5 mins just elongating the slots so they fit nicely onto your terminals.

[robsco]

It could just be your rev counter needle. If I tapped the underside of the dash of my first Cerb, the needle would bounce from the stop with no ignition on.

[fatjon]

In my experience, freezing cold in the pissing rain next to the M18, freezing cold in the pissing rain outside Santa Pod and freezing cold in the pissing rain outside work, it is not the fuse that actually packs in. You can pull it out and test it and it will be just fine. It's the holder that the fuse goes into that burns up and the wire connections to the holder that burn up as a result of all the arcing inside. My wires were burnt and oxidised for 6" either side of the fuse holder. Since I ripped it all out from alternator to battery terminal and replaced it with a 6mm good quality cable and one of those gold plated car audio fuses in a water tight housing with proper cable cable clamps it has never left me wet and miserable again. Incidentally, the terminal at the alternator was similarly frazzled.

Of course this is not the only cause of weird rev counter problems but I thought I would add this as it seems to be a permanent fix for one the Cerberas most annoying failures.

[ukkid35]

fatjon said:

Big Fuse. Change it before you get bitten.

Changed it as recommended, haven't seen any bouncing since, which is only about 300 miles.

Could have been that the old fuse wasn't seated properly, but who cares the cost of a new fuse is trivial.