Faulty rev counter

[markyc100]

Hi

I have a 1994 3.2 911 target.

The tachometer is faulty, as soon as you turn the key to the second position before it starts, the counter goes right round to 7000 rpm. When you start the car it does the same, when you stop the car the tachometer goes back down to 0.

Any advice appreciated

[markyc100]

[markyc100]

[silverlock]

Pull the gauge and check the ground wires. Bad ground = pegs at max RPM.

Do any work in the frunk or adjacent gauges that might have pulled the ground out?

[jbell959]

Originally Posted by silverlock

Pull the gauge and check the ground wires. Bad ground = pegs at max RPM.

Do any work in the frunk or adjacent gauges that might have pulled the ground out?

Bad ground will not peg the tach. If it pegs when power is applied, a component on the driver board has failed.

[markyc100]

Whereabouts is the driver board located ?

Is it likely to be this as opposed to a faulty tach?

hanks for the help

[markyc100]

update, i have unplugged the dme relay and then the dme, the problem with the tach is still there, goes right round the clock when the ignition is turned on and stays the same when started.

[markyc100]

I have connected a volt meter to the black and purple wire at the back of the tach.

Ignition on its reading 12.4v

Once started it drops to 10.8v and drops as the revs increase.

I am not very technically minded so would be grateful of any input as to how it all works and any help to resolving the issue.

[JSV798]

When my 1978SC did this it was the voltage regulator failing allowing charging to rise to 18 volts. Suggest you measure output voltage of alternator. I did this by buying a plug-n voltmeter for the cigarette lighter.

Roy

[jbell959]

Originally Posted by markyc100

Whereabouts is the driver board located ?

Is it likely to be this as opposed to a faulty tach?

hanks for the help

The driver board is inside the tach. It needs to come apart to diagnose the issue.

Originally Posted by markyc100

I have connected a volt meter to the black and purple wire at the back of the tach.

Ignition on its reading 12.4v

Once started it drops to 10.8v and drops as the revs increase.

I am not very technically minded so would be grateful of any input as to how it all works and any help to resolving the issue.

Connecting a voltmeter to the purple/black wire doesn't tell you anything. That is the tach signal, which is basically a square wave. You want to connect your voltmeter to the 12V and Ground wires to make sure your regulator is working and the voltage isn't spiking when you rev the engine.

These tachs will work fine up to 20V, so your issue is more likely a damaged driver board, which could happen as a result of a bad regulator, so you want to check that before you repair the tach and put it back in.

[markyc100]

Thanks for that help

I have just tested the power and gnd going into the tach and its 12.7v with ignition on and 14v running.

Is this pointing to a faulty tach?

[markyc100]

could i try a generic cheap and cheerful rev counter before i send mine away to make sure thats the problem?

[jbell959]

Sounds like your regulator is working fine. If you have 12V and the tach pegs with the purple/black wire disconnected, then it's definitely the tach. I wouldn't waste your money on a cheap tach to test it. You need to have it repaired.