Rev Counter stability

[Deleted]

Hi

I have just joined the ranks of the proud P5B Owners and all in all its been a great step.

One small irritation being that the Rev Counter needle swings wildly at all engine speeds, rendering the instrument all but useless - not to mention an annoyance and distraction.

Any ideas as to what may be causing it ? - poor earth connections ? - back feed from another electrical source.

All ideas to trace and cure greatfully received.

[Phil Nottingham]

Welcome to thsi forum and P5 onership. rev counter instablilty is quite common now as the transistors degrade, if you are lucky it may be acoroded connection.

Use SEARCH set to 1000 days and see pictures!

Tell us more about your car

[johnwp5bcoupe]

It is quite a basic device and is inductive coupled if you get stuck email me and I will have a look at it for you after 45 years in electronics I may be able to have a go

[Colin McA]

My rev counter went a bit wild for a while.

The Ignition coil had slipped and the wires melted on the exhaust manifold. If the wire contacts an earth the rev counter goes erratic.

Colin

[johnwp5bcoupe]

As the pickup point for the rev counter is point C on the diagram which is the hot end of the ballast I cant see it working from this point because that would mean the rev counter goes from A +ve and point C which is also +ve as it goes to point A anyway if point C was the other side of the ballast I could see a pulse caused by the drop across the ballast otherwise I cant see it working

The Ignition coil had slipped and the wires melted on the exhaust manifold. If the wire contacts an earth the rev counter goes erratic.

If this had happened Colin I would have thought you would have been loosing the ignition as one side goes to earth via the points and the other side goes to +ve and either would kill the ignition

[Colin McA]

I was a bit worried when it happened as the RPM as through the roof bu the engine was clearly not doing 7,000rpm.

I put it down to gremlins but the only thing a miss is the coil wires getting melted. I still need to sort it properly

Colin

[Deleted]

Problem solved - many thanks for replies and particularly for the wiring diagram.

Connection onto ballast resistor was the fault - wire not correctly crimped to connector and hence low grade contact.

Renewed/cleaned and rev counter working pretty much as new.

Thanks to all for assistance