Rough Idle

[AlistairCliff]

Hi everyone.

I have a 1967 MK3. It recently had a new electronic ignition system, carbs tuned etc. 

Recently, a squeak has appeared at or around the alternator. When that squeak happens, the engine simultaneously slows momentarily and the ignition warning light flashes. Moments later it speeds back up again. However that repeats every few seconds with the result being a very rough idle. 

Am I right to assume that alternator is faulty. I haven't checked what is happening to the output voltage when the episodes happen as wasn't sure what to expect. Presumably the voltage dropping could be both cause and effect?

All input gratefully recieved.

Alistair

[johny]

It certainly sounds like the alternator so you could remove the fan belt and run the engine for a short while. Obviously the ignition light will be on all the time as the alternator is not being driven but at least you could see if the squeak and tickover fluctuations have stopped....

[AlistairCliff]

Thanks Johny - that's a really helpful.

Would that fix the tick-over fluctuations by forcing the car to use the battery (reliable and constant) rather than a dodgy alternator voltage?

A

[johny]

yes thats the idea and it might even be worth disconnecting the alternator electrically as well to be completely sure thats where the problem lies. Its ok to run the engine just on the battery although of course it will go flat after a while but without the fan belt fitted it will overheat before then😁

However never do the reverse and disconnect the battery to leave the running engine powered from the alternator as this can upset the voltage control....

[AlistairCliff]

Thanks Johny,

Huge help.

I should have said dynamo instead of alternator - as haven't converted. Is the principle the same? 😎

A

[johny]

Yes same applies and if you disconnect the dynamo electrically make sure to remove both its connections and dont let them touch earth. However the dynamo works in conjunction with a separate voltage regulator which can make it a bit more difficult to locate the fault.

With the fan belt removed you can spin the dynamo by hand to hear and feel for any bearing roughness.... 

[AlistairCliff]

Fantastic.

Thank a lot.

[johny]

Let us know what you find as hopefully it'll obviously be the dynamo at fault but as I say could just be the voltage regulator doing something strange and suddenly increasing the load of the dynamo. The squeak is probably the fan belt slipping as the load its driving goes up...

[PeteH]

A worn fan belt can give similar issues?. V/wedge Belts are designed to drive on the sides of the belt Not the base of the pulley. Worth a check?.

Pete

[Pete Lewis]

any charge takes power from the engine so if the control box is fluctuating and asking for inconsistent charge the idle will respond as the dynamo takes up its energy 

vee belts can get sqeaky  a dust of talc  or a light rub with an oily dipstick can reduce the squeak of the belt   but it would be telling you to change it 

its the kevlar strands that can make a metalic squeak

its not related the the odd control box operation   you may need a new control box  

if thedynamo  brushes and bearings  are fine  the box sounds the culprit 

when the dynamo does a consistent charge the idle would have been affected but it would be normal not fluctuating as you have noticed 

so chicken and egg realy can be the brushes worn out or a duff control box   or even  both 

Pete