charging problem at 3500 rpm

[chapelfarmer]

hi - can anyone cast light on this? My car (mk1 Sprite 1293cc positive earth, newish dynamo, newish control box) charges perfectly up to 3500 rpm then suddenly loses charge (ammeter into the negative, red light on). could this be a slipping fan belt? (it feels tight although it is old, much like me!) or is it something more scary?

thanks JOHN

 

[Guest]

Just throwing this out, but a wire/ componant might be vibrating enough at that RPM to short/ground out.

What about if you rev higher?

 

[jlaird]

If old I think I might change fan belt first and see, they cheep enough and you should always have an extra under the front seat.

 

[Sarastro]

I assume that this is without any extra electrical stuff turned on, correct?

It seems hard to imagine that a slipping fan belt would cause this, although I suppose it's easy to check. In my experience, when a fan belt slips you know it, because of the screech, and it seems unlikely that it would suddenly slip so much that the output, at 3500 rpm, would drop essentially to zero. When the red light is on, that indicates that virtually all output has stopped.

Since the output is OK at lower speeds, it's unlikely to be the generator.

That just leaves the control box. It's possible that the regulator relay is sticking, or some other flaw. The resistor (which is located on the underside of the box) could be burned out or otherwise disconnected. The fact that it's new doesn't matter much, in my view; I never assume that new components are good components until it's proven to me.

 

[Deleted member 8987]

If the belt is oil-soaked, won't squeal, but at low speeds with high load (headlamps, heater motor, etc. on) should duplicate it.

 

[chapelfarmer]

Many thanks for all the advice! I'll get a new belt and try some of the theories next weekend and post the results.

 

[chapelfarmer]

problem fixed - new belt, right tension (not too tight). job done! a cheap car fix (at last!)