Can any dynamo / electrics experts diagnose this?

[bstark]

This is slowly doing my head in ...

The car is a sixties Fiat 500 running a dynamo, separate regulator, points and wasted spark ignition.

All was well until a few weeks back the ignition light came on at speed on the motorway. Car drove fine but at anything above about 2500rpm the ignition light came on.

Have since had the dynamo checked out and apparently brushes, springs and bearings are all good.
Have fitted new regulator.
Tested in garage and light goes out once started and stays out even when revved hard - hooray!
Go for a drive and almost immediately the light comes on when revved - pretty much only goes out at idle or at less than 55km/h in top (guessing about 2400rpm), so presumably load related?

Electrics are not my thing and short of spending time and a chunk of cash replacing the apparently sound dynamo with a new one to see if it works I'm at a loss. Is this something that could be caused outside of the dynamo and regulator, eg. The wiring between or the battery or something?

Help appreciated!

Bob

[Auntieroll]

Check the tightness of the drive belt.

[bstark]

Yep, done that...

[bstark]

Just to make absolutely sure I've just tried tightening the fan belt to the point of over-tight and it's actually made the problem worse.

In garage, light went out, as soon as I drove forwards the light went on, and then stayed on even at idle.

[Steve_D]

I think a session of stripping and cleaning all the major electrical connections is called for.

Steve

[oakdale]

Check the engine earth strap.

[Locknut]

The ignition light comes on when the battery positive flows to earth via the bulb and the dynamo. The light goes off when the dynamo positive output matches or exceeds the battery positive. (two positive inputs to the bulb result in no light) I would check the battery and dynamo connections and remember that in some systems the line from the dynamo to battery is looped via the starter, so check that too. Also check that the correct wattage bulb is fitted in the ignition light.

[bstark]

Thanks for those ideas.

oakdale said:

Check the engine earth strap.

This one has got me most excited as I can imagine a situation with this that matches all the symptoms.

The engine mounting on the 500 is very soft and positioned centrally at the back, allowing a lot of torsional movement of the engine under load.

The earth strap goes from underneath the dynamo mounting on the far left of the engine (middle of cooling fan) to the far left of the engine bay (one of the rear panel mounting bolts). If engine movement under load is pulling the strap and breaking the connection that would explain the repeatability under load, and why it could be initially fine at high revs but no load in the garage after refitting the dynamo. The problem first showed itself under high load cornering hard which would also fit.

Heading to garage as soon as I get home to check!

[bstark]

Well, checked, cleaned and where needed replaced all the contacts to the dynamo and the earth strap. Several were pretty ropey and the earth strap contact was really poor.

Put it all back together and... No change...

Check the voltage at the battery and it rises to about 15.4v and then drops straight back as the red light comes on with revs. Same story if you take the voltage from the brush terminal on the dynamo but with a higher voltage. This was all in the garage so no real movement to confuse things.

So it all points in my mind to the dynamo itself (despite assurance from the local specialist that all is good) and it is the brushes floating as the rotational speed increases.

Grrr...

[Locknut]

Does this car not have a control box for the dynamo? That would have to come under serious suspicion.

[bstark]

Locknut said:

Does this car not have a control box for the dynamo? That would have to come under serious suspicion.

Yep, replaced that with a new one (100 quid gone) when the dynamo specialist said the dynamo was fine. Symptoms are identical with the new regulator fitted.

[LordLoveLength]

What about the control box earth?

[RC Developments]

Surely over 14.5v is overcharging, sounds like the regulator box is at fault, is it the same as original? Try isolating the dynamo by running it directly to the battery.

This page gives a good guide on how to diagnose the problem.. http://www.howacarworks.com/electrical-systems/tes...

Edited by RC Developments on Wednesday 25th June 23:09

[bstark]

Yep, same box as the original.

The dynamo is at the opposite end of the car to the battery so connecting direct isn't so easy,.

Does 15.4v at the battery sound way too high then?

[bstark]

Have just checked regulator earth - fine, all the wiring between regulator and dynamo - fine, and all the fuses - fine...

[RC Developments]

yes 15.4v is definitely too high, max off any dynamo or alternator should be mid 14V. Doesn't have to be a check off the battery just something with a permanent live will be ok i.e starter motor. Maybe contacts in the regulator are closing up under load?

[bstark]

I think I may admit defeat and hope I can find a local auto-electrician who can remember what a dynamo is...

[Steffan]

bstark said:

I think I may admit defeat and hope I can find a local auto-electrician who can remember what a dynamo is...

Many of them will I can still just remeber magneto ignition (and crap it was). Just confirm how much better alternator power is although I do appreciate that originality precludes such changes. Best of luck in your endeavours.

[Locknut]

My memory of the dynamo system is also like a fading dream. At the risk of drawing some criticism I think I would take the cover off that control box. You should find two or three coils in there and each coil has a spring-loaded contact on the end. I would make sure that these contacts are clean by inserting a thin piece of fine sandpaper between each and giving them a few strokes. There is a method of adjusting those contacts but I never mastered that.

[Steffan]

Locknut said:

My memory of the dynamo system is also like a fading dream. At the risk of drawing some criticism I think I would take the cover off that control box. You should find two or three coils in there and each coil has a spring-loaded contact on the end. I would make sure that these contacts are clean by inserting a thin piece of fine sandpaper between each and giving them a few strokes. There is a method of adjusting those contacts but I never mastered that.

In view of all the work the OP has done it does seem probable that either the Dynamo is failing or the control box is failing. I appreciate the control box is new and the Dynamo declared good but something is causing the fault to continue. I would be doubtful of those two items the OP seems to have covered the poor contacts probabilities. Best of luck to th OP electrics are always tricky on older cars.