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Alternator Woes..

Bob Tucker

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Ive just returned from my annual French jaunt.

DFDS cancelled my ferry to Dieppe at the last moment as their loading ramp broke down.

The replacement journey from dover to calais got me to the campsite too late to get in.

A cycle wing stay broke, luckily without any damage.

& finally the alternator packed up....hence this post.


A helpful village mechanic managed to find a new one after much searching.

It is a Bosch, that looked identical to the unbadged one I had fitted, & it does charge OK......

but when I start the engine the ignition light stays on until I rev the engine to 3k revs.

No good at all for a sneaky early Sunday morning run out.

The ignition light is an LED if that make any difference.


Does anyone have any ideas?

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Simple test

Can you make a 12v bulb on a fly lead -much easier if you can put a crocodile clip on one ends -or ring terminals

Clip one end to warning light terminal the other to batt + start the car does light go straight out ?? If yes then LED not able to excite alternator - if it stays on its alternater fault

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Snap for the above post Bob, a resistance of some sort is required to supply a feed to alternator field; LED is far too little. With your old set-up can you recall having a 5 watt lamp in parallel with the LED ignition light? May have been tucked away out of sight somewhere -- could have been your french fault --- the 5 watt lamp had failed.

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The drive pulley is the same, but when I installed my tuned redtop, I was advised to fit a smaller alloy

crankshaft pulley, due to the considerably higher revs the engine was capable of.

I fitted the same small pulley to the standard engine I now have, & with the LED ignition light it all worked well

until I had the Bosch alternator fitted.

I can refit the bigger pulley, but after a couple of hours trawling, I have found an article that says

that specifically the Bosch alternator I have fitted, needs at least a 2 amp load to get it "excited".

I plan to disconnect the lead from the alternator, and put a 5w bulb in the circuit to try that before

I replace the crankshaft pulley.

I can do it once all the grandkids move back home.....we're now down to just one, so fingers crossed!

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Update 1.

I used an interior bulb (5w) on a long flylead.

Disconnected the small brown wire, connected one end of the flylead to the terminal & the other to the wire.

with the ignition onThe LED lit up, but the 5w bulb didnt .????

At 3k revs the LED went out.

My flylead wasnt long enough to connect to battery +ve so thats the next job.

Also find the voltmeter that I knew was in my electrics drawer.....except it wasnt.

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Needs to be in parallel with the LED, Bob. The LED draws so little current the 5 watt would never glow, although it's in the current path.


Try again ---- Led Bob --- makes a change from Bob T.

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It's all bloody witchcraft to me....


But thanks for the continuing assistance......

You wouldnt believe how much grief I'm getting for

"spending all that money, & you let them sell you a duff alternator!"


Edited by Bob Tucker
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  • 2 weeks later...

Congratulations and thanks to you all....


We're down to just one junior house guest now, so I sneaked a couple of hours play time yesterday.

With a lead from the small alternator terminal, via the 5w bulb, to live battery....

voila! Both bulbs extinguished even while the engine was just turning over..

And I had 14.3 V at the battery at tickover..


Just the broken cycle wing bracket to fix now...

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