Non-work­ing al­ter­na­tor

Car Mechanics (UK) - - Help! -

I have a Mar­lin kit car fit­ted with a circa-1975 Alfa Romeo en­gine (105). The al­ter­na­tor was not work­ing when I bought the car – I don’t think the al­ter­na­tor was orig­i­nal to the Alfa en­gine; it is a Lucas 16acr – so I have fit­ted a re­con re­place­ment. I have no volt­age out­put at the al­ter­na­tor ter­mi­nals – it only has two wires go­ing to it, large to main ter­mi­nal and a small one. The al­ter­na­tor is spin­ning clock­wise viewed from the front. The charge light is not on at all, even with the en­gine not run­ning. Mike The al­ter­na­tor wiring on the Lucas 16ACR unit is quite sim­ple and can be eas­ily checked. The large wire go­ing to the main ter­mi­nal should be per­ma­nently fed di­rectly from the bat­tery; this is the feed wire and the one which will sup­ply the charge volt­age to the bat­tery. The small wire should have a volt­age when the ig­ni­tion is on; this earths through the al­ter­na­tor to al­low the ig­ni­tion warn­ing light to il­lu­mi­nate and the feed stim­u­lates the reg­u­la­tor into ac­tion. When the al­ter­na­tor be­gins to put out a charge, this will pre­vent the ig­ni­tion light from earth­ing through the cir­cuit and ex­tin­guish the bulb.

The al­ter­na­tor does need to be earthed to the en­gine to cor­rectly op­er­ate. You will need the ig­ni­tion on to test it. First, re­move the wires from the al­ter­na­tor and check if ei­ther has a feed when dis­con­nected – the small wire may not be show­ing a feed when con­nected as it does earth out. If there is no feed to ei­ther, it would ap­pear you have two prob­lems: if there is no feed to the small wire, the first thing to check is the ig­ni­tion bulb – it could be that sim­ple; if there is no feed to the large wire, there must be a break in the cir­cuit from the bat­tery.

As this is a kit car there may be dif­fer­ent op­tions, one of which may be that the wire goes via a fuse; this would be a large fuse and may be a fused link within the feed wire. Re­move the bat­tery ter­mi­nal and, us­ing a con­ti­nu­ity me­ter, check for any breaks. You should also en­sure the en­gine earth is in good con­di­tion, al­though I would ex­pect this to af­fect the starter if it was at fault.

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