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Spitfire Alternator Problem


Gnbickley
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Hi everyone,

I'm a complete newbie when it comes to Classic Car maintenance and I'm hoping to tap into the wealth of experience on here to help me out.

I've got a 1972 Spitfire MkIV which has been running fine most of the time. About a month ago I carried out the 12000 mile service and everything went ok. Recently though my ignition warning light has stopped illuminating at all, I've changed the bulb and it has no effect I've cleaned all the connections and checked the wiring and that seems fine. Last week I was driving home in the dark with the lights on, heater on, windscreen wipers on and as I got home the indicators stopped blinking they just came on and stayed on. half an hour after getting home I had to go out again but she wouldn't start as the battery was flat.

I've checked the battery and it's holding charge fine once it's charged even driving the car during the day with no lights or heater on the battery is fine.

Having read some of the threads on here I bought a multi meter and checked the voltage across the battery with the car off and I get 12.5V with the car running this drops down to around 12.4V. putting the multi meter positive probe to the brown wire terminal on the Alternator and the negative to the battery negative and I get 12.38V at idle. If I rev the car up to 2000/2500rpm the voltage goes up to 12.4v.

From what I've read I think it might be a problem with one of the Diodes but I'm not sure where these are or how to test them. Am I right in thinking that the Diodes are inside the control unit which is inside the Alternator? How would I test them to find out if they're the problem?

Cheers

Gary

 

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the diodes are inside , most are accessible with a few screws out, ,some carry the brush gear which is sprung to make contact, these may have worn down

the diodes are often a dome headed one embeded in a heat sink  you can check with ameter on resistance across the wire thats visible and the sink.

if meter reads  swap the leads over ,if it gives a reading the same in both tests its failed they should only pass voltages in one direction.

old trick was to snip the wire on the duff diode to disconnect it

for what they cost its not worth doing too much messing around 

Pete

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NO!!! Don't try that! Remove the alternator from the car, then take the cover off the end. Here's what's inside. Diode pack is bottom left and can be bought separately for even less money. As Pete says, on the resistance range, measure one way then reverse the probes. The meter is actually putting a voltage across the diode and the diode should only pass current in one direction.

https://www.ebay.co.uk/itm/LUCAS-ALTERNATOR-ACR-REPAIR-KIT-15-16-17-18-LRA100-RECTIFIER-REGULATOR-BRUSHES/182293663157?epid=1587044608&hash=item2a718c8db5:g:06IAAOSwmLlX6RsG

Here's another link about testing car batteries.

http://carbatteryworld.com/car-battery-voltage/

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Cheers Doug.

Pete :lol:

Just about to start taking the Alternator off and I've had a thought, I've only got two wires coming out of the connector a thicker brown one and a slightly thinner yellow and brown one. Is that correct? how is the Alternator Earthed?

Cheers

Gary

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through its body mounts to the engine and an engine earth strap  

but with only 12.5 volts as a max its sounds more likely brushes worn out than diodes  youre just measuring the battery voltage , seem there no output worth recording from the unit,  a diode fault normally gives some charge and things like glowing warning lamps appear.

pete

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Ok so I took the Alternator off last night and tested all the diodes and they all seem to be fine they read 1 going one way and 440 odd going the other way. So I'm going to assume it's the brushes. Plan will be to replace all the internal bits with the kit Doug listed to above as the inside of the alternator was a bit of a mess. 

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Alternator2.jpg

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Right I've finally managed to get everything undone (not having an all imperial tool box is not helping!) and get inside the Brush box and the pictures show what I found. I'm assuming the large number of chunks of black plastic that were in the bottom are a bad sign? and the gouge on the bottom of the black plastic and the groove at the top are also bad signs?

Is this fixable/worth fixing or should I just get a new alternator?

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You normally have to split the case halves to access the shaft and bearingthe front takes all the belt loads and rhe tail one

Gets the brush dust,  they are sealed bearing but they do fail and suffer lost dried lubrication

Whilst its half in bits why not take the next step  or they will fail and the earlier work is wasted

 

Pete

 

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The bearings are a tad more difficult to do than the electrics. They get noisy when they've had it. My alternator is slightly noisier than I'd like so I bought some new bearings last year. They are still sitting on the mantelpiece taunting me, but my alternator is no worse.

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14 hours ago, Gnbickley said:

Pete,

You said earlier about checking the shaft bearings, whereabouts would I find them are they below the slip rings?

Hello Gary

                   I have a Lucas 15ACR which I fitted new (quality) bearings and brushes and skimmed the commutator it was on the car for about 5000 miles but not needed now as I have a Denso 40amp one fitted as the EFI pump runs all the time and the 15 ACR   was a bit marginal at night and heater on.

If you have no luck with yours you can have it for the cost of postage and a Donation to children in need.

Roger.

ps I am sure I have a set of bearing in the garage I just can not find them!(the supplier sent 2 sets and said keep the second set not worth posting back!)

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