Strange electrical problem

Don't know if anyone else think this but I'd be suspicious of the alternator's voltage regulator - on my PI (1970) I think this is a separate unit but many alternators have this built into the assembly (under the plastic end cover). Got a spare alternator? My first thought would be to swap it over for a day and see...

These cars have a marginal charging circuit at the best of times (headlamps on, heater on and nowt else often brings the voltage down to 12v or lower, basically draining the battery slowly) - later alternators had a slightler higher current rating (17 amps as opposed to 15 amps??) so it's a delight to see that voltmeter go so high - but too high will burn something out, perhaps the non-relay wired lighting circuit.

Or I may be talking pish - anyone else?

-- Ken Davidson

The regulator in the altenator has an intermittant fault I suspect. Sort it quickly or you will be paying out for a lot of bulbs motors and stereos!

Andrew

The only thing it can possibly be is a faulty regulator. Had just this on the SD1, which may have a similar alternator. It can be easily replaced for about 30 quid or less. You'll need to find a decent auto electrics place, motor factors or Lucas agent for the part, though.

I agree its the alternator,well worth a check with a volt meter on the battery. Jas.

It's unlikely to be a short - look for an _open_ circuit instead, probably on the sense wire (the thin one) from the alternator. One a Lucas, first culprit would be the multi-way plug on the alternator itself.

I doubt it's a regulator failure either. Possible, but rare for them to go this way.

-- Die Gotterspammerung - Junkmail of the Gods

I'm not quite sure how any external open circuit can result in more than

14 volts or so appearing at the battery? If the battery was disconnected from the alternator with the engine running, then you can get high voltage, but not under load as Jim says is happening.

It's not that rare, and the only logical explanation.

No, I see the way this would work. If the sense wire tells the alternator regulator what the state of the battery charge is, and it goes open circuit, then the regulator will think the battery is low and boost the charge to compensate.

The fault might be in the regulator unit but not in the regulator logic. There will be a circuit board linking the connectors to the regulator control. A bad connection or cracked printed circuit would give an intermittent open circuit. And the extra heat from the over voltage could expand something and temporarily repair the open circuit.

Many thanks for the suggestions. I haven't got a spare alternator, so this one will have to come off and be examined under a magnifying glass. I'll check the continuity of the sense wire too - after 34 years of flexing as the engine moves on its mountings, I can't rule out metal fatigue inside the insulation.

And yes, I can't leave it. It has already blown a dipped beam filament....:-(

Jim

It's not something I've experienced myself, but if number a plug lead runs too close over the top of the alternator it can cause some strange faults. Cheers, Bill.

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PO Box 1232 Calne Wiltshire SN11 8WA United Kingdom

I'd go along with that. You should be able to get a replacement regulator pack for a lot less that 30 quid. However many auto electrical places will deny that you can get them separately and try to sell you a refurbished alternator, so beware. Keep the duff regulator in case you need to exchange an alternator at a later date, so you can retain the good one!

This is UK, so it'll be a battery-sensed alternator, rather than a US two-wire machine-sensed one.

If this connection goes high-impedance, the alternator "sees" the output voltage as having dropped, so it ups the field current to compensate. This boosts the output voltage and starts popping bulbs. If it's a smallish alternator, then the maximum voltage you can generate, even at maximum field, is about 20V (more on a motorway).

-- Die Gotterspammerung - Junkmail of the Gods

Not necessarily.

On my old P6, which would be about the same age, disconnecting the battery sensing wire did not turn the alternator hard on - or stop it charging either. So I *guess* it merely fine trimmed the output.

I suppose it might on a fully charged battery with no load from accessories etc on a modern car, but on a PI with a probably 55 amp alternator it would be somewhat less than this - I saw about 17 on the SDI.

In article , Dave Plowman writes

Depends entirely on the age of the alternator. Some of the early Lucas ones (as fitted to Triumphs of the late '60s) had a remote voltage sense wire. If this falls off the regulator goes to maximum.

Many reconditioned units will have the sense and feed terminals hard wired within the regulator housing, at least that's been my experience. Cheers, Bill.

-- Rarebits4classics .......just what you've been looking for

PO Box 1232 Calne Wiltshire SN11 8WA United Kingdom

I said 15 or 17 in the first message - of course I was thinking of the ACR model number not the current rating. The PI should be only 28 amps Dave, or maybe 36 if using the 17 ACR from later cars - not a helluva lot! Easy to see why turning the radio on with the heater motor will make things a tad marginal whilst driving at night!

Ages ago: know I shouldn't have done it but we had one battery between a TR7 and my brother's Alpine years ago. I used to start the TR7 with the battery, disconnect it and drive to work (with no battery), borrow a colleagues, do the same again to get home. Kept this up for a week until "pay day" - didn't bugger up / burn anything out but I guess I was lucky? Worked fine - no weak spark, low lamps etc.

-- Ken Davidson

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Certainly the alternator fitted to my '72 3500S was the most pathetic device ever designed by Lucas. It was meant to be about 35 amps, but wouldn't run the headlights and rear window without discharging the battery - and it was common enough to need both of all the time in town on a cold winter's day. Nor would it stay on full charge for more than a couple of minutes even with a flat battery. After replacing the alternator with a genuine exchange unit to try for a cure, I gave up and made a new external regulator to the design of the older ones which had an eternal one originally.

Yes - I've never been convinced that sensing the voltage a short distance from the parallel feed that is the output makes any practical difference. On the P6 it had more validity, as it had an ammeter, so the alternator to battery cable wiring was much longer than usual. But even with the rear mounted battery, the sensor wire didn't go to it, but to IIRC the starter solenoid.

I'm not sure if it's used these days or not. You might just as well use the warning light feed.

My P6 had this and it didn't.

Well known problem on these cars was wear on the alternator pulley and belt as a result the belt would bottom out on the pulley and slip after a cold start when the load was highest.

Quite common symptoms. Well known problem on Lucas ACR alternators was arcing at the big blade connectors on the 3 pin plug termination, the cure is to remove the rear plastic cover from the alternator and clean the Lucar blades and and relplace the large spade connector inside the plug. While the back of the alternator is off also check the bolted connection for the solid state regulator box is clean and tight,