Non P6 electrical issue

dmcsweeney

dmcsweeney

Active Member
Hi all,
my father had a bit of an electrical melt down last week that I need to try and sort out this weekend and I was wondering if someone could shed a bit of light on it. While out working with our back up row crop tractor (an old fordson dexta) the wiring loom and the rear of the alternator caught fire. He stopped it, extinguished the fire, bump started it and carried on working (as you do :roll: ). I found it parked in the workshop on Sunday with the rear of the alternator melted, and about 6 inches of the loom burnt out, including both the main alternator lead and the warning light cable copper completely broken away, and of course he'd left the battery master switch on :shock: . He later claimed that the battery was flat anyhow. The loom is very basic, and there was no where in the area where it could have shorted, so what am I looking at (I had no time to investigate), dead short in the alternator (Lucas 45amp 3 pin) or could a internal short in the battery cause it?
Regards,
Dave
 
dmcsweeney said:
dead short in the alternator (Lucas 45amp 3 pin) or could a internal short in the battery cause it?
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If the battery shorts internally as far as I can think that won't have any effect on the wiring connected to it, if the alternator wiring shorts to earth, possibly inside the alternator, then you'll have battery power running straight to earth, and that will easily melt the wiring, as you would know if you've ever shorted the live post on a battery straight to earth with a spanner.
 
Thanks Harvey, wasn't sure as he had bump started it before the fire, and said that he thought the battery was knackered when he started, that the alternator might have been sending current out to the equivalent of a dead short. Didn't get a chance to look over it properly but both components are probably for the bin anyhow!
Regards,
Dave
 
If the battery had dead shorted, so was putting nothing out, then the alternator won't have the input through the ignition switched live (warning light) to multiply. But running an alternator without the battery connected, which is effectively what he's done, even if it was OK before, normally knackers them, (although I can't remember why that should be, bearing in mind it needs the supply to excite it) so as you say he's probably in for replacing both.
 
harveyp6 said:
If the battery had dead shorted, so was putting nothing out, then the alternator won't have the input through the ignition switched live (warning light) to multiply.
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Doh, forgot that part!

harveyp6 said:
But running an alternator without the battery connected, which is effectively what he's done, even if it was OK before, normally knackers them, (although I can't remember why that should be, bearing in mind it needs the supply to excite it) so as you say he's probably in for replacing both.
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That's exactly what I've always been given to understand, but strangely, as the tractor is only used ocassionally, it spent years with no battery fitted and the loom unplugged from the alternator, being dragged behind one of the other tractors when needed and then left run all day, and the alternator has not been replaced as far as I'm aware. Probably explains why it went south! It's been back to being towed lately because I made the mistake of buying one of those poxy calcium batteries which has given nothing but grief since day one.
Regards,
Dave
 
dmcsweeney said:
with no battery fitted and the loom unplugged from the alternator,
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I think that's the key point, unplugging it means there's no problem, but as I said, I can't remember why running it connected, but without a battery, causes problems. Electrics are witchcraft anyway, and the less I know, the less I get asked to fix other people's problems. :LOL:
 
harveyp6 said:
dmcsweeney said:
with no battery fitted and the loom unplugged from the alternator,
Click to expand...

I think that's the key point, unplugging it means there's no problem, but as I said, I can't remember why running it connected, but without a battery, causes problems. Electrics are witchcraft anyway, and the less I know, the less I get asked to fix other people's problems. :LOL:
Click to expand...

I seem to recall (IN THEORY!!!) it's because the battery acts like a "cushion" and keeps the voltage level on the rectifier parts within certain boundaries. With no battery (or with no reasonably heavy load - headlights, HRW etc), the voltage spikes generated can become so large they can actually "overstress" and breakdown the rectifier parts etc. I'm sure it was once said to me (maybe not so much with 100A+ alternators) that if the battery did become O/C, putting on a reasonably heavy load (headlamps and HRW) would keep the voltage to safe levels and reduce the risk of damaging spikes being created.
 
Modern alternators have voltage/current control built in and can be run without a load. Running the alternator without the battery does produce current because the field coils are wound on permanent magnets, and therefore immediately produce current.
If I were to guess at a cause, I would suspect a short in the wire loom itself.
 
I would suspect the rectifryer stack in the regulator, shorted diodes would cause a dead short to the battery causing the frying you experienced however also possible that it is a short in the wiring however that is pretty academic now as that peice of wire will be one of your melted ones.

Graeme
 
Thanks gents,
I'll go through the full loom (will take all of about 5 minutes as it consist of the starting, charging and oil pressure warning light at the moment :roll: ) and will investigate/replace the alternator and battery at the weekend. Hopefully I still have a spare alternator about somewhere!
Regards,
Dave
 
DaveHerns said:
with no battery fitted and the loom unplugged from the alternator
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- does it run once started without electricity ?
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It's a tractor, so most likely a weasel, which will run quite happily without battery or alternator once started providing it has a cable stop rather than solenoid, which at that age, it will have.
 
harveyp6 said:
DaveHerns said:
with no battery fitted and the loom unplugged from the alternator
Click to expand...
- does it run once started without electricity ?
Click to expand...

It's a tractor, so most likely a weasel, which will run quite happily without battery or alternator once started providing it has a cable stop rather than solenoid, which at that age, it will have.
Click to expand...

As Harvey rightly pointed out it is a pull stop, so electrical shut off solinoid. The dexta was launched in about '59 so the electrics are basic to say the least, even the starter pinion is manually engaged (you push a lever down which slides it forward and simultaniously energises the solinoid)! It's this simplicity and bullet proof mechanicals has kept it on the farm the last 34 years, in addition to the fact that it is light, compact and when fitted with narrow row crop wheels ideal for vegetable planting, spraying and harvesting. Our much later Ford 10 series is the same, no electronic engine control or electro mechanical interface apart from the duel power (splitter) in the transmission. We've considered the later 40-series replacements (from about '93 on) but have steered clear so far because everything started to go electrical/electronic including the gear shift, PTO, 3 point linkage, spools etc, so in the even of an electrical failure you're stuffed!!
Regards,
Dave
 
dmcsweeney said:
... even the starter pinion is manually engaged (you push a lever down which slides it forward and simultaniously energises the solinoid)!...
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Oh dear, now you reminded me of REO M34 trucks. The starter was engaged by pressing a pedal right to the accelerator. It took some practice to press the starter pedal with your right foot, while with your left you pumped franticaly the accelerator in order to feed the 6 cylinder beast up front.
Later M35A2 were turbo diesels and easier as you just had to keep both pedals to the metal until it started.
 
OrganDoctor said:
Modern alternators have voltage/current control built in and can be run without a load. Running the alternator without the battery does produce current because the field coils are wound on permanent magnets, and therefore immediately produce current.
If I were to guess at a cause, I would suspect a short in the wire loom itself.
Click to expand...

1) This is an old Lucas alternator (which probably uses the usual Lucas smoke to run! :p)

2) If the field coils are permanent magnets, then how do they regulate output voltage? (Unless I am misinterpreting!)

3) Call me a sceptic, but I wouldn't trust running an alternator (new or old, with or without modern day "fail-safe" controls) with no load
 
Some old alternators have external votage regulators (I guess, but have never owned one) The alternator generates AC current from the permanent magnet fields and stator. Then this is rectified to DC and regulated for current and voltage.I assume the AC potential remains constant rising to the demand of the DC requirement.In any case, no load conditions reduce the current production to zero on the DC side.
 
Hi, probably only has one. Fuel is sprayed onto a electrically heated coil of wire which
produces a flame in the inlet manifold. A piece of burning rag or rolled up newspaper
applied to the air intake would be an alternative. To be honest, if it is a little used
machine Just booster pack or a battery to get it going will be enough and dispense
with the alternator and recharge the battery every so often...Very Redneck!!

Colin
 
OrganDoctor said:
Some old alternators have external votage regulators (I guess, but have never owned one) The alternator generates AC current from the permanent magnet fields and stator. Then this is rectified to DC and regulated for current and voltage.I assume the AC potential remains constant rising to the demand of the DC requirement.In any case, no load conditions reduce the current production to zero on the DC side.
Click to expand...


Hmmm... interesting! :)
 
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