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TR4/4A TR4a positive or negative earth?

tdskip

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So all TR4a were positive earth, no? I'm about to send the TR4A IRS distributor off to Jeff at Advanced and I'm wondering if I should convert over to negative earth at this point? Generator and starter are out for rebuilds anyway, so seems like now would be the time.

What do you think?
 

Andrew Mace

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Actually, I think most if not all TR4As were negative earth. At least that's what is indicated by the 4A wiring diagram in my Leyland Repair Operations Manual for TR4 / 4A / 250 / 5 / 6. On the other hand, I've seen references (can't for the life of me remember where) to available positive ground alternator options for the 4A.

Regardless, conversion is little more than repolarizing the generator (if keeping same is your desire) and then swapping the leads on the coil and the ammeter.
 

sail

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My 66 has a generator and negative earth. I read on a British forum but can't find it right now that a few had positive earth and also that an alternator was an option but did not see any mention in my books of either.
 

TR3driver

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AFAIK, nearly all TR4A were negative ground from the factory, and again nearly all equipped with a generator. I just checked Bill Piggott's latest book, and he seems to agree. Apparently the Lucas 11AC alternator was part of a "police package" (!) that was even available on the TR4 (and would therefore be positive ground). Perhaps this is the reference to a positive ground alternator you recall, Andy.

The 11AC alternator, unlike most (later) alternators, is relatively easy to configure for positive ground operation. It is externally regulated, so the only internal polarity-sensitive components are the diodes. The diodes are pressed into heat sinks, so all it takes to configure for positive ground operation is to install the "positive" diodes into the ground heat sink and vice versa.

My early Stags have 11AC alternators, and the external control circuit is oddly complicated with a relay cutout mechanism in one box and the actual voltage regulator in another box. Apparently, Lucas felt the rotor might overheat with full field current and the engine stopped, so the cutout box contains a resistor that supplies a reduced field current when the ignition is on; plus a relay that operates on a low voltage AC signal brought out directly from the windings, to only apply full field current when the engine is running.

Altogether now ... Emm Eye See; Kay Ee Wye; Emm Oh You Ess Ee !
 
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tdskip

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Well you just never know with these cars. My solid axle TR4A is positive earth....but then again the car had been messed with as well before I bought her.

Both the TR4As have generators, and the 1968 is a super original 50K mile car that doesn't appear to have been messed with much.
 

Banjo

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My 66 TR4A solid axle was negative ground, with a generator. As was the other 66 solid axle parts car I had.
 

HerronScott

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My 1966 and 1967 TR4A's are both negative ground and definitely generators.

Scott
 

DanB

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Actually, my 1966 TR4A was Positive earth when I got it in 1980. I ran it that way a while, then converted to negative ground so I could run a cassette radio. I have also converted it to a mitsu alternator with internal regulator. The first alternator I put on was a Delco, but it is a little big physically, so there was not much adjustment room for the belt.
If you don't know which way your car was grounded, the control box will need to be re-polarized before you hook anything up. If not, you have a 50-50 chance of either being right or setting the car on fire. This is the reason I went with the alternator. I burned the control box up because I hooked the battery up backwards. Everything else works fine either way.

Dan
 

TR3driver

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Actually, when you swap ground polarity, you should also swap the wires on the ignition coil, and ammeter.

And you can hook everything up, just don't start the engine until you repolarize the generator. One way is to pull the wire off the 'F' terminal at the control box, and brush it against the 'A' terminal. Then put it back on 'F' and fire it up.
 

Andrew Mace

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I have heard of, and actually witnessed*, instances where the generator would "repolarize" itself...and I'm sure there's a logical explanation as to how that might work!? :wink:

*Did it myself once, many years ago, on a friend's MG-TD, after noticing that someone had installed the battery for - ground when it had previously been +! Everything continued to work just fine, including the now-correctly reading ammeter.
 

TR3driver

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Andrew Mace said:
I have heard of, and actually witnessed*, instances where the generator would "repolarize" itself...and I'm sure there's a logical explanation as to how that might work!? :wink:
Yes, but it does involve a certain amount of electrical violence. Basically, the generator starts out generating the 'wrong' polarity, until the cutout contacts close. The relay doesn't care about polarity, it will close for either +12 or -12. But instead of having nearly 0 volts across the contacts at that instant, it's nearer 24 volts! Basically the generator fights the battery, causing currents far higher than the wiring, generator & cutout contacts are designed to handle; but the battery should win quickly. If it wins before anything burns up (which is apparently usually the case), then all is well.

Had the same thing happen on a friend's MGA. He didn't notice a problem until he realized that the ammeter was reading backwards! Since it had worked right before the battery replacement, the car must've been positive ground before and inadvertently converted to negative. He put it back to positive (a decision that he later regretted) but we repolarized the old-fashioned way that time.
 

LBCs_since_1988

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TR4A=negative earth, generator. I believe MG kept positive earth through '66 if not into '67.
 
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