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GM Alternator Conversion in a TR4A

Written by Rick Pam, using information supplied by Jeffrey Barteet

This conversion, done by Leonard Steinberg and me in a 1966 TR4A, went very smoothly, using Dan Masters' Instructions for wiring (the "DMI's") here . We were helped greatly in the mechanical mounting by a posting on the triumphs-digest list from Jeffrey Barteet on July 11,2000, in which he detailed his experience: (Editor's note: link that was here is now broken) . Thanks to both Jeffrey and Dan for sharing their experiences. Made ours go a lot more smoothly.

Rather than repeat the DMI's, I'll add just my own comments and experiences to fill in some details:


We all have our philosophical approaches to how "original" to keep our cars. Ours is: if it's unreliable, modernize it, but keep it appearing original if possible. What this means is that we went with a 3-wire alternator, not a 1-wire, so that the ammeter and charging light on the dash work the same as before. We did not add a voltmeter, as many people often do.

The Alternator Itself

Asking for a "70s-80s" vintage GM alternator wasn't specific enough for the auto parts stores we tried. Dan Masters suggested asking for a model "7127" AC Delco, 60A alternator (the smallest common rating), and that did the trick. We got an aftermarket rebuild, 3-wire with internal voltage regulator (I specified the internal regulator just to be sure). You also need to specify the orientation of terminals 1 and 2 on the rear of the alternator--a 3 o'clock mounting (as viewed from the rear of the alternator) gives good access to the terminals without running into the exhaust manifold. Also bought a pigtail connector for Terminals 1 and 2. Caveat: we had changed to stainless exhaust headers, so look at your exhaust system carefully to determine if the 3 o'clock mounting will work, or if you need a different one.

Note that in the DMI's, Dan says you can get a GM alternator for around US $30 + $5 core, whereas our cost was $55 plus $10 core . However, he lives in Tennessee, while we live in the San Francisco Bay Area. We purchased the alternator in mid-2000, at the height of the Silicon Valley boom, so we're used to things costing twice as much as in the rest of the US.


The DMI's at here worked perfectly for the wiring. The car had already been converted to negative ground (see here). Details of our experience:

  1. Alternator Case Connections
  2. The two terminals on the side of the alternator case had dual labels: labeled 1 and 2, and alternatively R and F. The DMI's refer to terminals 1 and 2, so the R and F were confusing, seeming to refer to "Regulator" and "Field". Turns out this aftermarket model uses a universal casting, for both internal and external regulator models, and the R and F are for external voltage regulators. Bottom line: the internal regulator is the way to go, use the 1 and 2 designations, ignore the R and F.

  3. Wire splicing
  4. We found an old, non-working Lucas control box/regulator, and used it for a terminal block in place of the original, working, unit. Removed the insides, so that all terminals have no internal connections, with the exception of the short copper wire loop connecting terminals A and A1 (shortened and resoldered this connection). The external terminals have just the right number of male spade lugs so they can be used to jumper the various wires, per DMI, as follows (letters are the terminals on the old Lucas control unit base):




    (2) Brown/Blue wires


    Fusible Link


    Fusible Link
    Large Brown/Yellow


    No Connection


    Small Brown/Yellow

    A big advantage of this method is that most of the original spade connectors on the ends of the wires that connect to the Lucas control box don't have to be changed. Only the two original connectors at the generator need to be changed for the alternator: a ring terminal for the Large Brown/Yellow, and a connection to the alternator pigtail connector for terminals 1 and 2. For the black wire that Dan says to "remove and discard", we covered it with heat shrink tube for insulation, rather than cutting it off. Thus, since we saved the original, working Lucas control box, this changeover is easily reversed should we ever have a brain cramp and want to return to the original generator/control unit. And, as noted, the ammeter and red charging light on the dash function exactly as before the changeover.

  5. Fusible Link
  6. The potential to put 60A from the new alternator through wiring that is sized for the 22-25A of the original Lucas generator is covered in the DMI's. It won't happen under normal circumstances--the original Lucas ammeter goes to +/- 30A, and I've never gone off scale--but could happen if the battery is seriously discharged. Rather than upgrade the wiring, we put in a fusible link in series with the alternator output (the original Lucas large Brown/Yellow wire from the generator output). The link is installed between terminals A and D in the table above. It's standard to size a link two wire gauge sizes below the wiring you're trying to protect. We used calipers to measure the diameter of the Large Brown/Yellow wire as ~.100", which is about 10 ga (= 0.102" ) , so the fusible link is sized at 14 ga. Caution: our measurement is only an estimate. If anyone has a different experience as to the gauge of this large Brown/Yellow wire, let me know and I! 'll up date this. The link itself was easily available at auto parts stores; we had to add our own female spade connectors to mate with the Lucas control unit. Thanks to Dan for this suggestion.


In a prior escapade, the fan belt had been retrofit to the narrow style that fits the stock pulley on GM alternators: the mechanical fan had been removed, replaced by a harmonic balancer, and the water pump pulley had been changed, both accommodating a narrow belt. The narrow belt is 9.5 x 975 mm .

For the mounting, here is where Jeffrey Bartett's experience was very handy and accurate. The relevant description from his post, plus my comments:

"Step 2 ( Modify alternator )

"I put the alternator in place and found that the mounting boss on the GM alternator was too thick, causing the pulley to be about 1/2" too far forward of the water pump and crank pulley. Rather than modify my TR's mounting bracket, I cut a small piece of the GM alternators' boss off with a hacksaw.

Comment: we used a grinding wheel instead.

"A portion of the boss I removed was built up with some webbing on either side, and this is the portion I cut off. ( Look at a GM alternator and you'll see what I mean. ). The alignment was perfect then, and I attached the alternator with a good 3/8" bolt to the stock generator bracket which attaches to the block."

Comment: In this mounting scheme, the main mounting boss on the GM casing mounts IN FRONT of the stock generator bracket on the block. Then for mechanical stability, we used a 3/8 x 7-inch long bolt to pick up the alternator mounting boss, plus BOTH holes in the 'ears' of the stock generator bracket on the block. We used a length of 3/8" SCH 40 copper pipe (.493" ID) as a spacer, cut to length to fit between the two mounting 'ears' of the stock bracket, so that the ears won't get squeezed when the 7-inch bolt is tightened.

"Step 3 ( Modify stock tension bracket )

The slotted tension bracket for the adjustment of belt tension didn't align with the GM alternator's other 'ear', so I simply increased the amount of offset it already had with the use of a vise and a large crescent wrench."

Comment: Instead of increasing the offset as Jeffrey did, we used a vise to squeeze the slotted tensioning bracket and REMOVE the offset (this was a new bracket from Moss since the old one had fallen apart when I removed the generator). In this way, the tensioning boss ('ear') on the alternator mounts IN FRONT of the slotted tensioning bracket. So there you have it -- you can try either increasing the offset or removing it entirely. The mounting ear/boss on the alternator has female threads, so a bolt can be inserted from either side. Make sure you can fit a wrench over the bolt head.

All in all, the retrofit works quite well, and the favorite pastime of revving at idle to prevent the ammeter from showing discharge is now only a memory.

Rick Pam -
Jeffrey J. Barteet -


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