TR6 Another eletrical question [uugh!]

Hey Forum,

Back with another question, so I installed a new alternator which had bad diodes (according to bench test from Orielys) I assumed the alt was defective, returned for warranty exchange. Got to thinking, before I put the new one on, could it be the 73'tr6 that caused the blown diodes. The previuos Alt also hade a rectifier problem, I assumed it was age, as the car did not start when I bought it and had been sitting for 10 years. Now I am begining to wonder. Most all of the electrical in the car was functioning when I got it running and it was all original untouched. I have since had a few gremlins with turn signal relay and door/key buzzer. I fixed them and really have not had any issues other than instrument bulbs that are out.
So just to be sure I started to check all the wiring for corrosion or loose connections. The four spade plugs on the battery + terminal wire were pretty corroded and I cleaned them up. Did some ground resistance test on the fuse panel and found resistance at these locations. (See Pic) The other terminal plugs were open/infinite. Question being, should there be ground resistance at these points?
Sorry for long-winded question.

Additionally, the sense wire from the original Lucas alt had melted. I chased it back to clean wire and taped it back, since not used on new alt. Does anyone know where this wire originates from? Thanks-Brian

Yes, you would expect to find some small resistance to ground at those points. Basically you are seeing the loads on the circuit, which for the red wire will be all the tail and marker lamps. For the green, you're probably seeing the voltage stabilizer and gauges (assuming the points are open and the wipers/heater/turn signals turned off).

The corrosion at the positive terminal could possibly be causing diodes to pop if that is where the output wire (s) from the alternator are connected. If the connection was closed and then opened up with the alternator working hard. the output voltage might go way high before the regulator has time to turn down the current. Normally, the diodes should be able to handle that condition (known as "load dump"), but it puts a lot of extra stress on them. So if they are the least bit weak, pop goes the diode.

Many alternator rebuilders also say that you have to charge the battery first. IMO this is another case of reducing strain to avoid popping weak diodes; they should be able to handle whatever the alternator can dish out. But it's a thought.

I believe the small brown wire originally went to the connector hanging off the side of the positive battery cable. But I'm not at all certain of that. I'd try disconnecting all of them, and see if the small brown one gets disconnected. Another possibility might be at the starter solenoid.

Thanks Randall, The positive cable plugs were pretty corroded and I did have the new alt wired through the two brown wires in the circuit. I plan on running an 8awg wire direct to the battery for extra safety margin, so with this additional wire there really can't be a load dump can there? Only other wire on the alt is the voltage gauge light wire, is there a chance this circuit could cause diodes to pop?

PanP said:

so with this additional wire there really can't be a load dump can there?

Click to expand...

Make sure the ground path is good as well. That's not usually a problem, but it might be. I had a case once where the ground strap bolt was loose, and the ground strap would alternately weld to the bolt, and break free from vibration. Didn't damage the alternator, but sure made it act funny.

Only other wire on the alt is the voltage gauge light wire, is there a chance this circuit could cause diodes to pop?

Click to expand...

An intermittent short to ground in this wire might possibly damage the diode trio (assuming your alternator uses a diode trio). But I don't see how it could damage the main diodes.

I recently replace the original fuel pump on my TR3A with an electric fuel pump. Things started going badly when I hooked the power wire on the pump the ignition. A couple of panel wires came off and I thought I had put them back on correctly but the car now turns over when the key is turned without using the starter. I have been using the wiring guide in the maintainance manual but is not very informative as far as the control panel is concerned.

Can you give me any suggestions on how to properly wire the panel? Where I can find a better diagram? Where proper colored wires can be obtained? Would a wiring harness be a good source of wire? The wiring harness on the vehicle is new a few years ago but not hooked all the way to the panel.

Any advice or suggestion will be appreciated
Bruce Beesley 11/26/16

You really should start a new thread with new questions. This thread is about a 73 TR6, which as you might imagine is wired quite a bit differently than a TR3A.

Is your car an early or late TR3A? Late 3A diagram is in Practical Hints 6th ed https://drive.google.com/open?id=0B2H2NJt34OffNjQ4MTkyNjItZTVmYi00M2U2LWJhYWUtMjEyZDgxMzUyNzUw
Earlier cars are closer to https://drive.google.com/open?id=0B2H2NJt34Offc1R3Szh4WG1ybDA

(In both cases, I've hacked the drawings up to show the left hand steer connections. But the gauges in the instrument panel still aren't in the right places.)

Or you can use the appropriate diagram from https://www.advanceautowire.com/tr24a.pdf

There should be a white/red wire from the starter solenoid on the firewall to the starter pushbutton on the instrument panel. Nothing else should be wired to the same terminal. The other side of the pushbutton switch gets linked to the load side of the ignition switch right next to it. I would start by hunting down that wire and getting it right, then check to see what else doesn't work.

If you can't read the color codes, you may need to do a little detective work. Disconnect the small wire from the side terminal on the solenoid (which is the white/red I mentioned above) plus both wires from the starter pushbutton and temporarily connect a jumper from the one you disconnected at the solenoid to the hot terminal on the solenoid (or one of the brown wires at the control box if that is easier). Now get under the dash with a test light and look for a disconnected small wire that has power on it. Disconnect the temporary jumper and check again. If the power went away when you pulled the jumper, that's the wire.

Later wiring harnesses used PVC insulation, which is usually still readable if it hasn't been too hot. I often rob bits from an old harness. But, the PVC gets hard with time and heat, so it may not be the best source. I believe British Wiring will sell you color-coded wire by the foot, if you want
https://www.britishwiring.com/Wire-s/43.htm