View Full Version : Is this normal?

04-04-2010, 06:31 AM
Hi All,

Hope everyone in the northeast is taking advantage of the beautiful weather this weekend.

Question regarding my electrical system: At idle (700 - 900 RPM) if I so much as turn on the headlights or hit the horn button, it's enough to kill the motor. Tapping the brakes will cause the generator light (which is normally on at idle) to dim significantly.

I realize the stock generator leaves much to be desired but, this does not appear normal. I would not expect either of these to draw that much current. Plus, shouldn't a healthy battery be able to make up for the insufficient generator output?

The battery is quite new and fully charged. What should I be looking for?


04-04-2010, 07:28 AM

You need to test your alternator output, but first is the belt tight? If so, check the output with no load, then after you turn on the lights and report back to us.

Batteries that appear to be fully charged, but with a dead cell (or two) can cause this as well, but it's rare for a new one to exhibit this condition.

04-04-2010, 07:40 AM
Do you have the original generator or have you replaced it with an alternator? Sounds to me that you have a generator. At idle, your "ign" light should not be lit. There should be a slight charging current to maintain the battery.

Generators, unlike alternators are very sensitive to rotor speed. So I would agree with Paul. Check for belt slippage and if you have a generator and you are sure the control box is properly adjusted, consider increasing your idle speed. if that does not work, report back and we can consider other causes such as a bad control box or necessary adjustments.

04-04-2010, 08:26 AM
Your battery may be new,but you are not keeping 12volts in the system.If you have bad connections of your main power wires ,the voltage in the harness may be border line.The coil "starves" for power when the lights go on and the engine dies.You must take a volt ohm meter and find your bad connections,they will fell warm to the touch also.
MD(mad dog)

04-04-2010, 09:35 AM
Your external voltage regulator could be bad. I run an original generator/voltage regulator set-up and it's fine for me. I never get the red light at idle....can't say as much for the MGs I've owned!

Geo Hahn
04-04-2010, 10:44 AM
I run my generators with a belt tension that is not very tight (maybe just this side of loose) and never have a problem.

If it were me I would start with the standard set of Lucas fault tests for the Control Box.

04-04-2010, 10:48 AM
You've got multiple problems, Bob. The car should run just fine on even a moderately charged battery, without the generator at all, so that is one problem. As MD says, you need to find out why you either aren't getting full battery voltage to the coil, or why that isn't enough to keep the engine running.

Then you need to work on what is happening with the charging system. It could be the control box, or the generator itself, or even both of them at the same time.

Get a voltmeter (cheap digital DMM (https://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=90899) from Harbor Freight will do) and start by checking the voltage directly at the battery posts. If you don't find at least 12.5 volts here, then your battery is either defective or not fully charged.

Now turn the key on (without cranking the engine) and check the voltage between the coil hot terminal and the distributor housing (or any good ground on the engine). If this isn't within about 0.5 volts of what you found at the battery, you need to chase down the bad connection in the wiring. I would probably do my next test at terminal A1 on the control box, since it's easy to get to.

Here's the relevant portion of Dan Master's wiring diagram

04-04-2010, 11:21 AM
...consider increasing your idle speed...

Thats what I had to do when I upgraded my electric fan... that and my hot cam is just too rough at low idle. 1000-1100rpm won't hurt anything but your city gas mileage. Just make sure everything else is g2g 1st, so you don't end up with a road-side fire-ball.

04-04-2010, 04:13 PM

Thanks. I have the original generator and external control box (i.e. stock) for a TR4A. I suspect the "multiple problems" as suggested by Randall. If I may try to divide and conquer, I suspect I have issues in two areas:
a) charging system: insufficient input into the battery while engine idling. I plan to test both generator and control box
b) faulty wiring: I performed the test Randall suggested above. I got 12.78v across the battery but only 8.09v on the coil (+) terminal. Obviously, there is a voltage leak somewhere.

WRT a) I am confident I can track down a procedure to test (i.e. Moss videos).

WRT b) any ideas what to try next? Also, I am running a Pertronix electronic ignition with a 3.0 ohm coil. Could the electronic ignition be the source of a voltage leak like that? I wouldn't think so but...

Appreciate any further input.


04-04-2010, 04:26 PM
Actually, you shouldn't expect the generator to actually charge at idle. Just not in it's nature. If the ammeter comes up to 20 amps or so at a fast idle (1500 rpm) just after starting the engine, then it's probably OK.

The Pertronix is not likely to be the source of the problem. If it were defective and drawing that much current, you'd see the smoke leaking out somewhere!

Did you check at terminal A1 on the control box, as I suggested? If the voltage is OK there, then the problem is most likely the ignition switch itself (but I would make more checks to confirm that before replacing the switch).

If the voltage on A1 is too low, check at terminal A. If it is different than A1, then the control box is defective and should be replaced (those two terminals are supposed to be internally connected together).

If the voltage on A is also too low, I would next check the brown wire where it attaches to the starter solenoid (again because it's an easy place to look). The idea is to keep doing a "divide and conquer" on the circuit between the battery & ignition coil, until you find where the voltage drop is happening.

Just for clarity, that circuit is the hot battery cable to the solenoid, brown wire from solenoid to ammeter on dash, brown/white wire from ammeter to control box terminal A, brown/blue from control box A1 to ignition switch, white from ignition switch to coil.

Another way to attack the problem is to simply feel along those wires, connections &amp; devices after a drive. Wherever the voltage drop is happening, will be hot! (Burned fingers are sometimes the price we have to pay <<GRAEMLIN_URL>>/grin.gif )

04-04-2010, 06:51 PM
If that 8.09 volts you read at the + of the coil and your Pertronix red wire is on that same terminal, it's borderline voltage for the Pertronix Ignitor.
From the Pertronix package:

04-04-2010, 11:02 PM
Thanks Randall, this is really helpful. So, I want to pursue the testing on the control box (terminal A1, etc) as you suggest. Do I do as before? Namely, ignition switch on, engine off? What reading is considered OK and what is considered too low? Am I still looking for a reading within 0.5 volts of my battery?

Then, is it the same procedure as I "divide and conquer"? I assume I keep going so long as the voltage remains within 0.5 volts of my battery until I isolate the culprit where the voltage drop occurs(?)

Thanks again,


04-04-2010, 11:38 PM
That's the idea, Bob, you should find basically full battery voltage all along the circuit, from the battery all the way to the coil. A very slight drop (0.1 volts) per connection is about the limit; anything more than that indicates a problem.

By "divide and conquer", I mean jump along the circuit (testing at points that are easily accessible) until you find a point that doesn't have full battery voltage (within 0.5 volts). When you find that point, you now know that the problem is between this point, and the last place you checked. So you start working back the other way until you start finding full voltage again. Thus, if A1 is low, check at A. If A is also low, check at the starter solenoid. It will probably be fine, so next check at the brown wire on the ammeter (behind the dash, not so easy to access). Etc, etc. Or, you can just let us know what you find at A1, and someone will suggest a next step.

Oh, one thing I forgot to mention and probably should : Pertronix warns against leaving the key on for an extended length of time (like 10 minutes) without the engine running. For the purpose of this test, it would probably be best to disconnect the red wire to the Pertronix, and install a temporary jumper from the other coil terminal to ground. That removes any danger of overheating (and damaging) the Pertronix module. The coil should withstand several hours of testing, but it would be best not to leave the key on for more than several hours without giving it a chance to cool.

Radio Shack sells a package of "test jumpers" (aka "clip leads") that are handy things to have around. I keep a couple of them stuffed in with the DMM that I always carry in the car. https://www.radioshack.com/product/index.jsp?productId=2062661

04-05-2010, 11:23 PM
Well, thanks to you guys I'm actually starting to feel like I know what I am doing (always the first sign of danger!).

The schematic above is really helpful. I got the DMM out this eve and did a few quick tests:
Battery = 12.45 volts
Term A1 = 7.85 volts
Term A = 7.85 volts
Coil (+)= 7.10 volts

So, clearly something is happening b/t the battery (+) and Term A of the generator control box. Next I'll check everything b/t those two points and "divide and conquer". Quick question, what the likelihood the ammeter itself is causing the voltage drop? The ammeter actually reads correctly but, I ask only because each time I go out for a drive I've always noticed the inside glass of the ammeter fogs up for the first 10 minutes of the ride (almost like it's getting hot inside the gauge).

In this regard, would it be OK to bypass the ammeter (i.e. "jump" the N and NW wire together) and take another reading at Term A on the control box?

Thanks again

PS: I used to fear electrical issues but, now, this is actually kinda fun

04-06-2010, 12:16 AM
Could well be the ammeter, Bob. It's pretty simple, just a coil of solid wire inside, but I've heard of the coil breaking away. If it was just barely touching, then the ammeter would still work (since the current is still there) and the heat from the resistance could even be the cause of the fog. Might also explain the generator light.

But I would be more suspicious of the connections to the ammeter. Broken wire strands, or corrosion inside the 'Lucar' terminals would do the same thing. If it's something like that, though, you'll probably notice when you look back there. Recently overheated copper has this funny red color to it, instead of the usual "old penny" brown.

Bypassing the ammeter is a reasonable test, but don't forget it needs to handle at least 20 amps or so. Those Radio Shack clip leads I mentioned won't carry that much current for long.

Might also be worth a quick check at the solenoid terminal; again just because it's easier to get to than the ammeter.

BTW, you can find a full schematic at https://www.advanceautowire.com/tr24a.pdf (which is where I got the snippet posted above).

04-09-2010, 09:39 PM
Hi Randall,

I was able to remove the ammeter this eve (obtained access by removing the glove box). Sure enough, the rubber covers on the Lucar terminals were all melted from heat damage. I cut the covers off the wires, cleaned the connections (on both the wires and the ammeter), inspected, and all looked normal. Surprisingly, no damage to the wires or connectors. I reconnected everything and...12.3v at terminal A and 11.5v at the coil (+)! Much better than the 7.85v and 7.10v I got the other night.

So, two more questions:
1. could simple corrosion on the terminals have caused this? Or, should I suspect something more?
2. there were also two eye-connectors with black wires connected to the ammeter securing brackets (not the ammeter terminals) under the knurled nuts. One eye-connector had one wire and the other eye-connector had three wires off it. Is this correct/normal?



04-10-2010, 10:23 AM
Could be just corrosion on the terminals. Still, if it were me, I'd try a test before putting it all back together. Leave the gauge hanging out (making certain the wires/terminals cannot come in contact with anything metal) and turn on the headlights for 5-10 minutes. Feel the wires &amp; terminals; if you can feel they are warm then you still have a problem.

I'm not intimate with TR4A wiring details, but your black wires don't sound original to me. Someone likely decided the gauges needed to be grounded and added ground wires to everything.

04-10-2010, 12:07 PM
I think the black wires are the grounds for the gauge lights due to the wooden dash if I remember correctly. It's been a long time though so I'd have to go crawl under the dash to confirm.


04-10-2010, 03:00 PM
I just looked and have one black off the knurled nut. Sat on another pair of glasses in the process. rats

04-10-2010, 03:42 PM

I would call this project a huge success thanks to everyone's knowledgeable guidance. Not only did this solve the original issue but:
-I have a 62% increase in voltage at the coil
-engine now fires up instantly
-my headlights, tail lights and brake lights now burn bright even at idle
-this was a serious safety concern of mine idling at a stop light at night
-my turn signals and horn actually function at idle

I did reassemble every thing last night, including the black (ground?) wires. I made certain every connection was clean and clear of each other so nothing was touching by accident. I plan to try the 10 minute headlight test tomorrow and feel the terminals/wires to see if they get warm. That will be the final test.

BTW, here is what I suspect happened. I recall two years ago or so, I accidentally left my headlights on after I parked in the garage. 45 minutes later my 9 year old son came in and said "Dad, there's smoke coming from the dashboard of your Triumph". At the time, after shutting the lights off, there appeared to be no other ill effects. But, now I suspect there was mild corrosion on the ammeter terminals which, after 45 minutes, generated enough heat to seriously compromise the connections and cause further problems.

Let's hope with the removal and full cleaning this is the end of it.

Thanks again,


04-10-2010, 04:15 PM

Here's a picture of the back side of a TR4A dash showing the black grounding wire for the center gauges. Sorry for the quality (focus especially) but I only had my cell phone with me.


I checked the wiring harness in the car and there is a another eyelet connector with 3 black cables which would be the ground for the gauges. Not sure which post it would have been connected to but the ammeter would be a likely suspect.

Glad to hear that you have your problem fixed!


04-10-2010, 05:47 PM
I think the black wires are the grounds for the gauge lights due to the wooden dash if I remember correctly. Ah yes, that would explain it. Sorry, I forgot the 4A had a wooden dash and separate gauge lamps. Apologies to all.

04-11-2010, 10:03 AM
This is just one more story illustrating that contact oil/dielectric grease are necessary for an old Lucas system to soldier on in the 21st century. Just a little dab makes a huge difference on a 50 year old spade or bullet connector....
MD(mad dog)

04-12-2010, 12:17 PM
And indeed, this is one of the areas where Lucas electrics fall short in quality compared to most. All other car makers plate their terminals with tin or similar, to prevent corrosion. Lucas just left theirs bare copper, which corrodes slowly just being exposed to air.

04-12-2010, 08:46 PM
The prime reason I scream: "SOLDER!" :wink: