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Triumph TR6 - coil resistance

What should happen to a coil's resistance as it gets hot? Should it increase, decrease or stay the same? Also, a 6 volt coil should have a resistance around 1.5 ohms and a 12 volt coil around 3 ohms, is this correct?
C Wiebe


1.2 and 1.5 for non internally ballasted aprox. depending on primary windings? There are different specs.

Internally ballasted are about 3. Should say on outside of coil internal ballast.

Resistance might increase slightly as coil warms never tested myself? You would need to have a very high quality meter to notice if things are good. Coils do fail at temp. due to internal connections.

If your having a problem the description of that may be of more help.

Bill Brayford

My car runs okay for about 15-20 minutes (after choke off) until it really warms up, then it misses and sputters at lower revs, and gets worse the longer it runs. I thought it may be the coil but I want to make sure I get the correct 6 volt coil for my ballasted ignition.
C Wiebe


2 things. What year and model car are we talking about thats running 6V? Could you toss a first name out for yourself.

The 2 originals as known are HA12 early and 15C6. Most have been replaced by many variations made for the car? Internally ballasted or external. Sports coils, super coils etc.

As for your problem:

Coil may be faulty?? But I would check these first.

Fuel starvation problem?? Open the gas tank cap and try. Your vent system may be faulty. Vaccum builds in tank.

Old gas, water, Rust junk in tank gum from old gas.

Your carbs have a bowl drain. May screw out or wiggle out under each. Drain in a clean glass jar. The ones your wife uses for preserves are great unless you get caught. Look for any junk.

Replace your fuel filter. If your tank is full of scum or rust particles the filter will clog in a very short time.

Vacuum leak. If the car is EGR equipped . You likely have a thermal vac. valve. in the emissions system. Could be on the rad hose whatever since I don't know car or year. From your description thats the place to start. The device opens only when engine is warm.

Sounds as if when engine comes to running temp. and TVR opens you have a Vaccum fault to atmosphere. Usually consistant time frame. Bad hose or EGR. Easy to test just block them.

Garbage in the fuel will build up and create blockages due to flow. Not always constant time frame.

Let us know


Bill Brayford


Thanks for all the input. My name's Chris. I have a '76 TR6. The coil it has(had) is a Lucas 15C6. I measured the resistance in the car as 4.4 ohms, I read somewhere it should be around 1.3 (later out of the car it measured at 1.9!?).
I bought a coil from a local parts store that said "do not use with external balast" I asked the guy if it was a 6 volt coil and he didn't know (their book listed the same coil for 69-76). Its made by Niehof. It resistance was 3.3 ohms. The car ran the same (poorly). I jumped the resister wire to give it 12 volts and the car still ran poorly. After the car was hot the resistance on this coil was 11 ohms.

There is no thermal valve. The car runs the same if the vacuum to the EGR is attached or not.

I haven't checked out the fuel filter or tank vent yet.

I feel strongly it's something on the ignition side; however, everything (except the coil) has been replaced or checks out fine.

I'm ready to pull my hair out, for more than 2 months I've been working on this!

C Wiebe

Hi Chris

Nice to meet you. Daughter is home for summer attends U/Vic for last 3 years. Brought home husband to be for summer also attends U/Vic but resident of Pender Island BC. Now that we have covered reasons for tearing hair out :)

The coil should be for a 12 volt system. Coils are really simple just HV spark providers. Your 76 has an external ballast wire. You get 12+ on ignition and about 8.5 at run. I don't think using it with the external ballast should harm it but I can't be positive. The output of spark would of course drop with both in line at run?

Do not rule out a bad part from your parts store. If they are any good they should replace for free. Thats the way the system works.

Hook up new coil to full 12 volts ignition run. Bypass resistor wire. Replace the little wire to distributor if not done. Can cause problems.

Try the other items. Its way to late here so off to bed will sleep on it.

Bill Brayford


From experience a coil will either work or not. They do get old but don't usually cause your problem.

Have you removed the plugs after a period of bad run. You want to look for any plug fouled or a different color from nice even grey brown?

When the engine is causing problems what does the exhaust look like. Black, white, light grey?

Have you done or had done a compression test?

I am still betting on fuel problems or a vaccum leak. Try the carb cleaner sprayed around manifold when the problem occurs.


Bill Brayford

Thanks for the advice Bill.

The plugs look okay, a little on the lean side (carbs are set full rich, points to a vacuum leak?)

Exhaust looks fine (eg no color at all)

No compression test.

I've sprayed carb cleaner everywhere, I can't find a leak.

I figure there's got to be a pretty big leak to affect mixture at anything above idle.

At temprature (when it runs poorly) the temp compensators are full closed)

I will try a new fuel filter, check for vacuum leaks again and check the carbon canister to see if too much air is coming that way.

I'll keep you posted.-Chris
C Wiebe

Chris, Sorry to hear about your problem. Are you going to attend The car show this weekend At the Van Dussen Gardens?
Don K.

I'll jump in here, my 75 had an intemittant problem with the engine quitting without notice, only to start a couple hours later. It finally happen once for all and I was stranded 100km from home. The problem was a dying resistor wire and when it fully burned out I needed a tow back.

Best modification that I did was to by-pass that terrible resistor wire burried deep in the wiring harness which will only become a source of problems in the future due to age and heat. I attached a direct voltage wire right from the fuse box, piggy-backed a ballast resistor to the coil and keep a spare one in the glove box. A Ford/Mercury (for pick-up trucks) ballast resistor for years 1959-1969 provides the right Ohms and the mounting makes a very neat job. 5 years later and over 10,000miles it has yet to fail.

Steven, Thanks for the advice. I've already jumped the resistor wire to rule that out. There was no change in how it ran.

Don, I hear the Van Dusen show is a real good one. I've never been to one and again will miss this year's. It's sometimes difficult to go the shows on the mainland (I'm on Van. Island). One year I'll make it though! And I'm not going anywhere until I can fix my car :(
C Wiebe

Chris pick up your coil power from the white on the fuse box. Remove the others do not add on. Heres a sight with actual wiring for each car. Go to bottom of page TR wiring.

I went through this with someone else awhile ago. A coil can also be a resistive ground? Due to the windings a faulty coil wiring coating can cause problems as they warm up. What you suspected with HV coil. HV is not down in hot exhaust area though?

Looking at wiring diagram. It may be your starter solenoid coil connection White/yellow bleeding current to ground if you just added on testing to that circuit? Doing the above should bypass that?

From your posts you know your stuff. I tend to agree on electrical after latest.

I do better than I write so ask if I don't make sense :)


Bill Brayford

When I jumped the resistor wire I disconected all other power wires to the coil and attached a full 12 volts from a white wire at the fuse block.

I have finally procured a proper coil that requires an external resister. I will let you know how that goes when I get a few minutes to pop it in and try it.

If that fails. ??????
C Wiebe

Well, I got a new coil. Didn't change much. But on the test drive a noticed the choke nob would always come out a few millimeters and cause a slight fast idle. Since pulling the nob out while running made the car run worse, I decided to hold the nob all the way in while driving.

The car actually ran better! Finally a clue! I will fiddle in this area when I next get some time.
C Wiebe

OK here's another question.

Voltage at resister wire when car is cold = 5.65 volts

Voltage at resister wire after about 30 minutes running time = 12.35 volts

Is the voltage supposed to change?

My understanding was that it shouldn't.
C Wiebe

Hi Chris

Now I get the 6v.
The voltage at the resistor coil end of the resistor wire Pink/white should be 6.5V aprox.? That # is not written in stone!! I have seen 8.5 and just below 6. When the cars ignition switch is in the run position. That wire is bonded in to the white wire system from ignition. After 27 years who knows what it will be now but it should not go up. Heat usually increases resistance.

The white yellow wire should have full battery voltage 12+ hopefully at ignition starter turning only.

Are you taking the readings engine running?

I suggest if you havn't tried already. While still hot.

Shut the car off and then turn ign. to run read that coil end voltage. You should only have resistive wire at that point? somewhere around 6v?

Have someone start the car voltage should jump to battery while engine turns and then drop back?

If those 2 wires are still connected together at coil seperate them however is best for you so you can remove the white/Yellow while running. And check voltage.

Restart the car if the coil input goes high right away or stays high remove the white yellow while running and check. If the voltage dropped back you have a starter solenoid problem maybe ignition switch?

If it stays high white/yellow off its coming through resistive wire. My bet would be wire covers have somewhat melted together due to heat from resistor wire over time? Heat will build in wire and then shorting to next wire in harness you have 12 volts as the car and resistive wire warms up?

As problems go Chris this is definately a 10 :)


Bill Brayford

Hi Bill

I've checked the voltage when cold and hot (5.85 vs 12.35) but never while the engine was running.

If the current is bleeding through from another source within the harness, the only solution is an external resistor.

I fixed my choke and the car runs so much better but still will miss after about 30 minutes running.

Maybe this resistor wire thing is the culprit. I may also try to advance the timing a couple of degrees and see what happens.

C Wiebe

This thread was discussed between 10/05/2003 and 21/05/2003

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