Coil killer??

[markcro]

Late 1973 GT6 Mk.3
Not running a ballast resister so am using a 12v coil.

Look for opinions and if anyone has advice....

About a year and a half ago I replaced the HT leads, dissy cap, points, and coil, etc. She ran great for a while but then developed a nasty habbit of back firing and then dying if you touched 3,000 rpm or above. This meant popping the bonnet poking around until she would start again. Then repeating the process 5 miles down the road until you managed to limp home.
Checked timing etc., dissy etc. All seemed fine and this issue caused me not to use the car over the last year as it was so intermittant and unreliable.

Anyway even though the coil was brand new I started grasping at straws and about 8 weeks ago when I broke down I cabled tied a 12v coil that I borrowed from a friend to the chassis leg. And bingo! She ran like a dream. Would pull easily across the full rev range. Spent the day driving and drove 60 miles of country roads and motorway. She was purring.

So I gave back the borrowed coild and fitted a brand new coil today in it's proper position on the block. Took her out for a spin and she ran great for the first 40min and then developed a miss at 4k and was hesitant to pull past 4k. (I don't usually go around high revs but I wanted to fully test it).  She just didn't want to pull past 4k and gave the odd miss.

Got home and found that the new coil was too hot to touch. It was scalding hot. How hot should a coil get?
Has anyone relocated the coil to keep it cooler? Would this make a difference?
I need to test it more to see if this is yet another new coil that is dying. Any ideas?

Thanks.

[gallandro]

The typical coil killer is over voltage. Check whats comming out the alternator.  Also chek the points etc are OK.

[rotoflex]

Did the late GT6 Mk3 have a ballast resistor, or resistor wire?

[SpitNoir]

Hi Markcro. Did the borrowed coil have numbers on it to verify what it's internal resistance was? Or do you know whether it had been in use on a 4 cyl or 6 cyl engine previous to your using it?
What brand was it? Perhaps a very high-performance one that was better equipped to handle the heat?

And isn't it a bit iffy to use the full 12v on a points ignition, no matter what coil you're using? I can't really remember, but I thought that the condenser took a powder and gave the points a good thrashing when trying to control the extra volts.
Perhaps not, but there may still be the exact rating of the other (good) coil to take into account.
Then again, you can always test the over-voltage theory by installing, at least temporarily, a ballast resistor inline.

Best of luck. It's frustrating to have your ignition crap out on you.

Paul

[markcro]

Thanks Guys.  According to the owners manual the late Mk.3s actually used a 6v coil. I'm not sure if they used a ballast resistor or resistor wire (To be honest I'm not sure what the difference is as I'm still learning! ;) ).
Anyway when I got the car the ballast resistor had been removed.

The borrowed coil was from a V8 and didn't look to be anything fancy. When I originally stuck it in, it had a ballast resistor on it's body so I left that inline but the car wouldn't run at all. Would only run when I bypassed the resistor.

The new coil that I just replaced was a "Sports" model so was high spec. I was thinking that maybe it was putting out too much juice so the one that I have replaced it with is just a standard one.

Why would it make a difference to points ignition whether running 6v or 12v coil? (sorry if a stupid question!)

[Van]

Coils can cause strange problems which are hard to diagnose other than by replacement. Years ago I had a problem with tickover being rough, adjusted the timing, tickover was then OK but wouldn't rev. Replaced the coil and then it was OK. More recently the car would only run for half an hour or so, then splutter and cut out and had to be left to cool. Again it was the coil.

Full 12V should be fine on a points system, most 60s cars have that and ballast only really came in in the early 70s.

Exact type of coil should not be an issue as long as it's 12V.

I always move the coil away from the engine. On my GT6 I have it on top of the suspension mount, on the PI I had it on the inner wing. Heat from the engine increases the coil's resistance and it always puzzles me to see "performance" low resistance coils costing £££ screwed to the side of an engine whereas moving the standard coil costs nothing.

The 12V coil should still work in the normal place though!

This then leaves a number of possible causes. Voltage too high causing coil overheating quite possible - already mentioned. Some other part of the system failing at temperature - rotor arm or condenser. Distributor advance mechanism doing odd things.

Check the voltage first as you will have to drive for ages to identify other faults.

Then check everything else is in order like points gap and timing (is timing unchanged from where you left it?)

Is the condenser OK? Again these can be hard to test other then by substitution as they can check out on a meter OK but then go funny when they are hot/carrying current/higher revs.

After that it becomes time consuming as you have to run the car a while to make it go wrong. Next time you go out, take a new coil and rotor arm. When the car misbehaves, substitute these parts and see if it makes it work.

Van

[markcro]

Thanks Van. I will try those things. If the voltage is too high would that mean that this 1 day old coil is now screwed?  :-/
And yes the timing hasn't been touched, or contact gap since it was running sweet last week with the borrowed coil.

[Van]

You have posted since I started writing!
This is getting stranger as you are saying the coil that worked originally had a ballast resistor on it, but you bypassed it. If this coil originally came with this resistor then this is a less-than-12V coil. Yet it is the only one that works. This makes it seem like your car has got a ballast resistor after all. I'm not familiar with what the ballast resistor looks like on a Mk3 GT6 - you say yours has had it taken off but are you sure? It might be a bit of wire rather than a big lump, so it's less obvious. You need to check out what it looks like and where should be and have a look.
Van

[piman]

Hello Markro,

there is some confusion as some coils intended for a ballast resistor system are still marked 12V. The simple test is to measure the primary winding resistance, about 3 ohms for a coil that runs on 12V and about 1.6 ohms for one intended to be run with a ballast resistor.

If a coil gets very hot it is getting too much voltage, location etc is secondary; Triumphs have run cars with engine mounted coils without problems for years (as did MK 2 Jaguars, which tend to be pretty hot under the bonnet).

Alec

[Pete Lewis]

give us the  make/numbers/type on the coil  there are many 8volt ballast use coils that are actually marked 12v as the base voltage of the vehicle just to add to the confusion.

but fitting a 6-8v and power up with 12v will give a red hot coil, often melts points, blows electronic versions, and the excess HT wrecks rotors , and caps  all are possible
if unsure,borrow, get or buy a £5 multimeter and see what you coil feed actually is giving .  as said ...

12v =  3ohm coil
8v = 1.6 ohm coil
Pete

[cook1e]

Just to confirm the ballast resistor set up on MK3 GT6, early GT6 MK3s (ie the ones with Rotoflex rear suspension and basket weave plastic seats) have a ceramic type ballast resistor mounted on the side of the engine near the coil. Late GT6 MK3s (ie the ones with swing spring suspension, and cloth corduroy  seats) have a resistor wire incorporated into the wiring loom to form the ballast resistor.

[rotoflex]

cook1e wrote:

Just to confirm the ballast resistor set up on MK3 GT6, early GT6 MK3s (ie the ones with Rotoflex rear suspension and basket weave plastic seats) have a ceramic type ballast resistor mounted on the side of the engine near the coil. Late GT6 MK3s (ie the ones with swing spring suspension, and cloth corduroy  seats) have a resistor wire incorporated into the wiring loom to form the ballast resistor.

This is what I was suspecting.

markcro:  Earlier cars had the resistor implemented as a ceramic ballast resistor, & apparently later ones had a "resistor wire" instead.

So if your wiring is unchanged, & you have the "resistor wire" from the factory, you should be running a 6v coil.

I don't have a wiring schematic for a late GT6 utilizing the resistor wire, but I suspect that because of Lucas standardization of wiring color codes someone familiar with the resistor wire implementation in the Spitfire can tell you what color wire to look for, where it would be located, & perhaps suggest a wiring diagram for a Spitfire incarnation utilizing the resistor wire that might correspond to your GT6 (at least as far as that part of the ignition hookup is concerned).

[markcro]

Thanks again everybody for the replies and apologies for the very late reply. Unfortunately my GT6 lives 20miles away at my sister's house and with work and everything else going I have been mad busy. Anyway I went out there today and took the following measurements:

- Output from the alternator (measured on battery terminals): 14.54v
- Voltage going into coil at idle: 11.6v
The voltage going into the coil raises to 13.5v at 4,000 rpm. (didn't check any higher).

-- The original coil that I confirmed had this issue was a Lucas Sports coil (golden colour).
Lucas DLB105
Coil resistance: 2.9 Ohms

-- I then replaced this with a borrowed coil and she ran sweet for the 80 miles that I drove in the one day. That coil was a Lucas CL-72 12v  (Japan 112K7).
Coil resistance: 1.5 Ohms

-- So then the new coil that I just fitted that started showing hesitation at 4K after 40minutes of being out of the box is  Lucas DLB101.
Coil resistance: 3.3 Ohms


So all of this confuses me even more!  :-/   So the coil that she ran perfect with is an 8v one as it has 1.8 ohm coil. So that means that it was getting over voltage from the 12v feed but yet ran sweet?

The last Sports coil and now the new standard coil were having issues, yet are getting their designed 12v ??


I am thinking of going electronic ignition as I believe that these don't need a ballast resistor? Or my other option is to try and wire back in the ballast wire and buy yet another coil (6v) one.

At this rate I am getting average of 50 miles per coil as this would be my 4th coil!  :X
(5th coil if I include the borrowed one!)

The first thing that I will do though is to replace the condensor and rotor arm just to elimiante these. Even though these were replace about 300 mile ago...

[TimW]

cook1e wrote:

Just to confirm the ballast resistor set up on MK3 GT6, early GT6 MK3s (ie the ones with Rotoflex rear suspension and basket weave plastic seats) have a ceramic type ballast resistor mounted on the side of the engine near the coil. Late GT6 MK3s (ie the ones with swing spring suspension, and cloth corduroy  seats) have a resistor wire incorporated into the wiring loom to form the ballast resistor.

Thats very interesting as my very late MK3 (200 from end of production) had a ceramic type ballast resistor mounted on the side of the engine. :-/ :-/

[Pete Lewis]

going electronic only replaces the mechanical points and has no bearing on the coil used or its voltage that stays as designed unless you wish to change something  be it  6/ 8 or 12v .

electronic units do best with a 12v direct feed

you have measured yours as being a 12v circuit witha 3 ohm coil  so any ballast has be removed /by passed

is the coil polarity correct,  the terminal that links to the points should be the same polarity as the battery eaarth ..on yours - negative

ign feed to the +positive.

if you can still measure the resistance of the 12v  primary        whats failed on the coil ????


just some thoughts
Peter

[markcro]

peterhlewis wrote:

going electronic only replaces the mechanical points and has no bearing on the coil used or its voltage that stays as designed unless you wish to change something  be it  6/ 8 or 12v .

electronic units do best with a 12v direct feed

you have measured yours as being a 12v circuit witha 3 ohm coil  so any ballast has be removed /by passed

is the coil polarity correct,  the terminal that links to the points should be the same polarity as the battery eaarth ..on yours - negative

ign feed to the +positive.

if you can still measure the resistance of the 12v  primary        whats failed on the coil ????


just some thoughts
Peter

Thanks Peter,

The +ive on the coil is connected to the feed coming from the alternator and the -ve goes to the distributor. So that should be correct.

That's the thing, I'm not sure what has failed on the coils if anything!?! The symptoms started about a year and a half ago. I could potter around all day at below 3k rpm. But as soon as I touched 3k or beyond she would be very reluctant to pull past 3k, start backfiring and missing and eventually die. Stop at the side of the road for 2-3 minutes, restart, drive 3 miles and die. Do this all the way home.
And then some days it may take an hour or so for this to happen but always reluctant to pull past 3k.
Swapped the coil- and bingo- rang for 80miles and pulled with pleasure across the full rev range.
Bought the new coil, ran well but then noticed a slight misfire at around 3k and reluctant to pull. So I went home before she started the dying act again. Haven't been out since.
And all coils including the new one were scalding hot..... :-/

[drofgum]

markcro,
Having followed this thread I wonder if you have checked the voltage at the coil with the engine running. The coils being so hot along with the other symptoms makes me think that you might have a case of too much voltage because of an alternator problem. It wouldn't take long to check ( you might want to check at 3K rpm to see what happens at that speed )and also to check the meter on another car to be sure it is reading right.
                                                                          All the best,
                                                                          Paul

[Pete Lewis]

another obscure idea...  do you have a electronic rev counter  if so disconnect it and see what effect that has  they (depends on type) are a good cause of a misfire.

if its mechanical cable driven  forget this  clue.


give the dizzy rotor a goos side shove, is there any play in the spindle ,, nobody oils the inside of a dizzy and the top bushaand spindle  wears out and yuo cant control the points gap when its running.

make sure the rotor  has no rivet in it if its a lucas copy,  delco are less problem for rotors but classics for worn spindles  

Peter

[markcro]

Hi Paul, these measurements were taken with the engine running and up to temperature at idle. (Didn't take it out for a run so the coil was not scalding hot):
- Output from the alternator (measured on battery terminals): 14.54v
- Voltage going into coil at idle: 11.6v
The voltage going into the coil raises to 13.5v at 4,000 rpm. (didn't check any higher).

Thanks Pete, I'll check to see if there is play in the rotor. It's a Delco dizzy and she is still running a mechanical rev counter.

Cheers,

Mark.

[drofgum]

Mark,
Those seem like good figures, but did you check the meter on another car? Inaccurate meters are not unknown. The voltage drop of about a volt to the coil seems rather a lot, but it isn't likely to be the cause of your coil problems.
                                                                        Best of luck,
                                                                        Paul

[markcro]

1317 wrote:

Mark,
Those seem like good figures, but did you check the meter on another car? Inaccurate meters are not unknown. The voltage drop of about a volt to the coil seems rather a lot, but it isn't likely to be the cause of your coil problems.
                                                                        Best of luck,
                                                                        Paul

No, I don't know anyone with a GT6 to do a comparison. But I trust my multimeter, had just fitted a new battery to it as well.
I'll get the car out and give her a good kick in the guts and see how the coil goes. I'll move the coil back off the block for starters.

[shedmonkey]

OK here is a daft idea  the feed cable to the coil could be iffy IE as it gets hot its resistance increases thus the voltage to coil whilst running hot drops off? it could be caused by a semi dry joint or similar on the ign switch as the coil feed comes via that ? try running a car like they do up north IE  (a bit of hot-wiring!) by pass the ign  switch run a temp feed to the coil (12v if you are using the 3ohm coil)
Just an idea I could be way off the mark

-but I had a similar issue with a 2000, coil over heating strange voltages etc so rigged up temporary feed to coil and just started it by jumping 12v onto starter solenoid and seemed to cure the problem so replaced ign switch and rewired coil circuit new connectors etc etc

[markcro]

Thanks for the idea ajp. So If I run a temp feed straight from the 12v terminal on the battery it should do the job for this test?
I'm going to order new condensor etc today just to be sure.

[shedmonkey]

yes just run a good clean feed to the coil and see what happens,its worth a punt as  I must admit I don't really know why it should kill so many coils! another idea would be to monitor the voltage at the coil as you go for a drive -have the multimeter on your lap with some long leads ! see what the voltage does as it misfires (might be a bit safer to have  a passenger hold it for you!) -good idea to replace condenser etc gives you a clean sheet to start from so to speak -keep us posted will be very interested in finding out what the fault is

[Pete Lewis]

I would take a reading at the coil engine  off.   dizzy lead off and then check the input voltage . this still has the sounds of being ballasted but
checking with a digital meter with engine running could be misleading the base voltage at the coil
so check it with the dizzy link removed  or check the feed wire with disconnection from the coil ,  then there's no induced or did they call it back emf from outside influences .clouding a simple answer .

Pete