Burning coils

[cudaman]

While waiting for the tow truck, I thought I would make use of this time by asking why the heck my sbc '69 corvette wants to burn up HEI coils within a couple of minutes after turning on the headlights? I can drive for miles and miles over many different days, but twice now it has smoked coils both times I've used the headlights. Any thoughts? Anyone wanna buy a corvette?

[Mingus]

That's a strange one. I assume you have driven it with the lights on before these last two times with no problems?
If it has just started out of the blue my first thought is that it is some kind of ground issue. The ground wire might have come off on the headlight loom. This would cause them to pull ground from (or through) whatever they can. If the HEI has a weak ground, it will burn out the coil. The HEI (if it's a coil-in-cap model) gets it's ground from the distributor base through the harness plug to the ground strap below the coil. Make sure the ground strap is there under the coil and the connector on the wire from the distributor base fits tightly there and in the base. I think there is also a ground wire on the coil itself that attaches to the top of the coil yoke and the strap attaches to the bottom of it.
I believe 'Vettes have a ground circuit that runs through them because of the fiberglass body. You should make sure the engine block and frame have a ground between them and the headlight looms are grounded. A bad or missing ground will do all kinds of funky things. My wifes' LeMans had a bad ground on the driver side headlight loom a few weeks ago. It was still attached, just not making good contact with the core support. With the low beams on, the light was really dim, and the high beam lit up slightly. Then when you put on the left turn signal the light would go full brightness as the blinker pulsed. Cleaned and reattached the ground and it was fine.
Since it is a '69 with an HEI it has been added on. It's possible the positive side of the circuit could be wired with the headlights but I think that's unlikely. Either the coil would have power all the time or your headlights would only function with the key on.
My best guess would be that it is grounding related.
Mike

[cudaman]

Thanks for the reply. As with many electrical problems, the ground is the first suspect. I thought I had it grounded pretty well after installing this latest coil, which lastest for less than 30 miles. Everything was good until the headlights went on. This could be a coincidence, I suppose, but the likelihood of two coils frying within a mile of turning on the headlights seems pretty low without some causation owed to the headlights, or at least the load created by the headlights. I guess it's hard to say whether it's something in the light circuit itself or if nothing else used in the car up to this point has created that kind of load (which could have caused the same issue, perhaps).

I have not had a chance to look at this latest coil, yet, because I has to go to work. However, the last one had a fried positive wire. I thought maybe the insulation had just worn enough to allow the wire to contact the grounded coil frame because that's where it was burned. However, that seems unlikely with a brand new coil.

I'll advise of my findings later.

[MrQuick]

lets back up a bit.


before this happened the first time....was there any work done to the vehicle? under dash, under hood or near the front bumper?

Knowing that a 69 Vette never came with HEI, how was the wiring hooked up? Was the B+ ignition wire jumped or properly replaced in the harness plug? Proper replacment of the resistor wire? Deletion of the "R" bypass wire? Right wires replaced?

vince

[cudaman]

I have not had a spare moment to look at the car. I just recently bought the car and have done nothing to it, other than replacing the intake and carb. I suppose I could have done something during that operation to precipitate this, but I don't know what that could be. No wiring was disturbed that I can recall.

I can tell you that the car is nearly original, based on appearance and information passed on from previous owner. The car was in his family for 25 years and in his hands for the last 17 of those, and he never turned a wrench on it. He drove the car sparingly, so it's possible that he never drove the car with the headlights on. Unlikely, but possible, I suppose.

I have absolutely no reason to believe that he withheld knowledge of any problems, either.

If there is any hacked wiring, it's been there for many, many years. So, either a problem has developed over time and has just now presented, or I unwittingly caused this when replacing the induction. The car was virtually inoperative before the induction replacement, so I never drove it for any distance and certainly never had the lights on while the car ran, so I have no datum to compare to.

I can tell you that no ballast is used, as it gets 12v in "on" and "run". There is no tach wire hooked up, just a single 12v source.

Curiously, I noticed that when I replaced the first burned coil, there was no ground strap from the coil frame (whatever that square surround is called) to the open space in the middle of the three-prong connector that goes back the the dist. body. There was just the black ground wire from the coil to the coil frame (again for lack of a better word). The new coil came with the black wire that I screwed to the frame, and also the previously mentioned missing ground strap that I've seen in all HEI's I ever dealt with. I installed this strap and its attendant blade connection, which now allowed the ground to go into the three-prong connector and back to the dist. body. So, if the blade connector was never in there for the first coil, how was it being grounded? And if there was a faulty ground caused by that condition, why would it still do the same thing after a (hopefully) good ground was established?

I'll get you more information over the weekend, I hope. Thanks.

[MrQuick]

Since you had a burnt coil bat+ wire I would say you have a short to ground issue when the headlights come on. Lights go on and doors pop open.
I'd like to see where that bat+ ignition wire that feeds the coil comes from.


vince

[cudaman]

I've only had about five minutes to look at this coil, but I pulled it out and did a cursory reistance check and found about 3 ohms between the tach and B+ leads. If memory serves, I thought it was suppose to be between 0 and 1 ohm, preferably closer to 0. Nothing visibly burnt this time.

Anyone ever heard of tapping a screw into the dist body and running a ground wire directly to the battery? Just a thought. I still need to check the ground wire from the engine first.

[Mingus]

Grounding the distributor body to the battery definitely wouldn't hurt. You can never have too good of a ground connection.

[Mingus]

I'd also trace the B+ and headlight wiring since you didn't do any of the wiring and the HEI was added on. No telling what they did. I've seen some really scary wiring over the years.

[Mingus]

One thing I've started doing to any older vehicle I have is to wire the headlights through a relay driectly off of the battery. That way all of the amps for the lights aren't going directly through the switch. It is only used to trigger the relay. Usually makes the lights brighter too. All newer cars do this. I had a switch smoke (literally) on me a few years back and have done this since.
Since your problem seems headlight related (both coils smoked immediately after turning the lights on) this might solve the problem. If there is some kind of weird loop going on between the coil and light circuit it would remove the load off of the switch.

[cudaman]

Can modules somehow cause this, also?

[EFI69Cam]

Since V=IR it could be that when headlights go on voltage drops and current goes up. Check alternator and headlight wiring condition.
The module could be part of this equation as well.

[MrQuick]

Since V=IR it could be that when headlights go on voltage drops and current goes up. Check alternator and headlight wiring condition.
The module could be part of this equation as well.

see what your voltage goes to with full load.

vince

[cudaman]

I just got a quick minute to check a few things. The car was not running, of course, but the supply wire for the coil shows 12.5v with the key on and lights off. Turn the lights on and it drops to 11.5v. That doesn't look unusual to me and shouldn't cause a problem. I checked the resistance from the distributor to the frame (grounded by engine-frame strap) and it shows between 0 and 1 ohm. I'm not sure if a module can cause this issue, or if the local NAPA can test it, but that's my next move. They're about $25 and non-returnable, so I don't want to buy one without knowing if has any relationship to the problem.

[cudaman]

Also, the resistance check from dist. to frame shows ~27 ohms with the lights on and key off. It shows near 0 with no lights. Is this a problem?

[EFI69Cam]

I just got a quick minute to check a few things. The car was not running, of course, but the supply wire for the coil shows 12.5v with the key on and lights off. Turn the lights on and it drops to 11.5v. That doesn't look unusual to me and shouldn't cause a problem. I checked the resistance from the distributor to the frame (grounded by engine-frame strap) and it shows between 0 and 1 ohm. I'm not sure if a module can cause this issue, or if the local NAPA can test it, but that's my next move. They're about $25 and non-returnable, so I don't want to buy one without knowing if has any relationship to the problem.

How does that 11.5v compare to the voltage at the battery with the lights on?

[Mingus]

cudaman wrote: "Also, the resistance check from dist. to frame shows ~27 ohms with the lights on and key off. It shows near 0 with no lights. Is this a problem?"

I think that is a problem. Maybe the problem. It should still be close to zero with the lights on or off. If the resistance is jumping up, it would draw more amps to make the coil work, and probably be what is causing you're coils to burn out. I'm not sure why it would be jumping with the lights on. Have you checked the ground wires for the light harness?

[dhutton]

It sounds to me like a poor ground from the engine block to the frame. You are reading a high reisistance because there is a voltage drop across the connection from the engine to the frame when the headlights are drawing current. It also sounds like the battery is not well connected to the frame. Measure the voltage from the block to the frame with and without the headlights on. I think you will see a voltage and it will increase when you turn on the headlights.

Also make sure the body is well grounded to the engine and frame.

[cudaman]

When I measure the resistance from the block to the frame, it is about 0. Is that not good enough?

Dhutton, are you saying to put my voltmeter leads on the block and the frame and then turn on the lights? I would be very surprised to see a reading.

Thanks, guys.

[Mingus]

I think he is saying to put the + lead on the + battery and then check the voltage with the - lead on the frame and then on the block.
On steel bodied GM cars the ground straps always went from the firewall to the frame and were usually bare braided cables. They should go to the frame on a 'Vette since grounding to the firewall wouldn't do any good. The battery is behind the seats on your car right? If so the ground strap is even more important because the battery isn't grounded directly to the block like most cars. Even if they are there, you should scrape any paint off where they mount and put some dielectric grease on the connection. That was the problem with the lights on my wifes' LeMans. The wire was grounded, but was not making good contact. Grounding the distributor body with a wire like you mentioned before is also a good idea. Otherwise it is grounding through the hold down bracket and bolt. If they are painted you can end up with the same problem.