Ignition cuts out

[Jimbeaux]

Hey Z folks, I'm having an electrical problem with my '72 240. Here's a brief history of the car, the problem, and the attempted troubleshooting.

The car: 1972 240, all stock except aftermarket AC, exhaust, radio. I'm second owner and added the air and radio upon purchase 7 years ago. AC professionally installed by custom shop that specializes in AC add-ons to classics, street rods, etc. Radio by me, very clean, careful install. Car has been fine for 6 years with all items working well until 4 months ago.

The symptoms: Cars runs GREAT until temperature reaches approximately 180 degs. At this point cars starts skipping, stumbling and the tach needle wavers +/- 1000 RPM. A few more degrees up to operating temperature and the tach goes wild, 2000 RPM swings, complete drop to nothing, but comes right back. Car dies, then runs, then dies, then runs. Idling at operating temperature it acts like this, 900 RPMS fine for 5 seconds, tach drops, car practically stalls 2 seconds, then back to 900 RPM for 5 seconds, repeat. It is very regular and cyclic. The only thing I've found is if I turn the fan switch on it dies immediately (only when at full operating temparature).

The troubleshooting steps: 1.) Complete tune, plugs, points, condensor, rotor, cap, wires, no change (didn't expect it). 2.) Cleaned all electrical connectors I could get my hands on with contact cleaner and small file. Cleaned grounds, alternator, voltage regulator, coil, ballast, ignition switch etc., no change. 3.) Changed to HEI per Kyle Hagemann's GM conversion write up only used Accel instead of MSD, the only changes are a noticeable improvement in power and response UNTIL AT OPERATING TEMPERATURE then problem persists. 4. ) Replaced Voltage regulator, same problem, Replaced Ignition switch, same problem, replaced Temp sending unit same problem.

This problem is driving me nuts. I'm 100% sure it's electrical. Fuse box is in good shape, everything electrical works on the car. Could it be something to do with a relay for the electric cooling fan installed?

Is there something I'm over looking? Any help would be greatly appreciated.

Thanks in advance for any suggestions.

Jim B.

Houston, TX

[2ManyZs]

First thing I would chek would be how the electric fan is wired. It isn't stock and it could be wired whereby it is taking its power form somewhere it shouldn't.

Since you say you have changed to the HEI it is possible that the fan could be drawing away from the ingition, any electronic ignition needs a consistent voltage to run properly. That's why a car with a weak alternator will run farther on a set of points than it will on electronic.

Another possibility would be a faulty ICU in the HEI itself. It should be a small square under the plate in the distributor. It should have a couple wires running to it. Not sure exactly what it looks like in the HEI but they are all similar. When one side of the ICU burns out it will try to run off the other side of it and overheat, bringing about similar problems as the ones you described.

Another possibility is an overheating coil, you didn't state as to whether or not this has been replaced or not. If not, try a different one just to make sure if you do not find any other problems.

There still is the possibilty of vapor lock with the symptoms you described. So if you find no electrical problems this might be your next step. Try blocking off the water to the intake, (the line that comes around the back of the head) and see if this helps. It is a slight possibilty since you say the symptoms are dependant on engine temp. It could be a faulty water control valve.

[Jimbeaux]

Wow, thanks for the quick response. To help clarify this is a link to the mod I did to the ignition http://www.sonic.net/~kyle/ go to Z tech page then GM HEI RETROFIT. I replaced the distributor with one I rebuilt (new pick up coil, good advance plate, working vacuum pull, new rotor, cap), new Accel coil and new GM ignition module. The real kicker is the problem existed before the HEI retrofit.

Your point about where the aftermarket fan is pulling power from is interesting. It's worked for 6 years but could something be "getting tired" that all of a sudden it's drawing too much? Can the alternator affect the ignition system?

As far as vaporlock, I'm familar with those symptoms and this is without a doubt electrical. Picture this, I start the car cold and just sitting in the driveway it runs great I keep my foot on the throttle to maintain about 1500 RPM. The tach stays perfectly steady, then the temp reaches about 1/8 of the way across the gauge (150 degs? or so?) and the tach needle starts wavering ever so slightly. The temp rises a little more (1/4 of the way 170?) and the needle starts bouncing up and down 500 RPM or so, the gauge reachs 1/3 of the way across (180 degs? this is where it always runs unless in traffic in July) the tach imeediately drops to zero, then back to 1500, then back to zero and back to 1500. This is immediate, not a slow drop I mean 1500 then BOOM zero, then BOOM 1500 again. It really acts like a bad pickup coil or module but the problem existed with the new points and condensor prior to this conversion. I think that's what baffles me the most.

I wish I could post a mpeg so you could see and hear the problem. HHmmmm.....

Thanks again,

Jim

[2ManyZs]

Hmm, is right, this is an odd problem to say the least. Well, I guess we can rule out the ignition control module.

Now what is going through my mind is a short of some type either in the tach itself, or in the wiring off the coil. Seems very odd the tach acts that way just when the engine develops a problem. I would suggest a thorough check of all the wiring form the coil to the tach and perhaps even trying it with the tach unhooked. I've never heard of one having this problem before but I guess there is always one to be the first.

Might want to double check the charging system just to be sure there isn't more than one thing that is contributing to the problem.

I'm not as good on electrical problems as some of the other members, so somewhere we'll find the answer. :stupid:

After reading his instructions on the site I'm wondering about another thing, is it possible your coil is one that is designed for the ballast resistor? If you have taken out the ballast resistor and used a coil that is designed for one that will foul the coil and cause it to burn out. I'm not sure what you have done so I'm just asking to get all the answers to questions that are running around in my semi-empty skull. Another question that may not be anything, but where did you connect the tach lead to? It should run off the + side of the coil if my memory is correct. If it is anywhere else this could be where the problem lies. It should have it own separate wire and not be piggy-backed to anything.

[Jimbeaux]

I'm going to triple check everything again. Your point on the type of coil might help. The instructions I used for the HEI retrofit say use a MSD Blaster II and does not work with all coils. I'm using a Accel Super Stock which I believe to be the equivalent 12v coil, I certainly could be wrong. (12v coil no ballast used)

That lingering issue though....the problem existed before the retrofit. Oh well as you said maybe some more insight will follow, thanks for your help.

It's Sunday night Father's Day, I'm closing the hood and turning off the PC then firing up the grill and cracking a cold brew!

[Jimbeaux]

Well it finally appears to be fixed. I worked on it Monday and it still had the same electrical problems described above. So I thought I'd try another new coil only I went with the MSD Blaster II instead of the Accel Super Stock. Ta da! It runs great.

As the problem existed before I upgraded the ignition in hindsight my thoughts are that my original coil was bad and the new Accel is either bad or doesn't work with the GM HEI retrofit.

Thanks for help and I hope this may help someone else someday.

[2ManyZs]

Ya gotta like a problem that is easily fixed and not too complicated. Turn a couple screws and unhook a couple wires and its done. Glad it turned out the way it did. It could have been a "head-scratcher" that took a while.