jpc647

Hello All. I'm relatively new around here, I have a 1972 Oldsmobile Cutlass S. It has a rocket 350, a th350 transmission, and electronic distributor in the car out of a newer Oldsmobile 350.

I was driving the car home today and it just quit. Like I turned the key off, but it would not start again. I took the air filter off, hit the throttle linkage a bunch, the car is getting fuel. I checked for spark, nothing. There is juice going into the distributor from the wire running from the fuse block, but nothing coming out. I checked this with a test light while waiting for the tow truck. It again has an electronic ignition, with the "coil" on top of the distributor. What should I check first? Is there a way to test the distributor? Or the coil? How do I start?

olds425

It could be a bad coil, a bad ignition module or the pickup coil in the distributor. Most auto parts stores can test the ignition module for you. The pickup coil plugs into the ignition module and can be tested with an ohmmeter. Unplug it from the module and test across the two leads, it should be 500-1500 ohms. Then test from each leg to ground and that should read infinite, if it fails any of these replace it. It requires distributor disassembly to replace it. I think the coil can be tested but do not know the procedure. Good luck!

jpc647

Well the problem is intermittent. If I let the car sit overnight or something it will fire right up. I've never heard of a coil on a GM doing this. It'll run for a few minutes and then quit. Anyone got any suggestions?

lshlsh2

My guess is your chasing a short.
Larry

jpc647

Any idea what kind of a short would cause this? Where should I start to look? Are there testing procedures for the coil, and module, etc? Where can these be found?

MDchanic

Make sure the two wires that go from the module inside the distributor are not crushed or shorting to ground against the edge of the vacuum advance or other nearby objects.

- Eric

lshlsh2

When the car stops working. Is it high humidity, like rain or mositure in the air? If so your wires just could be old and starting to wear out. How long have the wires been on the car?

oldcutlass

That is a common problem. Do you have a friend with a spare distributor. Or work your way thru each by trial and error.

MDchanic

Or just sub in a new module and / or coil - both easy to do, and it shouldn't be hard to find a buddy with used spares lying around the shop.

- Eric

jpc647

Well its a newer style distributor, like out of a 73-7x. I don't know. I don't know how to tell either.

jpc647

I don't know anyone around with a spare distributor, not one like this, therefore there are no extra coils or module's laying around.

How can I identify what year the distributor is out of? Where are the stamped codes?

MDchanic

Year doesn't matter.

If it's a non-computer HEI distributor, all the electrical parts should interchange.

If you want to try to ID the distributor, there are numbers listed in Section 2.361 of the Olds Parts Manual.

- Eric

rocketraider

Go ahead and change out the module. Since it runs when it's cold I expect the module is breaking down after it gets hot or all the insulating grease has dried out. Coil will do this too.

The only outright coil failure I ever had was an ACCEL that was doing this exact same thing- the car ran fine until everything got warmed up and then it would quit and have to sit till it was cold again.

When I finally figured it out (after spending considerable time on the phone with Mallory tech support, thinking the Unilite module had failed and ultimately replacing that only to have the same scenario all over again) I took a 4-pound hammer and mauled that coil. When I finished, it was near-bout flat.

gregplus40

The module could be failing from heat , as suggested above. If you can, get a hold of a can of freeze spray and shoot the module when it fails. Some times this will duplicate the cool down, and get it to operate. If so, it is almost surely the module. I had one do this once on an old Pontiac 6000. Got a module from Pull-A-Part the next day, went back to the road side where I had left it the day before and got it going. Be sure ,as suggested above also, that the dielectric grease is covering the entire plate, as this is how heat is transferred.

jpc647

Alright. Thanks. I took a shot in the dark and bought a new module. The car seems to stay running now. I let it idle in the driveway for about twenty minutes and took it up the street, everything seems to be okay. Should I coat the new device is the grease too?

Now my question is why? What would have caused this to fail. I know it's probably old and all, but are there possible causes that can be tested that would cause this to happen?

MDchanic

They just go. Not very often, but when they do, they do.
Heat (or poor heat sinking) will accelerate their demise, but sometimes they just go.


I assume you used a heat-transfer paste between the module and the mounting surface. You do not coat the unit with the paste. The paste is to facilitate heat transfer from the module to the distributor base, which acts as a heat sink. It would do no good to coat the rest of the module with the paste.

A previous poster mentioned "dialectric grease."
This is a grease that is used in spark plug boots to help keep moisture out, and to make them easier to remove. It is not the same as heat transfer paste, and would probably interfere with the transfer of heat out of the module.

- Eric

gregplus40

JPC647,
Sorry to mislead you, but by dialectric I meant that the properties of the paste or grease is electrically non-conductive, not the generic name given to the grease they put in auto light bulb sockets and such for moisture reasons. Some thermal greases are semi-conductive, and the use of a thin mica wafer would then be sandwiched as an insulator, not usually in automobiles except in some electronics such as amplifier outputs and such. The place you bought the module would or should know the difference and supply you with the correct material. As for as failure, some new modules fail due to imperfections in the silicone substrate, and some are forced into failing or being intermittent by the expansion and contraction of heating and cooling during operation and being off . Sorry for the confusion. Glad you seem to have made the problem change and or go away.

gregplus40

JPC647,
By the way, the layer of grease is used to prevent small air pockets from being trapped between the module and the heat sink, and as a transfer agent. More grease is not better. Too much grease making a thicker layer can actually have the opposite effect by insulating the modules surface from the heat sink, and causing the heat to remain with the module. A nice, covering, thin layer is optimal. Sometimes take a clean module, apply some grease, and spread it around either using the heat sink and itself as plates, or with a wooden stick (popsicle?) like peanut butter. You've seen the guys on TV use that clear stuff on cardiac resuscitators just before yelling "clear." Same process, only they need good electrical conduction to the skin.

BILL DEMMER

actually, the layer of thermal joint compound is to allow for as much thermal transfer as possible. the mating surfaces of the module and distributor are not totally flat and therefore leave gaps where heat can build up, thermal joint compound fills the gaps. you only use a thin, even layer. yes, it does eventually dry out, but, this doesn't materially change the performance of the compound as long as the joint is not separated. both surfaces must be totally clean before application of thermal joint compound.

as mentioned above, dielectric grease is not the same thing and should never be substituted for thermal joint compound, and vice-versa.


bill

Lady72nRob71

Sounds like one of the parts fail when heated - common electrical problem.
My old Ford had this issue early this year. It would run great cold, but die when temp got to 150 or so. When it cooled below 110, it started again.
It was the electrical pickup coil inside the disty. When it got hot, it's electrical output decreased and the ignition module could not detect the pulse. I verified this at home and installed another reman disty - ran fine after.
The previous disty was a reman unit that I installed 20 years ago to try to solve a noise issue (stupid on my part).

The wiring in a coil is insulated with a thin coating of dipped 'insulation' and heat can break this down and the thermal expansion can short out part of the coil, causing a failure.
My lawnmower had this issue twice in 12 years. My neighbor's mower had this issue once in 8 years.
I think it is a poor manufacturing process in the coil's wire insulation.
I have many other coils in other much older equipment that have never failed.

Run to Rund

Back in 1974, an Olds engineer sent me two HEI's to "test" and I had one quit on me while driving. They crimped the fine copper leads inside the cap, and one broke. After soldering it, all was good. So, you might check for something as mundane as that--an intermittent connection. Also be sure the power wire to the distributor is 12V all the time, don't use the resistor wire for the points ignition coil for "run".