Testing Ignition Coil (1984 GMC 1500 Sierra Classic)

Before you do that, check the rotor. It may be punctured and arcing. If it's bad, check the plug wires...

Al

Al,

Thanks for the response. Will the rotor show any physical signs of problems? Also, if it turns out to be bad, what's the best way to test the plug wires?

-- Christian

unless you have some fancy equipment, I look under the hood when it is dark outside. Engine running. Sometimes you will see what looks like fire flies jumping all over the place. Or just one wire is bad, like the coil wire.

He went ahead and replaced the coil and rotor. He's pretty much just throwing money at it, hoping something will work. Neither of those things did, unfortunately.

The coil on this one is integrated and doesn't have a wire that feeds the diistrbutor. I'm going to have him check now the wires that feed the coil to see if they have power.

-- Chrisitan

Future reference. The rotor can be visually inspected after removal, turn it over and look at the bottom side center, will look charred. Look for signs of "red dust" in the cap and on the distributor, This is caused by the spark shooting down through the rotor and arching off things, like the advance weights around their pivot pins..If there is a lot of it in there, the ignition module maybe damaged as well, and even the pick up coil for that matter. Don't buy cheap caps and rotors for GM. Stick with AC Delco, Standard Ignition, the Blue Streak line, (excuse me Standard Ignition is now Standard Motor Company) not the regular stuff, and of all places AutoZone carries a high end line that holds up pretty good., Gold, part number is DR2001G, sold as set only. .NAPA has two lines, red by Echlin, and blue by standard. Echlin stay away from on GM, fine on Ford or Chrysler.

Whitelightning

Thanks, Whitelightning.

Ignition control module is something I hadn't considiered. Can this be tested with a multimeter? If so, what numbers should I be looking for?

Also, when testing current going into the coil, is it ~12V? Is it constant or intermittent?

Any other thoughts on places to look would be greatly appreciated.

-- Christian

Just get yourself a Haynes P# 24064 manual. It has all the tests,illustrations,instructions for replacement etc. I don't see how you could fix this truck without the manual,except by continuing to throw $ at it and a lot of trial and error attempting to replace items without instructions.

Or take it down to your parts store and they should have a machine to test the modules.

Jape

A few things...

First, you have what's called a "High Energy Ignition", most commonly referred to as the "HEI".

There should be one wire coming in on the driver's side of the distributor cap (2 wires if the truck has a tach). That's the 12vdc "hot" wire that supplies power to the distributor. If you pull that wire off, and turn the ignition to the "ON" position, you should get

12+ volts when testing the wire and ground.

If the ignition module was cooked to begin with, I doubt any further damage was done while replacing the coil. But make sure that the little metal strap (sometimes a very short wire) is connected from the frame of the coil, and goes to what I believe (IIRC) the center leg of the 3-pin connector that plugs into the module and capacitor. Without that wire there, you'll cook every new ignition module immediately, having never gotten the truck started.

I'd go ahead and look at the ignition module, and count the pins on it. I don't know which year they went from the older 4-pin module to the newer 5-pin module. The 5-pin version is what's on my '86 Chevy. The

5-pin version uses a very basic computer connected to the distributor and a knock sensor, and it's sole purpose is to interupt (delay) timing. It has been known to be problematic, to the point that GM issued a service bulletin on modifying the system to render it functionally the same as the older 4-pin version. I don't know to what degree the computer can fail, or what the symptoms are, but it's worth investigating. Bypassing it is as easy as jumpering two wires together behind the glovebox. Search this group for my name as well as "ESC" (electronic spark control) if you'd like to know more.

Provided that the wire supplying voltage is giving you 12v+ when the key is "on", I'd say you're pretty much down to the ignition module, or the little capacitor. If you end up replacing the module, get some heat-sink grease, sometimes mistakenly referred to as dielectric grease (something entirely different). The stuff you want is a thick, white paste and the bottom of the ignition module needs a coat of the stuff before you reinstall it back in the distributor.

Hope this helps, and as always, please report back regardless of whether you fix it or not!

~jp

CMM wrote:

I want to thank EVERYONE who helped me out with this. Your advice was the key to solving this puzzle.

We hooked a light up to each side of the wires going into the coil. With the other end of the test light grounded, the bulb lit up for each side-- neither would flash with the engine turning over. NAPA store tested the ignition control module and it was faulty. Put a new one in and it's running like before.

-- Christian