Servicing GM Direct Ignition System

Warning: DIS coils generate considerably more power than many stock distributor based ignition systems. DIS ignition can kill you. Do not touch DIS components or sparkplug wires with the engine running.

The DIS Brick is far more reliable than distributor-based ignition however there are 2 costs to this reliability. The first is that the parts are expensive and the second is that it appears more complicated than it is which scares allot of people off.

Scaring people off is likely a good thing in light of the first cost. DIS parts add up really fast. The PIM alone is $120+ if you fry it. Then add coils at $30-50 each. If you also manage to damage the ECM then add another couple hundred dollars. As you can see the last thing you want to do is replace DIS parts unless they are truly bad.

The truth is that DIS is very simple. The Fiero DIS "brick" consists of 5 parts. 2 coils, the PIM(Primary Ignition Module), the mounting plate, and the Crank Position Sensor which is hidden under the mounting plate. There are no adjustments at all as the PIM and the ECM control the whole system. Parts in DIS systems mostly work or they don’t.

How DIS works. In very short form.

The only moving part in DIS is the crankshaft notch wheel. The crank position sensor generates pulses as the cuts in the notch wheel pass by it.

Spark timing is controlled by the ECM and PIM based on pulses from the crank position sensor and a number of other factors. (Proper operation of the MAP sensor is critical. The ECM uses the MAP sensor to calculate Vacuum Advance for the spark amongst other things.)

Coil selection is done by the PIM based on the input from the crank position sensor. That I can tell the ECM has no clue which coil the PIM is firing.

Spark plugs fire in pairs. One plug of the pair is in the cylinder under compression and the other is in a spent cylinder under exhaust.

Disassembly

The brick is held to the block by 3 bolts. 2 bolts are 10mm hex heads; the third is a 13mm hex w/ stud. The hex w/ stud at first glance looks like a nut on a stud but it is not.

The coils and PIM are held to the mounting plate by 4 5/16(they are really metric but 5/16 6 point sockets fit just fine) nuts on carriage bolts. NOTE: The coils can be removed from the brick on the car but there is a trick to it.

The Crank sensor is on the back of the mounting plate and it held by 2 hex head screws. Carefully inspect the sensor’s O-ring for damage as soon as you pull the brick. The sensor hole in the block tends to have rust that will damage the O-ring. If there is oil above the O-ring when you pull the sensor then at the very least the O-ring is bad.

Service & assembly

If the crank position sensor shows any damage to the face check the inside of the engine. Nothing should ever contact the face of the sensor. If the O ring on the sensor has leaked replace it. This sensor is about 20$us. I really recommend replacing it if the thing it old enough for the O-ring to go bad. When the sensor is replaced you MUST use dielectric grease on the terminals. Pack the grease into the terminals and leave a small blob on the tip of the terminals. This grease MUST be checked and replaced as needed anytime the sensor is separated from the PIM.

The PIM (Primary Ignition Module) and mounting plate must be cleaned and heat sink compound applied. FAILURE TO DO THIS EVERYTIME THE PIM IS SEPERATED FROM THE MOUNTING PLATE CAN DESTROY THE PIM. THE PIM IS OVER 120$US TO REPLACE. Heat sink compound is available at Radio Shack and most other electronic suppliers. If you get the little tube from Radio Shack use the whole tube. Make a good gob in the center of the PIM and let it spread itself when the PIM and coils are tightened. The mounting plate does not have to shine but it must be free of all dirt, scale, or corrosion in the PIM and Crank sensor mounting areas.

While not required I strongly recommend packing the coil primary terminals with dielectric grease. These are the 2 small holes in the bottom of the coils. These are supposedly weather packed terminals but the extra protection is a really good idea. Also pack the secondary posts where the terminals exit the towers after the coils are tightened.

Tighten the coils and PIM in a star pattern. If you tighten unevenly or over tighten the nuts you could damage the PIM. Go slow to allow the heat sink compound to spread.

Before attaching the brick to the block make sure the crank position sensor hole is clean. It tends to have rust around it that will damage the sensor’s O-ring. Oil the O-ring before pushing the sensor in the block. The 3 mounting bolts should be put back where they came from. WARNING! The TOP bolts penetrate the crankcase and MUST be wrapped with Teflon thread sealing tape prior to installation or you WILL have an oil leak.

Before connecting the brick to the wiring harness pack the terminals on the harness with a small amount of dielectric. Again this is insurance. The Fiero DIS brick is mounted low. It isn’t hard to swamp it in a big rain. Swamping the brick can ruin both the PIM and the ECM. ((Oddly as I was preparing this document, TNN’s Shade Tree Mechanic aired a show about a swamped GM that did exactly this. Too weird.))

Servicing coils and PIM without removing brick from car.

It’s generally easier to work on the coils and PIM with the brick removed from the car but that creates other problems. The first is that you disturb the crank sensor, which can damage it or its seal. Second is that you unseal the mounting bolts. Both problems can result in oil leaks.

The coils and PIM can be removed with the brick in place but you need to make a small tool to hold the carriage bolt heads. The tool is a 6-inch or longer piece of 1/2 to 3/4 inch wide flat stock. This needs to be pretty stiff and not more than about 1/8 to 1/4 inch thick. It doesn’t matter what kind of metal is used. Brass aluminum or steel are all ok. To make the tool bend about 1 1/4 inches of metal 90 degrees. You’ll use the bent part to hold the bolts.

To remove a coil, pull up on it as you alternately loosen both nuts. This will keep the bolt heads seated in the mounting plate. Note: If you are only replacing the coils do one coil at a time. This saves you having to reapply heat sink compound to the PIM. ((If the PIM hasn’t been off in a long time it doesn’t hurt to check it out and clean any crud from around it.))

Once the coils are off check over the PIM as above. Clean the mounting plate as needed, etc, before reseating the PIM. Don’t forget to always apply fresh heat sink and dielectric compounds every time the PIM is off.

Only the top left coil bolt is close enough to the block to stay in place by itself. Pull up the other bolts one at a time and reach under the plate with the tool to hold the heads in place. The top right bolt is easiest to reach from the right side of the plate. The lower ones are easiest to reach from the bottom of the plate.

Tighten all 4 coil nuts until they are just touching the coils. Final tighten the nuts slowly in a star pattern. You want to go slow to let the heat sink compound spread. The stuff is thick and spreads slowly. Go around the coil nuts a couple times to make sure they are tight. (They can loosen as the compound spreads after the first pass.)

Notes.

Allowing the heat sink compound to spread itself is important. As the compound spreads it drives out air from between the parts. If we were to try and spread the compound by hand we’d end up with air trapped between the parts. Air doesn’t transfer heat as well as the compound so the less air in there the better.

Unlike HEI the DIS PIM is rather unlikely to die. If you think the PIM is dead always try replacing the crank sensor. The crank sensor is $20 the PIM is $120+. If you do replace a PIM you might as well replace the crank sensor at the same time. ((GM says the crank sensor passing a resistance test of doesn’t mean its good. So if you think it’s bad just replace it. Almost no-one has the machine to test them fully.))

The main thing to fail in DIS is the coils and that’s often because a sparkplug or wire is bad. A damaged plug or wire can fry DIS coils by creating increased current demand on the coils. This increased demand increases heat in the coils. The coils should be replaced in pairs. When coils are replaced you should also replace sparkplugs and wires.

The only test I’ve found for the coils in GM documents is secondary resistance. (Resistance between towers.) The resistance should be between 5000 and 7000 Ohms. Problem is that even if the coil secondary resistance is ok the coils can still be bad. Mainly you could have a bad primary winding or the insulation could be failing neither of which you can easily test for sure. (Failed insulation doesn’t always result in spark leakage that can be seen.) My own testing of the Primary shows about 1 Ohm. At such a low resistance it’s hard to tell if it’s got a problem other than an open circuit. (The only true test of insulation resistance requires a special machine called a Megger and few people own one.)

DIS uses a 0.060 inch plug gap. Make sure all plugs are as close to this gap as possible. Mismatched plug gaps will unbalance the DIS coils and hurt performance. Unbalanced plug pairs are a good source of rough idle problems and miss. NEVER over gap sparkplugs in DIS systems to prevent fouling. You may prevent fouling but you’ll cook the coils.

Stay with standard sparkplugs. Some brands and types of “improved” plugs cause problems with DIS. DIS ignition is already much stronger than traditional distributor ignitions. The benefit of specialty sparkplugs in DIS is minimal if noticeable at all.

When investigating possible ignition problems always check the MAP sensor. If the MAP sensor or its vacuum line has a problem it can affect ignition timing. Always keep the MAP vacuum line as short as possible to reduce MAP response time.

The location of the DIS brick on the “Iron Duke” makes it run hot. This has been known to cause problems when aftermarket coils are installed. Aftermarket coils usually draw more current from the PIM resulting in even more heat. This can be enough to push the PIM to failure. The PIM usually will start working correctly when it cools down but permanent damage can result. Some people have not had this problem, others have. It may depend on the exact PIM. There’s no way to predict if yours will have problems or not. You’ll know if you have this problem when about 8-10 miles after installing the coils the car dies.