The Fiero Ignition System

by Oliver Scholz

The following is a description of the Fiero's ignition system. It applies mainly to the V6's, but also to the 1984-1986 4 cylinder engines. The 87/88 4 cylinder engines are too different, but 87/88 L4 owners may still want to read on, since they'll see how their engine is superior to the prior years. I'll describe the basic operation, problem areas, and how to fix them.

Let me warn you that this may get a little technical...

Everything revolves around the crankshaft (literally). This is the base for all timing. When the crankshaft is a few degrees before Top Dead Center (TDC) on the compression stroke, you want the spark to ignite the air fuel mixture, at the precise timing as determined by the ECM. To achieve this, the timing chain connects the camshaft to the crankshaft. Now, the timing chain streches over time, resulting in a timing that can shift with engine load changes. The valve timing changes too, but a few degrees change in the valve timing are not as critical as a timing change in the ignition system. This is one area where timing gears are better: they don't stretch. 88's don't have the problem at all, since the DIS module picks up its timing directly via a pickup on the crank. There is no stretching or wear. If your t-chain is stretched, well, this is not an easy job and is best left to the professional. But let's continue.

The camshaft drives the distributor shaft. This is the next area where problems can arise. If the distributor shaft gear wears out there is a bit of play between the distributor shaft gear and the corresponding gear on the camshaft. So, if the engine load changes, the timing can jump a bit (depending on the wear). This can confuse the ECM and can lead to the dreaded "my idle jumps up and down".

Now, provided the shaft is rotating with the camshaft, there are 6 (or 4 on the L4) little magnetic "tangs" that rotate around the pickup coil. Each time the tangs of the magnetic pole piece pass by the tangs of the pickup coil, current is induced in the pickup coil. Now, if these tangs are bent, the timing is again incorrect. So make sure these are all in the proper position and don't stick! I've seen engines where mechanics have somehow bent those tangs that the pole piece touches the tangs while rotating...

If the mechanic really used too much force, he can also get the pole piece loose, so that it can be rotated without the shaft moving!

Anyway, this signal is picked up by the ignition module. That is, if the pickup coil is working correctly. I'd recommend changing the pickup coil every two years or so. If it looks rotten, it probably is. It's also cheap to replace, and since you have to remove the distributor, this is an excellent chance to reset the timing, clean everything up, clean the gunk off the shaft (there is usually quite a bit of gunked up oil on the shaft). You should also use this opportunity to remove any play in the shaft by inserting the proper washers.

There was an article on rebuilding the distributor in a recent FOCOA magazine. If the contacts are bad, the coil has internal shorts, if the wires are corroded, the signal coming from the pickup coil will be weak and this can lead to a variety of hard to diagnose engine problems. In short: replace it unless it's new. It's cheap.

Now to the ignition module. This is the prime suspect in Fiero ignition problems. Don't leave home without one. I'm serious. I can name a dozen people that have been stranded because this bugger suddenly quit working. Remember: The distributor (especially on the V6) is subjected to a lot of heat, and this tends to wear out electrical things quickly. Also, stay away from aftermarket modules. Granted, they are cheaper, but you get what you pay for. My aftermarket module lasted a whopping 3 months before quitting. And guess when those buggers quit? Not while you are on your driveway, that's for sure! Don't forget to apply the heat dissipative grease that comes with the module! If you do, run to your dealer and order another one, since you are going to need it.

Applying that grease is vital on the Fiero's engine. Now, the module has two connectors, with 6 contacts total. One of them is 12V. If there is a bad connection on this line, you get all kinds of weird problems. Engine runs, engine stops, engine misses, etc. Once the ECM has 12V, ground and a proper signal from the pickup coil, it will drive the ignition coil. This will generate spark. These two signals are on the 2 way connector on the distributor module.

Next culprit is the ignition coil. It can have shorts and opens, the procedure for checking them is simple and explained in the Helms, Haynes and Chilton manuals. If you get a new one, get one of the performance coils. The stock coil is okay, but a bit on the weak side at higher RPMs. An often overlooked point is the ignition filter. It is next to the distributor and feeds the ignition signal to the tach. If you disconnect it you won't get a tach reading, but it may cure your problems. Simple to check, simple to fix.

The secondary side of the ignition coil goes to the distributor again, this time the mechanical part on top. There can be a variety of things wrong with it, ranging from cracked to worn parts. Unless it's new, get a new cap and rotor, they're inexpensive.

Needless to say, your ignition wires and plugs need to be in good condition. You can measure the wires (see Helms, Haynes, etc.) and take a look at the plugs, and regap them.

Again, simple and described in the manuals.

Back to the part I left out. The ECM connection on the distributor module. The distributor sends a reference signal to the ECM. The ECM reads this signal ("Ref") and determines the current RPM from it (it can be verified with a Scantool and is independent from the tach reading on the instrument panel!). Below a certain RPM (as during cranking) the ECM stays out of the ignition module and lets the module do its thing. During this time the ignition module grounds the EST line. If it sees a voltage there if it doesn't expect it to, it will set a code 42.

Once the RPM rises high enough, the ECM sends a signal to the module informing it that it wishes to "take over". This is done by the "bypass" signal. It switches the ignition module from internal timing to ECM-controlled timing. With ECM-controlled timing, the ECM waits for a reference pulse, delays it and then sends a signal on the "EST" line to the ignition module to fire the spark. Clear as mud?

Again, if the RPM is too low, the distributor module works on its own and generates its own timing. No advance. Once the ECM takes over, it delays the signal it receives from the module and sends it back.

What's the fourth input for? Well, it is a ground connection that makes sure that the ECM and the distributor module have the same, good ground.

So, there are plenty of things that can go wrong with the Fiero ignition system, but the ECM is rarely one of them, and it's expensive to replace. Mechanics are afraid of electronics, so you often get the "if the engine isn't running right, replace the ECM"-attitude. If that's not it, well we don't know (but made a lot of money by replacing that ECM).

Troubleshooting the Fiero ignition system can be frustrating, but most of the things I described can be done by the shadetree mechanic.

You need a minimum of tools, including a timing light, and if you troubleshoot systematically, you will usually find the problem and even if you replace all the parts, it will be cheaper than what the shop will charge you for replacing the ECM...

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