Fiero Proud

General Information

The Fiero speedo is fed by the VSS sensor, which is mounted on the transaxle. This sensor produces an output whenever the rear wheels turn, no matter in which direction (so driving backwards won’t turn back your odometer). This output is a sine wave of 4000 pulses per mile. So, in effect, if you are driving at 54 mph the frequency the VSS generates is 60Hz. You can use this fact to test if your VSS is broken by hooking the VSS connector up to a 60 Hz source. My speedo tester is designed to do this, but a cheap transformer should work as well. Since the AC in the US is 60Hz, this should work fine. But make sure the output voltage of the transformer is not too high!

Details

Anyway. The signal from the VSS is sent to the speedometer board. On this board the signal is buffered (amplified), turned into a square wave and the frequency is divided by two. This buffered signal is fed to the ECM and the cruise control module (87/88 4 cylinder Fieros don’t have a cruise module, instead the ECM does that job as well).

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The information in this article can be used to troubleshoot or repair your tach if you have some knowledge of electronics, but it can also be used to modify or adjust your tach if you are doing an engine conversion or try to put a backlit V6 tach into an L4 or other car. And this modification can be done by anyone who can use a soldering iron. If you find this topic interesting, read on…

General Information

The tach is used to convert the impulses generated by the ignition system into a deflection of a needle on the gauge. The needle itself (like the speedo is driven by an air core meter. The tach I took apart (from an 85GT BTW) used a proprietary GM part (Part #2505 4339) to drive the air core meter from the ignition pulses. As luck would have it, this part is pin to pin compatible with the National Semiconductor LM1819 Air-Core meter driver. This is where things became interesting…

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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.

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I don’t know about you, but every second Fiero owner I come across has cruise control problems. Even with my three Fieros, two had a non-functioning and an intermittent cruise respectively. Reason enough to give the Fiero cruise control system the treatment it deserves and hopefully get you cruising again. Even if your cruise is working fine now, it can start working intermittently in the near future, so it may be useful to remember where this article is.

First, the Fiero used two fundamentally different approaches to the cruise control. The 1984-1986 4 cylinder and all V6 Fieros use a separate cruise control module. It is located behind the carpet on the driver side center console. To access it you need to remove the radio console and flip the carpet aside. Fortunately these units don’t go bad very often. I have never seen one break. But you may need to access the module connector for troubleshooting. The second system involves the 1987/88 4 cylinder Fieros with DIS. On these cars, the ECM takes care of the cruise control functions. The advantage is that you can troubleshoot the system with a scantool. The scantool tells you whether or not brake and other switches are on or off. The downside is that you need access to a scantool and if the problem is in the ECM, replacement is expensive. But again, the ECM hardly ever goes bad, the far more frequent reasons for cruise control problems lie in the mechanical parts of the system. Before we get into those, let’s have a very general look at how the cruise control system works.

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The symptoms for a bad TCC are obvious: If your car comes to a stop and the engine stalls, but restarts (in neutral), and immediately stalls again if you put it in drive, your TCC is shot. You can verify this by disconnecting the plug facing forward on the transmission near the oxygen sensor. If the symptoms go away, your TCC is bad. (If the symptoms stay your TCC may still be bad). This is caused by sediment sticking to to the electromagnetic portion of the solenoid. The part number of a replacement solenoid with an improved seal is P/N 8652379.

Replacing the TCC seems difficult at first, but it can be done with the transmission still in the car. The procedure described is for the V6, but the L4 will be similar. You’ll need an assortment of wrenches and sockets, 10mm, 13mm and 15mm, and a couple of short and long extensions. Do yourself a favor and get a gasket scraper.

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While browsing through the 87 Helms manual, I stumbled across the drawing of the instrument cluster on page 8C-8 (see picture). As you can see, Pontiac obviously planned to use the center right indicator light for a low coolant indicator. On cars without the voltmeter this indicator is used for the battery “idiot light”. But the GT had the voltmeter as standard from 86 on, so this light was no longer needed.

Curiously, the same “Low Coolant” indicator appears even in the 87 Owner’s manual! Check out the pictures on pages 2A-1, 2C-1 (Not in the magnification, but on the smaller view below), 2C-8, etc. It looks like they went ahead quite a bit and cancelled this feature late. Reason enough for me to check into this further.

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Three of our four Fieros have the 4 speed transaxle, and each of them had shifting problems from day one. All standard methods like shifter cable adjustment as described in the shop manuals, clutch or clutch pedal adjustments brought no notable change. On one of the cars it was impossible to shift into first without flashing the backup lights on. Adjustment of the backup switch was not possible, since the only adjustment that kept the backup lights off during shifting kept the backup lights off permanently. The 5-speeds don’t have this problem, since the backup switch is mounted on the transmission on these cars.

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This article is a compilation of some information I have collected on GM’s delay wiper system (as used on the 85-88 Fiero) from various sources. It is by no means complete. I appreciate any and all information you might be able to share about this error prone bugger. Some of this information may or may not be relevant to other GM vehicles, like the Firebird, Camaro, etc. Use your own judgment.

The basics

On the 1984 Fiero, GM had used a separate control module with a gazillion (well, about 8 or 10) control lines to convert a dumb wiper motor into a system with delay. The new system consists of an electronic module located inside the wiper motor housing, the wiper motor and a delay switch located inside the steering column. The delay module is accessible by removing a single screw from the wiper motor housing, then removing the cover.

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