Pontiac's electrical blunders

- or how those electrical gremlins got into our cars -

by Oliver Scholz

Many have blamed Pontiac of making the Fiero a car that is known for its share of electrical problems. So far there has only been empirical evidence for this in the form of unexplicable electrical phenomena.

Over the years I have come to an intimate knowledge of the Fiero's electrical system, and there are at least four separate engineering flaws that can cause all sorts of weird behaviour. Not all of these flaws are present in all years, but I'm sure the symptoms will sound familiar to many of you, and maybe it will help some of you to at least explain your car's strange behavior.

The dancing oil pressure gauge syndrome

This is not strictly an electrical blunder. Actually it is the result of a mechanical design flaw that causes an electrical problem. In short, the location of the oil pressure sensor on vehicles with the V6 and air condition is less than ideal. On those cars, the sensor is mounted vertically on an extension tube. The drain hole on the connector lets water enter the sensor, which lets it rot internally.

The effect is the "dancing oil pressure" needle which many of us experience. If you have an 85, you are lucky, because the oil gauge in the tach has a mechanical damper, which hides the problem from you. If you live in a dry climate, you are in luck too. But if yours has failed, it will fail again unless you cure the root of the problem: seal the vent hole in the oil pressure sensor connector. GM has designed a plug to do this, but a bit of silicone will do just fine. Additionally, a shield (P/N 1009 6127) should be installed and sealed with silicone to protect the sensor from water. On the 1988 V6 GM recommends relocating the Oil Pressure Sensor to just above the A/C compressor by using some adapters (see TSB 88-8-22).

The pegging temperature gauge plague

If you have this problem, I'm sure you know what I'm talking about: the temperature gauge wraps to hot while cranking and gets stuck there. Only hitting the instrument cluster will get it unstuck. Often this is credited to a bad temperature gauge, and a replacement gauge is the only cure. Well, this is not entirely true.

The cause is another engineering bug. You see, the ignition switch has a position without a detent, it's called "bulb test". This position is right before you turn the key to "crank". In this position, several circuits are grounded to light various warning lamps. The reasoning is to check if any important bulbs are burnt out before this can cause any problems. Hence the name "bulb test".

Someone at Pontiac (intentionally or unintentionally) decided to use this circuit to test the temperature gauge instead of the idiot light. Maybe this is because the wiring colors are the same for both the warning lamp and the temp gauge. The result is that the gauge goes to "hot" during cranking. But this doesn't test the gauge's accuracy, and when you drive the car for a few minutes, you'll see the gauge move anyway if it's working. So this test doesn't really prove anything. On the other hand, if the idiot light is burnt out, that you won't notice.

The gauge has a built-in damper that prevents sudden movements, so it also prevents the gauge from wrapping around to the top and getting stuck there. Once the damper fails, you get the problem I just described. The cure is to rewire the circuit to the way it was meant to be. If you do that, the idiot light will light when you crank, the temp gauge will read the current temp, but will not move during cranking, and the gauge will no longer get stuck. It's a simple procedure that's described on the Internet, and will cost you close to nothing.

The phantom courtesy light mystery

Do your courtesy ("floor") lights go on when you turn on the radio? Your radio only works when the dome lights are off? The problem is only a blown fuse, but the cause lies in yet another Pontiac engineering flaw. Back in 84, the cars had a common fuse for the Radio, Cigarette lighter and interior lights. For some reason, the interior lights, trunk release and power door locks got a separate fuse later (which in itself is a good idea). But once again, someone made a mistake, probably on a monday morning, or possibly a friday afternoon.

So the courtesy lights were moved to the "BAT" fuse along with the radio and cigarette lighter, but the remaining interior lights remained on the "CTSY/LID" fuse. All these lamps share a common connection to several switches to ground though. So all these lamps go on if you open a door, or turn on the IP dimmer all the way. The problem is, if one of these circuits blows its fuse and none of the lamp switches are on, the other circuit can draw power through the courtesy lamps. So, if the BAT fuse blows (a dime stuck in the cigarette lighter often does the trick), the radio still gets some power through the lamps. If you turn up the radio, the radio draws more current, and the lamps glow brighter. If you open the door, the radio is shorted, and the lamps go to full brightness.

This can be a problem if you are tracing a current drain in one of these circuits (they're both hot at all times), and unless you know it's just a blown fuse, it can be plain annoying. You can separate these circuits by either opening connector C200 (white and orange wires going into it, near the fusebox), but this only helps for troubleshooting. If you want to permanently cure the problem, cut the orange wire going into splice S210 and move it to splice S125.

The curious brake warning light explanation

The last problem was discovered by Bruce D. Walters. It involves the brake warning light and the trunk release. Pontiac's original intention was to have the trunk release inoperational if your emergency brake was not pulled (this applies to manual cars only). The brake switch provides a ground path for the trunk release relay, and if the switch is open, the relay does not work.

The problem is that the e-brake switch is also used for the brake warning light, and that the trunk release circuit is hot at all times, while the brake warning light circuit isn't. So, if your ignition is off and you have not pulled the e-brake, what happens is this: Current will flow through the primary side of the trunk release relay through the brake warning lamp into the car, even though the ignition is off. This will cause the brake warning lamp to glow and will provide some power to various circuits, which is not a good idea. As a side effect, the trunk release will work with the e-brake OFF if the brake warning light was triggered by the warning switch built into the master cylinder.

The cure would have been a mere diode to separate those two circuits. And the diode is even there, it's just in the wrong location: right before the chime box. The diode doesn't hurt there, but it does no good either. The best cure would be an inline diode between the brake warning lamp and brake warning switch on one side and the e-brake switch and the trunk release relay on the other side. This would allow the e-brake switch to trigger the lamp while still locking the trunk release when not pulled, and still prevent current to flow from one circuit to the other.

Unfortunately, this is a major task, so Bruce has come up with a great idea: he has modified the brake warning lamp socket to include a diode. A simple fix, which does the trick by allowing current to flow only through the lamp in the desired direction, but not from the trunk release relay to the ignition switch.

If you have found any other electrical engineering flaws, I'd be glad to hear them!

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