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Thanks to Wayne Williams for forwarding this tip from
Paul McKibben / fierosails.com / Norcross, GA

TEMPERATURE GAUGE REPAIR

TEMPERATURE GAUGE - STOP IT FROM PEGGING ON START

The Fiero temperature gauge is infamous for "pegging" (violently moving to its maximum range) when the engine is started. This sudden acceleration of the gauge needle can be more than just annoying; over time it can loosen the needle such that it becomes inaccurate or falls off. Mine fell off.

Why does the gauge "peg"? The factory wiring applies a full 12 volts across the temperature gauge when the ignition key is turned to "Start". This momentary connection is called the "Bulb Test" position and shows at a glance that critical warning lights are functional. Unfortunately it appears GM made a mistake and connected the Bulb Test feature to the Temperature Gauge instead of the Temperature Warning light.

The fix is simple and requires no wire cutting, splicing, or soldering. Just swap two wires on the sensor and two wires at the instrument cluster. That's it! The result is the Bulb Test is applied to the Temperature Warning light and the Temperature Gauge works normally.

The temperature sensor has a two-wire electrical connector. One of the wires feeds the temperature gauge and the other feeds the high-temperature warning light. They are very easy to swap. The sensor for the V-6 is on the cylinder head near cylinder #5 just below the ignition coil. The sensor on the 4-cylinder engine sticks straight up on the head next to the coolant filler neck.

Here is a picture of the V-6 sensor:

Pull the plastic connector housing from the sensor (you must depress the locking tab), remove the wires from the connector, then swap their locations when re-inserting them. The tangs on the wire connectors (inside the plastic housing) MUST be depressed prior to pulling the wires from the connector housing. I use a small jeweler's screwdriver to do this. The tangs should be pulled up again prior to re-inserting them into the plastic connector or else the wires may pull out over time. Details of the wire connectors shown below.

Here is a picture of the connector housing:

Here is a picture of the wire connectors which slide inside the above connector housing:

Wiring at the instrument cluster requires swapping wires at locations #11 on the left side of the instrument cluster and #13 on the right side of the instrument cluster. The instrument cover must be removed to do this and if you can't get enough slack in the wires it may be necessary to remove (or lift) the dash. (You could also splice additional length to the wires.) Removal of the instrument cover requires pulling five screws on the top and two underneath the cover. The wire in location #11 is on the left side of the instrument cluster behind the speedometer in '88 Fieros. (The wire on mine was light green.) It may be elsewhere in other years but is still #11. Look for a large bundle of wires and then look for the identification number on the connector.

At least in '88 Fieros the wire in location #13 is on the right side of the instrument cluster behind the tachometer in the large bundle of wires. (The wire on mine was dark green.) It may be located elsewhere in other years but it is still #13. Look for the identification number on the connector. Unplug #11 and #13 (depress the tang on the end of the wire connector while pulling gently) and get enough slack in each wire so they can be swapped. The wrapping tape on each wire bundle may have to be removed. I replaced my tape with nylon wire-ties. When enough slack is available, plug the wire from #11 into #13 and vice-versa. You may wish to re-route the wires for neatness. Test the revision by starting the engine. If all is well, re-assemble. If not, re-read these instructions carefully and check your work.

A diagram of the factory wiring (from the '88 manual) and the revised wiring are shown below. (The diagram in the Haynes manual which presumably covers all years is the same except for wire colors.) The wire colors below are for the '88 Fiero. I believe wire colors vary from year to year and between engines; I have not researched the differences. The swap is simple and wire colors are not needed to complete it.

"Ralpy" from Pennock's Fiero Forum has provided the following information for the 1984 Fiero:

"I looked up info on '84s in the factory service manual. Here's what it shows:"

"From gauge thru ignition switch to sending unit the wire is Dark Green. The wire occupies position 11 in connector 205 which is located behind left hand side of instrument cluster."

"For the Warning Indicator Light (both temp and oil pressure) the wire is Tan all the way from light to temperature/oil pressure switches. The wire occupies position 6 in connector 206 which plugs into the cluster on the right hand side."

Thanks Ralpy!

The plastic connector housing at the temperature sensor or the plastic head on the sensor itself may be broken due to long-term heat exposure. If you want to replace them, the connector housing is AC Delco #PT110 available from NAPA for about $18. They are available in the chain auto parts stores too as CAR-PAK #92-7098. The sensor is part number 25036809 for about $21 (plus shipping) from www.gmpartsdirect.com. Non-GM (AC Delco) sensors are about $10 in parts stores. You can also find both these parts in salvage yards. They are most plentiful on Tech 4 engines although not all Tech 4s had this particular sensor/connector. If you replace the sensor, note that it relies on electrical contact with the engine. Therefore do not use any insulating material on the threads such as Teflon tape or pipe dope. There are compounds which will seal and conduct electricity, but you probably can get by without it. The sensor threads are tapered and the body is soft brass so clean threads and a good snug fit should be all you need. The sensor requires a 21 mm deep socket to remove although an adjustable wrench may work on the 4-cylinder engine if you have enough room to swing it. There is definitely not enough room on the V-6.

I generally have a few AC Delco sensors and connector housings which are pulls from wrecked cars. They are $6 for the sensor and $4 for the connector housing, shipping included (within the US). The sensors have been tested and the connector housings are in good condition (no junk). Contact me via fierosails@yahoo.com

ZEROING THE GAUGE

If your temperature gauge is pointing in some odd direction or seems erratic, you can test it easily if you have the appropriate resistors. I'm not sure but believe you can still get resistors at Radio Shack. Resistors come in standard values and I don't believe you can get them in the two values mentioned below. Try 47 Ohms and 1400 Ohms to get you in the general vicinity of the numbers below.

The position of the needle on the temperature gauge shows the resistance of the element in the temperature sensor. In other words, the gauge is an Ohmmeter. The gauge is supposed to read 100 degrees F when the sensor resistance is 1365 Ohms and 260 degrees F when the sensor resistance is 55 Ohms. To test the gauge for accuracy, pull the plug at the sensor, stick one end of a resistor in the plug and ground the other end of the resistor to the car. Start the engine and check the gauge reading. (You may be able to do this with the ignition "On" and not start the engine; I haven't checked. And if you don't get any readings, try the other location on the connector.) Assuming the test resistor is near one of the values mentioned above, your gauge should be in the general vicinity of either 100 or 260 degrees F. If not, the needle has probably shifted.

To zero the needle on my gauge I simply flicked it with a finger until it was accurate. But if the gauge wiring has not been modified it will continue to "peg" on start and will quickly become inaccurate again.

If zeroing the needle will not result in accurate readings with good test resistors then it's probably time to replace the gauge.

TESTING THE SENSOR

There are two functions in the sensor, the gauge resistor and the high temperature switch. The gauge function can be checked by putting an ohmmeter across the appropriate pin and the body of the sensor, then changing the temperature of the end of the sensor. A candle or lighter should be enough to determine if the sensor is functioning. If you want to verify the accuracy of the sensor, you will need an accurate thermometer and a stable heat source. One possibility is to use a candy thermometer and a pot of oil on a stove. Attach the wiring to the sensor with clips or a spare connector. When the oil is 100 degrees F (sensor immersed in it) the resistance should be 1365 Ohms. When the oil is 260 degrees F the sensor should be 55 Ohms. If your sensor misses these values by a wide margin (say 25%) you may want to replace it.

The temperature switch should close at 257 degrees F and open as the temperature falls back to 230 degrees. You can test this in the hot oil bath as mentioned above, first changing the ohmmeter to the other connector on the sensor. The oil will be hot to do this test and with wires hanging on the sensor it will be easy to have an accident. Be very careful!