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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
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.
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.
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.
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.
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:"
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."
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."
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.
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 firstname.lastname@example.org
ZEROING THE GAUGE
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.
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.
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
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
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