Temp Gauge Not Working?

[otive84]

The engine temp gauge does not work.

I replaced the sending unit and tested it for the correct OHM resistance, which looked promising.

Tested wires from the sending unit to gauge, and good connection.

Had gauge serviced and working correctly.

There are 12 volts at the gauge whenever the key is on.

Is this correct, as the Ohm resistance should decrease the voltage depending upon temp?

An additional green wire is connected to the gauge, and I assume it goes to ECM.

Also, I obtained a used ECM and installed it, which made no difference.

Does anyone have an idea of what the problem may be?

 

[86 Silverbird]

What engine do you have?

Is it a rally set of gages or a digital electronics instrument panel?

When you replaced the sender, did you use teflon tape?

If you did, the sender won't likely work.

Remove the tape and try it again.

This is a diagram of the 1985 rally gage cluster showing the temperature gage.

 

[otive84]

Thank you for the suggestion.

The engine is the 305.

The gauges are the original from 1984 and are not digital.

It does not have fuel injection and uses a THI Rochester carburetor.

 

[86 Silverbird]

Temperature gage operation and testing.

How it works.

Coolant Temperature Gage

The Coolant Temperature Gage is operated by two coils. Battery voltage is applied to both coils. The cold coil is grounded directly, and Hot coil is grounded through the Coolant Temperature Sender. This Hot temperature sender path has 54 ohms resistance at 260 F, 130 C (hot coolant) and the temperature sender resistance becomes greater at lower temperatures. It is approximately 1284 ohms at 100 F (40.6 C). This causes the current through the temperature sender in one coil to vary as the coolant temperature changes, and this moves the gage pointer.

Tests and checks.

COOLANT TEMPERATURE GAGE ALWAYS INDICATES COLD

Disconnect the Coolant Temperature Sender connector and jumper the DK GRN (35) wire to ground. Put the Ignition Switch in RUN. If the Coolant Temperature Gage reads hot, replace the Coolant Temperature Sender.

If the Coolant Temperature Gage does not read hot, check DK GRN (35) wire for an open. Replace the Coolant Temperature Gage if the wire is good.


COOLANT TEMPERATURE GAGE ALWAYS INDICATES HOT

Disconnect the Coolant Temperature Sender connector and place Ignition Switch in RUN.

If Coolant Temperature Gage reads cold, replace the Coolant Temperature Sender.

If the Coolant Temperature Gage does not read cold check DK GRN (35) wires and Ignition Switch, terminal 61 of connector Cl for a short to ground (see schematic). Replace the Coolant Temperature Gage if all are good.
The schematic above is correct for a 1985 model car and I think it should be accurate for your 1984.

 

[86 Silverbird]

A little more information.

Testing the ECT (Engine Coolant Temperature) Sensors and Connections

​

ECT Temperature vs. Resistance Values

​

ºC​	ºF​	Ohms​
100​	212​	177​
90​	194​	241​
80​	176​	332​
70​	158​	467​
60​	140​	667​
50​	122​	973​
45​	113​	1188​
40​	104​	1459​
35​	95​	1802​
30​	86​	2238​
25​	77​	2796​
20​	68​	3520​
15​	59​	4450​
10​	50​	5670​
5​	41​	7280​
0​	32​	9420​
-5​	23​	12300​
-10​	14​	16180​
-15​	5​	21450​
-20​	-4​	28680​
-30​	-22​	52700​
-40​	-40​	100700​

Use a Digital Volt Meter (DVM) set to ohms to measure resistance. It is also a good idea to get a " reference" from the meter you are working with. With the DVM on the ohms scale, touch the two meter leads together and note the ohm reading. It may not always be perfectly zero, but may be within a tenth or two. Now when you take an ohm reading, you will know what the meter will show when there is really no resistance.

The sensor in the cylinder head has only one terminal. This sensor is for the temperature indicator on the dashboard. Place one test lead on the sensor terminal and the other on a known good ground. Compare the reading to the table. If your car is cold from sitting overnight, the reading should be close to ambient temperature. (When reading resistance, it does not matter which lead goes to which terminal)

The sensor at the water pump has two terminals. This sensor is for the temperature input to the PCM and has nothing to do with the gage in the dashboard.

If the sensor seems to be ok, you may also need to test at the harness connector for proper lead conditions. Use your test meter set on the dc voltage scale to do this. You will need the key in the RUN position, but don't have to start the car.

For the one lead connector at the head, place the red test lead on the connector terminal and the black test-lead to a known good ground. With the key ON, you should read battery voltage (+12vdc or close to it). You can also ground the lead and see if the gauge in the car deflects, to full hot.

If you get no voltage, switch the meter to ohms to see if the lead is grounded. No voltage or no ground mean that the lead is open.

If the gauge is at full hot all the time, the lead is grounded back toward the gauge. It could be possible for the lead to be pinched and grounded toward the gauge and broken and open back toward the sensor (like in the case of the wire getting caught somewhere during some major engine work). Physically tracing the wire from the sensor into the harness should locate the problem.

You can test the black connector lead by using the ohms scale on the DVM. Place the DVM black lead to ground. Place the DVM red lead to the black lead of the connector. If the lead is ok, you will get an ohm reading close to zero. If you get no reading or a very high one, the lead is open or partially open.

 

[86 Silverbird]

I was just doing a little reading and came across another possibility.
It appears some 305s have a single wire sensor (should have a dark green wire) on the drivers side head, which is just for your temp gauge and nothing else.
They also have a single wire sensor, in the passenger side head (dark green with white stripe wire), that controls the secondary radiator fan, which is also controlled by the acc switch.
When you checked out your temperature sender did you check the correct one, on the drivers side head?