I've decided to try fixing three instrument panel displays on my 1989 Ranger -- the temperature gauge, the oil-pressure gauge and the fuel gauge. As a start, I verified that the temperature gauge in the dashboard works, by simulating the operation of the sensor with resistors. Then I replaced the temperature sensor, but the new sensor produces too much resistance -- not what apparently is the required range of 47.5 ohms when cold down to 9.7 ohms when hot. To proceed, I need to find out what the manufacturer's spec for the new sensor is -- not just "fits your vehicle."

The next mystery is voltage: I tested the leads that go to the temperature sensor and the oil-pressure sender (which is a switch rather than a sensor) by connecting a multimeter between those connectors and ground. The leads show only about 6 volts when the engine is running and slightly less when the ignition is on but the engine is not running. Why? Shouldn't they be about 12 volts? I have not yet found a way to test the fuel-gauge lead in the same way, but I suspect that it also would have low voltage.

If the voltage to the temperature sensor and oil-pressure switch should be about 12, what would cause it to be less? The alternator seems to be working, and the battery is good.

 

Welcome to the forum


You should have 12volts on temp and oil sender wires
Test battery voltage first then test wire to Battery Negative(key on) then to engine as ground
Its possible Ford is dividing the voltage at the gauges but never seen that
If you Ground the Sender wire in engine bay with key on, temp gauge should go to HOT, oil gauge would go to "normal" range

Fuel gauge uses pulsed 5volt from a voltage regulator behind instrument cluster
Anti-slosh module in the back of cluster can cause fuel gauge to stop working

Temp gauge used these OHMS, "C" 75, "H" 10, center line 25(200degF)

Temp sender can be 200+ OHMS cold, doesn't matter, but should be about 75-80 OHMs when engine temp is above 100degF, 130degF = "C", just below 1/2 is about 30-35 OHMs and 185degF

Boiling water is 212degF so you can test sender that way, OHMs should be about 20

The temp SENSOR uses 5volts, this is only used by fuel injection computer

 

Thanks, RonD.

Any idea why I'd have that low voltage (~6V) on the feeds to the temp and oil senders?

 

Ford could be dividing the voltage at the gauges, basically it means there is another resistor in the gauge pulling voltage down to make the Sender's resistance more accurate

I would do the test of Grounding the Sender's wire at the engine and see where gauge needle ends up, if it goes to "H" or higher then voltage is not the issue