Having my E fan running and light on w/engine off makes my temp gauge go up ?????????

[THUNDERZ]

So my car had been sitting for about an hour after taking a test drive to see if the new temp sensor would fix my high temp readings(which it seemed to for a little while and then it seemed to go back to reading hot), and I turn the key on and my cooling fan goes on because I have the fan thermostat turned all the way down to where I think it clicks on at 95 degrees, and I watch the temp gauge come up to halfway ....and I thought WHAT!!! theres no way it could still be that hot! and then for shits I turn on the headlights ( because I somehow thought that the draw on the electrical system was tied into it reading high) and the temp goes up almost to 3/4....so I double checked and I could make the temp go up and down with the lights....Anyone ever heard of something like this happening? I just replaced my temp sensor thinking that it was the problem(running hot), but noticing this, there seems to be other demons at work. Any ideas besides the obvious that it might be a short somewhere?

[mark]

Is this with a cold car or has the car been driven and is up to temp?

[THUNDERZ]

Is this with a cold car or has the car been driven and is up to temp?

 

 

Its with my car that was up to temp an hour before hand, but had been sitting since then.

[mark]

heat soak

[THUNDERZ]

heat soak

 

? Not sure what you mean.

[mark]

Car is hot, cut it off, the parts that were hottest heat the cooler parts. Your temp sender that was cooled by circulating water cooled from the radiator is on longer being cooled. Parts like the exhaust manuifold that are very hot act as a heat source. Your temp gauge goes up.

[strotter]

...so I double checked and I could make the temp go up and down with the lights...

 

This is clearly an electrical malfunction of some kind. Don't thermo-sensors (thermistors) work by increasing resistance with temperature? So there's supposed to be less voltage getting to the gauge with higher readings. Your headlights come on, the gauge sees less voltage - or less voltage difference. I don't know a great deal about electricity, but I would suspect that there's either:

 

1) a short between the gauge circuit and the headlight circuit, reducing the voltage on the hot side of the circuit when the headlights come on (that is the temp gauge circuit is driving the headlights), *or*

 

2) the headlight circuit is changing the ground the gauge sees when there's a load on it, that is the ground side of the circuit could be "pulling up". You could be grounding the gauge through a circuit that is going hot when the headlights come on.

 

No matter the problem, you're going to have to identify the bad leg of the circuit. Try this: unplug/disconnect the old wires in the existing circuit, both at the gauge and the thermistor. Get out a few feet of wire and build a new circuit for the gauge from scratch "in parallel" with the existing wiring. See if it works by dropping the thermistor in a pot of hot (boiling hot if possible) water. Headlights on, headlights off. If it works correctly, you've eliminate the thermistor and gauge as problems. Then, reconnect one of the original wires at a time, redo the test for each one. There are only a few wires involved, it shouldn't take too much time (other than the "digging around in the dash" time!) to figure out where the problem is.

[THUNDERZ]

This is clearly an electrical malfunction of some kind. Don't thermo-sensors (thermistors) work by increasing resistance with temperature? So there's supposed to be less voltage getting to the gauge with higher readings. Your headlights come on, the gauge sees less voltage - or less voltage difference. I don't know a great deal about electricity, but I would suspect that there's either:

 

1) a short between the gauge circuit and the headlight circuit, reducing the voltage on the hot side of the circuit when the headlights come on (that is the temp gauge circuit is driving the headlights), *or*

 

2) the headlight circuit is changing the ground the gauge sees when there's a load on it, that is the ground side of the circuit could be "pulling up". You could be grounding the gauge through a circuit that is going hot when the headlights come on.

 

No matter the problem, you're going to have to identify the bad leg of the circuit. Try this: unplug/disconnect the old wires in the existing circuit, both at the gauge and the thermistor. Get out a few feet of wire and build a new circuit for the gauge from scratch "in parallel" with the existing wiring. See if it works by dropping the thermistor in a pot of hot (boiling hot if possible) water. Headlights on, headlights off. If it works correctly, you've eliminate the thermistor and gauge as problems. Then, reconnect one of the original wires at a time, redo the test for each one. There are only a few wires involved, it shouldn't take too much time (other than the "digging around in the dash" time!) to figure out where the problem is.

 

 

I agree, but think I am going to run a entirely seperate circuit with a new cheap gauge and if needed a GM sending unit.

[lbhsbZ]

When you are running a car and coolant is circulating...the heads and block are always hotter than the coolant circulating through it. When you shut it down, the coolant will gain up to 40°. In an hour, this should cool off and be lower. If you are reading 3/4 on the gauge and you can put your hand on the engine...something is wired wrong.

[Pop N Wood]

Sounds like a voltage regulator issue

 

http://www.jrdemers.com/280ZX/overheat/overbase.html

[THUNDERZ]

Sounds like a voltage regulator issue

 

http://www.jrdemers.com/280ZX/overheat/overbase.html

 

Wow....great info! Thanks. It does sound like the gauge is bad, because I did measure the resistance at the sensor and it was reading around 170 degrees ( according to the chart that your link went to) when the gauge said it was 200+

[baddriver]

Is the car running or off at this point? If the car is off, it could be as simple as the reference voltage for the sensor is changing.

 

When the car is running, you can count on about 14 volts from the alternator at all times, but when you are running on battery, you get around 12 volts, and if you have a load (like an electric fan and a set of headlights) you can get anywhere from 8-11 volts.

 

It's true that the sensor changes resistance when the temperature changes, but it's the current, not the resistance, that makes the needle on the guage move. For a fixed resistance, if you change V in ohm's law, you get a change in I. The guage is calibrated for a specific voltage. If your guage reads normally when the engine is running, and only shows this behavior when the engine is off, it's probably fine. If the lights change the tempurature reading when you are running the engine, then you need to look for a voltage regulator problem or a bad ground.

 

For reference, every time I've ever found an electrical problem causing absolutely weird, impossible nutty things, it's been due to a bad ground.

[BillZ260]

Where is the sensor mounted? What motor is in your Z, V8?

[MrFancypants]

Wire your gauge directly off your battery and do the same test. That will rule out the car wiring and should be a relatively easy test.

 

 

 

- Greg -

[THUNDERZ]

Where is the sensor mounted? What motor is in your Z, V8?

 

 

The sensor is under the driverside exhaust manifold...the engine is an sbc 350.

[THUNDERZ]

Wow....great info! Thanks. It does sound like the gauge is bad, because I did measure the resistance at the sensor and it was reading around 170 degrees ( according to the chart that your link went to) when the gauge said it was 200+

 

 

The sensor resistance and the digital thermometer keep agreeing.

[deja]

The sensor is under the driverside exhaust manifold...the engine is an sbc 350.

 

Typically the temp sending unit is located on the intake manifold next to the thermostat housing on an old school SBC. I think you'd get a more accurate reading at the intake manifold. The fitting on the block is usually used for a thermal switch, like to enable the cooling fans.

[Pop N Wood]

Sounds like you need a new 5 volt reference source. From reading that link I would replace the guage in the dash.

[THUNDERZ]

Typically the temp sending unit is located on the intake manifold next to the thermostat housing on an old school SBC. I think you'd get a more accurate reading at the intake manifold. The fitting on the block is usually used for a thermal switch, like to enable the cooling fans.

 

 

 

I was thinking the same thing, cause it gets pretty darn hot under the exhaust manifold.