Temp Gauge Resistor

[Frog2Spider]

My gauge is showing about 30 to 35 degrees warmer than the engine really is. Has a new thermostat (tested to open at 192 degrees) and new fan control temp. sensor (tested to 'close' at 202 degrees).
The system is well 'burped', and the lower radiator hose gets hot.

By the temperature gauge, the thermostat opens at about 210-215 and the fan comes on around 235. Following the wiring semantic there is a resistor in the line between the gauge and the 2 temp. sensors in the engine head.

I have contacted a couple of vendors, and they show no listing for this resistor.

Anyone know where I can get a new one OR what the Ohms of this resistor should be (to cause the gauge to read properly) so I can pick one up at Radio Shack?

Thanks,
Dave

[zachmac]

Realize that the thermostat and radiator temperatures are NOT the same as where you are measuring in the head. You shouldn't be seeing as hot as you are, but the gauge should indicate something greater than the temp of the water passing through the radiator. Otherwise there would be no delta T between the two points and no heat transfer.

[Frog2Spider]

My tests of the thermostat and fan switch were done in a pot of water, on my kitchen stove, not in the car.

Thanks,
Dave.

[mattyd7]

My tests of the thermostat and fan switch were done in a pot of water, on my kitchen stove, not in the car.

Thanks,
Dave.

Zach is saying that the temp gauge in the car is showing the temp from the sensor located in the head. This temperature is different from the temp of the water at the thermostat, and much different from the temp at the bottom of the radiator at the fan switch. Even when thermostat is open you will be running different temps at those three locations.

So, just because temp gauge shows over 200F doesn't necessarily mean its time for the fan to come on, since the hot coolant might not be down at bottom of radiator yet, and such.

Regardless, yeah that is too high at the head so something is likely amiss. Have you used a IR gun to find actual temps at the locations to be sure the gauge isn't telling the true temp at the head?

[Frog2Spider]

The is my logic
1. The thermostat is operating correctly. I know this because:
A. It was tested 'of the car' and found to open at approx. 190 degrees.
B. The bottom hose, off the radiator, gets as hot as the top hoses.
2. The fan switch is operating correctly. I know this because:
A. It tested 'off the car' to close at approx. 205 degrees.
B. It comes on and turns off, keeping the engine from overheating.
This is telling me that the cooling system is all working correctly, and that my gauge is indicating temperatures higher than they actually are.
Whereas the resistor is the 'controller' to what the gauge indicates, it seemed like the reasonable place to start.
The temp. sensor and the gauge itself are the next suspects, however it has been said that some gauges are known to be + - 30 degrees out. This again indicates to me that the resistor is the fault.

AM I MISSING SOMETHING.?

Thanks,
Dave.

[spider2081]

The temperature resistor is in the second temperature sensor in the head. the second sensor is a switch that pegs the needle the later model Spider gauges. In earlier Spiders there is an over temperature warning light.
To see if this second sensor is causing the gauge to read higher than it should. Pull the wire off the second sensor in the head. If the gauge reading reads less the second sensor is usually the problem. The resistor is located in an inline fuse holder behind instrument cluster. I think the color is blue. The purpose of the resistor is to prevent the gauge from burning out if the over temp sensor switch closes.

[Frog2Spider]

Thanks spider 2081
I removed the sensor wire on the front sensor, closest to the radiator. The temp gauge went up as it has, showing a higher than expected reading.
I then re-connected that wire, and disconnected the wire on the sensor closest to the firewall. The gauge then showed no reading.

Does that tell me that the front sensor is bad?

Thanks for your help.
Dave

[zachmac]

The is my logic
2. The fan switch is operating correctly. I know this because:
A. It tested 'off the car' to close at approx. 205 degrees.
B. It comes on and turns off, keeping the engine from overheating.

AM I MISSING SOMETHING.?

Thanks,
Dave.

205 for the radiator fan switch seems high, should be about 195. I thought most were supposed to much lower, less than 200? The resistor in the holder will look like a fuse with a coil of wire wrapped around it. There was a recent post on how to repair them as they are hard to find.

http://www.fiatspider.com/f15/viewtopic ... e+resistor see this link.

[Frog2Spider]

Thanks zachmac!
Thank helps also ...
I now know what I'm looking for. Hopefully I can find, test and repair if necessary.

Dave.

[spider2081]

I think the over temp switch has a gray/black wire on it and the temp sensor for the gauge has a green /white wire on it. The position of the two senders in the head seems to change either from maintenance or from production year. I am not sure which. I would disconnect disconnect the gray/black wire and drive the car and see how the gauge performs. If it operate correctly with the gray/black wire disconnected then yes replace the over temp switch.

[Frog2Spider]

Well this was a learning experience, which is now finally resolved!
To re-cap.
The car never over-heated or even showed any signs of running hot, however, my temperature gauge started showing me temps, well into the 240 degree area before the fan would come on.

So I learned all there was to know about the in-famous 'Burping the Spider'.
Next, I figured the thermostat must not be completely opening. It looked like original, so I installed a new one. This didn't help. The gauge still showed me running around 240 -250 degrees before the fan would come on.
So I replaced the fan switch, but that didn't help either.
Next, I replaced the temperature gauge sensor. No change.

By now I had read a lot of information suggesting that it was not uncommon for these gauges to be 10, 20 or even 30 degrees 'off '. So I figured that I had a bad gauge, and started to look for a replacement.

I had looked at the wiring schematic to determine the 'indicating' temp. sender, and remembered that the wire coming from the sending unit to the gauge went through 2 different junction connectors.
Could the wiring be the problem?
I ran a direct wire to the gauge, from the sender, and wooppee! The gauge now reads as it should. At 190 the thermostat opens, and at about 205, the fan comes on.

I have no idea what could have caused this issue, but by-passing the connectors between the sender and the gauge, with a direct line, solved the problem. I realize the sensor that 'pegs' the gauge is now in-operable, but I don't see that as a problem.

If others have a problem with a gauge that is inaccurate, you might try this simple fix.

Dave.