Coolant Temperature Gauge Sits on Zero

[DeanG]

The coolant temperature gauge sits on zero. Before it died completely it would sometimes sit at zero, sometimes show the normal operating temperature (i.e., 1/4 up).

I replaced the thermostat and sensor. Voltage at the sensor measured at 9.6V which is a bit low given the 10 +- 0.1 V ideal.

What should be my plan of attack?

[Rusty Wishbone]

I'll be interested to see the replies on this one. I bought an Elan a few weeks' ago, and in that time, the temperature gauge has always pointed to zero.

One post I read (I can't remember where) suggests that this is most likely a problem with the gauge rather than with the sender unit.

[maz_r]

If you have access to a multimeter with a resistance setting (and I'm guessing you do, given the 9.6v reference ), then you could measure the resistance across the terminals of the gauge; I can't remember off the top of my head what it should read, but I think it is around 50 ohms. You'd need to pull the instrument cluster to get access, sadly, so not a quick job.

Similarly, you could disconnect the wire and then measure the resistance of the temp. sender (between the sensor connector and the engine block), which should vary with temperature between about 200 ohms when cold through to 35 ohms when the engine is very hot.

Given that you are seeing 9v at the sensor, I'd rule out a broken wire somewhere in the harness.

One thing you might try, but only very briefly is to short the sensor wire onto the engine block whilst someone watches the gauge to see if it twitches at all (in an ideal world you'd use a high wattage 40ohm resistor, rather than a direct short, so this is at your own risk!)

Finally, with the gauge removed from the car (which means pulling the instrument cluster ), you could feed a variable voltage to the gauge directly to see if the needle moves as the voltage is varied between 0v and 9v (or a 9v battery if you don't have access to a PSU, but don't expect the battery to last long!)

All this might point you at the thing to replace/refit...

Good luck

[HJ2]

There is more than one temp sensor.
One for the gauge and one for the ECU. Did you replace the correct one?

[mmatthej1]

and the answer is................. change the therostat!

My car had a zero temperature reading, until I popped into Halfords for a replacement thermostat, fitted it, then - voila - problem solved!

[HJ2]

Haha!

[Brit-Car-Nut]

Dean:

With the car cold and ignition turned on but not running, the gauge should register at the bottom of the scale.

Disconnect the lead at the temp sensor and connect a standard 12V test light between the wire and a good ground (earth).

The light should glow dimly and the gauge should move. It won't go to full scale because the light adds some resistance.

If you get glow but no change in the gauge, the gauge is probably bad.

The temp sensor gets it's ground from the engine block which gets it's ground from the braid between the chassis front crossmember to the cylinder head. The cross member gets it's ground throught the contact and bolts with the longerons which is connected to the ground wire from the battery.

If you don't get glow, then the gauge could be bad or the voltage regulator could be bad OR your ground (earth) might be poor.

Choose a different grounding point before testing anything else. If necessary, make a long wire that you can clip to the negative post on the battery and reaches the engine bay. That will be a trusted ground connection for any electrical tests you need to perform.

The voltage regulator also provides power to the fuel level gauge, so if the fuel gauge is working, the voltage regulator is probably good so you should check the connections at the gauge.

If the test light glows and the gauge goes up, then the sensor is suspect, but the ground to the sensor might be bad too. This could be the connection between the engine and the chassis. See viewtopic.php?f=37&t=21968#p276875 for the grounding points and braids.

Brass sensors need a good, glean grounding bond to the housing they are being threaded into. Sensors should NEVER be sealed with teflon tape or thread dope as it will prevent a good ground.

Sensors with tapered threads are tapered so the more you tighten, the tighter the seal between the sender and the housing thus preventing leaks.

Non-tapered sensors use a crush washer to keep things from leaking

On other earlier British cars that used Smiths gauges, the test light would actually blink as they used a bi-metal strip to open and close the connection (like an old mechanical directional flasher) when the power exceeded the 8 to 9 volt requirement.

Modern cars use transistor type voltage regulators that are usually very accurate but can't handle any kind of serious load as they overheat and fail. The voltage regulator used in the M100 is a standard electronic chip that can be purchased from any electronic shop or seller. Radio Shack probably has them too.

[Brit-Car-Nut]

and the answer is................. change the therostat!

My car had a zero temperature reading, until I popped into Halfords for a replacement thermostat, fitted it, then - voila - problem solved!

Actually, even if the thermostat was stuck open, the engine would have gained some heat after running for a while, and the gauge should have moved upward. It is also possible you improved the ground between the thermostat housing and the cylinder head when you cleaned up the old gasket and mating surfaces.

[DeanG]

Good advice. I will try the suggestions on Sunday (Saturday is to be devoted to the Seven) and post the results.

I gave the poor old M100 a wash today as it had gathered too much grunge sitting in the garage for so long. It is good to be able to drive it after so long being laid up. It ran almost flawlessly for the our 2011 coast to coast trip. Right after we got back home it has been one problem after another. Still not bad for almost 105,000 miles.

[dunnlor]

Dean,

I had an issue where the fan switch worked inconsistently due to the housing that the fan switch screwed into not being well grounded to the engine block. The housing was insulated from the block by a gasket and was grounded only by the two mounting bolts. With locktite or other materials on the bolts, the housing was not always a good ground.

I added a supplemental ground from the housing up to the intake plenum. Haven't had a problem since. I suggest examining the thermostat housing to see if its well grounded.

viewtopic.php?f=46&t=18336

Tom

[DeanG]

I seem to have a bit bigger problem (this seems to be a recurring theme with my car). I was getting it all clean - clayed - waxed. Shut the door. The signal lights started to blink. I got in and car wouldn't turn over. Playing around it happened several times. Other times the second attempt at starting the car was successful. I removed the alarm computer. Still the same issue.

[Brit-Car-Nut]

Removing the alarm computer will require reconnecting the starter and ignition feeds which the alarm system would inhibit if it thought the car was being stolen.

The blinking lights suggest it is an alarm issue, but you will need to connect the ignition switch to the circuits the alarm would disable before expecting the car to start.

You might consider disconnecting the battery, reinstall the alarm computer and reconnect the battery. Maybe that will "reboot" the alarm computer...