Electric Temperature Guages

[worzel]

Hi

Any knowledgeable person out there know how to test an electric water temp guage?

For some time now the one in my sprint (ok I know it's not an original fitment) has operated ok but the needle has never made a clean sweep across the dial- it moves in a series of steps. Today however, when I switched on the engine the needle shot round to the hot section. Switching off, the needle returned to the normal cold position. When I actually ran the car the needle was varying from normal temp to boiling.

Is this likely to be the guage, the sender or the voltage stabiliser and what tests might I try?

Regards

John

[paddy]

I think it's not uncommon for the gauge to get a bit stiff and then move in "steps" rather than smoothly.

If the fuel gauge is working OK I'd not suspect the voltage regulator.

Thinking of the possible failure modes of the gauge and the sender unit, I'd suspect the sender unit - the gauge might stop moving completely but I can't think of a failure mode where it would just indicate completely randomly.

If you have a multimeter I'd measure the resistance across the sender unit and see how it varies over time. I don't know if anyone out there knows what it is supposed to be at different temperatures, but you'd at least be able to see erratic behaviour, ie widely differing readings at different times for the same temperature.

If you can't measure it, I'd try replacing the sender - easier and probably also cheaper to try first.

Paddy

[stugilmour]

John, I agree with Paddy above that the sender sounds like the likely culprit rather than the gauge. Pretty cheap replacement part anyway.

I believe the gauge will indicate full sweep to boiling when the sender wire is straight to ground. The most recent failure mode you describe could possibly be caused by intermittent grounding of the sender wire somewhere down it's length, or perhaps a grounding fault within the gauge mechanism or at the sender connection.

You could test the gauge movement by having a helper disconnect the wire at the sender and ground it while you observe the gauge reading. This is how I have bench tested by stock Plus 2 electric temp gauge with the dash removed from the car. My gauge moved in a very smooth (but slow) progressive manner to "full boil". I would have to re-test mine to be sure, but I recall about 30 seconds to get up to an indicated full boil with the sender wire grounded (no sender unit in circuit).

If you see any erratic movement in the gauge during the above test, I think you have effectively eliminated the sender unit as the primary cause.

Perhaps repeat the grounding test using a length of scrap wire from the gauge directly to ground with the stock sender wire disconnected at the gauge. Any erratic behavior would lead one to suspecting the gauge itself, or perhaps the stabilized voltage supply.

The stabilizer should deliver ~10 volts. I understand the stock stabilizer cannot be easily tested with a voltmeter, as the voltage pulses in normal operation. The pulsing voltage supply does not cause a problem in service because of the damping in the gauge movement. If you have already changed to a solid state stabilizer, I believe they can be tested with a digital voltmeter. In any case, a modern replacement is pretty cheap and apparently they are known to fail. However, for the gauge to jump toward full boil the stabilizer would have to be delivering excess voltage; not sure if this is the common failure mode.

As Paddy already mentioned above, if you do not see corresponding erratic behavior in the fuel gauge, I would not suspect the stabilizer initially as the primary cause of your problem. However, I think the gauge would go toward full boil if the gauge is intermittently seeing over 10 volts supply. Perhaps check that your fuel gauge is supplied from the stabilizer as intended; if it is not, stabilizer voltage could be an issue and you are not seeing it on the fuel.

HTH Let us know what you find.

[worzel]

Hi

Thanks for the input- quite enlightening.

Sods law being what it is I checked the circuitry by connecting the feed to the sender to earth and the guage then registered boiling so that seemed ok. Replacing the sender connection I then switched on the ignition and guess what- the guage didn't move (correct as the engine was cold). I used the car and the guage is now operating correctly.

Apparently, according to an article in last month's Practical Classics, if a test lamp is connected to the disconnected sender feed,the ignition is switched on and the indicators operated the lamp should flash. Unfortunately this test won't work on my car because I've wired up the guage with a separate voltage stabiliser.

Maybe the problem was one of those "gremlins". It might have been a duff connection but they looked ok to me and none of the components in the circuit have been touched recently. I'll have to see if the problem returns- intermittent faults eh- don't we just love them!

Thanks for the help anyway.

Regards

John