Temp Guage Question C2 (67)

[Glenn B.]

I just had my cluster restored. It's back in and everything now works as expected - except for my temp gauge.

Gauge used to read about 170 deg when warm. Now reads about 140 - which I know is incorrect. I realize these gauges are not supposed to be accurate at normal temps - only in the danger zone.

At 40 ohms to ground - the gauge reads just past 250 deg - which seems right, at 80 ohms - it reads about 180 deg - whish is low - I expected 210.

Do I have a gauge calibration problem - or is this behaviour within normaly expectd parameters?

[John H.]

Re: Temp Guage Question C2 (67)

Glenn -

Sounds like a gauge calibration issue - the response curve on the factory print for the #1513321 sending unit shows an acceptable range of 200*-210* at 80 ohms.

[Glenn B.]

Re: Temp Guage Question C2 (67)

Thanks John, I'm afraid that's what I expected was the case.

Now I'm trying to decide whether the pain of taking out the cluser is worth it to correct the improperly calibrated gauge. I don't think my back can take that again for a while...

[Jack H.]

Plus

180F = 90 to 107 ohms. Also, keep in mind when you're playing with these things on the bench, the car's electrical system is NOT 12 VDC! Once the engine starts and the charging system kicks in, the alternator will drive B+ above 12 VDC to maintain the battery in a charged state.

Those who do benchwork on auto electric components (radio, WS wipers, Etc.) generally use an 'automotive' grade power supply. These are typically equipped with 13.8 VDC regulated output.

Remember, your temp guage is actually an ammeter whose face has been graduated in degree-F equivalents but it's responding to the current flowing through it. When supply voltage climbs over a fixed resistance value, the branch current has to rise in lock-step...and the temp guage IS reporting current...

[Glenn B.]

Re: Plus

Good point about the voltage Jack (not 12V). My sore back decided we are not going to tear the cluster out again (this year) to fix the goof-up by the cluster restorer.

I have not put my EE degree to use for some time, but I dusted it off and figured that I could more or less offset the miscalibration of the gauge by patching in a parallel resistance to the sender. I'll first determine the resistance value I need get the gauge to read 210 degrees. I'll then calculate the required parallel resiatance to add to acheive that same value assuming my sender is 80 ohms at 210 deg (I read acurately before - so I think this is the case).

My initial estimate is that I need about 56o ohms. If I do this on the gauge end of the circuit - I doubt anyone will ever notice.