P15 Heat Gauge Needle 'Zero' Position

[kbuhagiar]

Hello Folks,

On my 47 Plymouth, the heat gauge needle sits one mark to the right of the 100 degree position (what one might interpret as the 120 degree mark) when fully cold.

I have a couple of questions:

1) Can this have an effect on the overall accuracy of the gauge reading? In other words, should I interpret the gauge to be 20 degrees off over the entire scale (so when the gauge says 212 degrees, the temperature is really only 192 degrees)?

2) Where should the needle actually sit when it is fully cold - on the 100 degree mark, or slightly below (to the left of) that?

3) Can the needle be adjusted so it sits in the correct spot when fully cold (wherever that may be)?

Thanks in advance!

[greg g]

check the temp on the gauge with the temp of the coolant at the radiator and see if they agree when at running temp. Probably cause more problems trying to fix it.

[Dan Hiebert]

In our great southwest, that could just be the ambient temperature, but that's doesn't look to be where you hail from. Don't know if the sun could get your car that hot in San Fran if you leave it outside. The cold reading of the guage is not what's really important, its where it is when your car is running - "operating temperature". The needle could be off kilter when cold because it has already been adjusted for accuracy when hot. There are previous threads for that proceedure, but the most accurate involve having the guage out of the car. If you don't want to do that you can compare the coolant temperature at the radiator with the guage reading as noted previously, but remember that the guage reads the temp at the rear of the engine where the coolant has only made half its journey through the engine, but it'll be close enough to know how to interpret your guage reading.

Edited June 23, 2011 by Dan Hiebert

[Joe Flanagan]

As it happens, I just went through this situation with my own temp gauge. I think the needle on mine was sitting exactly where yours sits. 100 degrees is the low end of my gauge (like yours). There is a stop pin just to the left of that, which the needle is supposed to rest against when cold. My needle was sitting just a bit off that stop pin and I couldn't get it to move back. My capillary tube was broken and after two unsuccessful attempts at the Tod Fitch repair, I finally sent it off to John Wolf to be fixed. I just got it back yesterday and they somehow got the needle to sit where it's supposed to. I have monkeyed around with the needles on these things and all I can say is stay away from it. That is just my experience. Others may have done it successfully. The Tod Fitch repair has been done by Tod and others but for some reason, I couldn't do it. I found the needle/spring arrangement to be so delicate that there is virtually no room for error. It's like the inside of a watch. John Wolf is in Ohio. You might consider sending it to him. It might be pricey, though. I paid 150 to get mine repaired (shipping included). I'm glad it's done, though.

[kbuhagiar]

As it happens, I just went through this situation with my own temp gauge. I think the needle on mine was sitting exactly where yours sits. 100 degrees is the low end of my gauge (like yours). There is a stop pin just to the left of that, which the needle is supposed to rest against when cold. My needle was sitting just a bit off that stop pin and I couldn't get it to move back. I have monkeyed around with the needles on these things and all I can say is stay away from it. That is just my experience. Others may have done it successfully. The Tod Fitch repair has been done by Tod and others but for some reason, I couldn't do it. I found the needle/spring arrangement to be so delicate that there is virtually no room for error. It's like the inside of a watch.

(Sincere thanks to drdialtone for the use of his picture.)

I don't care too much about getting the needle to sit correctly; I'm more concerned with correctly interpreting the reading.

Engine is a TPI 350 Chevy with a pressurized cooling system.

On a 75-degree day at around 70 MPH, my needle sits just under the 212 mark (red arrow).

When I hit a long incline it may briefly go past 212.

Around town it usually sits just under the red arrow.

I am trying to determine if there is the 'fudge factor" of 20 degrees or so based on the starting point of the needle. If so, I have no quibble with the readings.

I have an infrared thermometer so I guess I'll take some readings this weekend to determine the accuracy.

Should I measure the temp at the thermostat outlet, or right where the temp gauge bulb is mounted in the head?

My capillary tube was broken and after two unsuccessful attempts at the Tod Fitch repair, I finally sent it off to John Wolf to be fixed. I just got it back yesterday and they somehow got the needle to sit where it's supposed to. John Wolf is in Ohio. You might consider sending it to him. It might be pricey, though. I paid 150 to get mine repaired (shipping included). I'm glad it's done, though.

John Wolf does fantastic work - I highly recommend his services. He rebuilt the fuel tank sending unit on my Corvette a few years back.

Edited June 23, 2011 by kbuhagiar

[greg g]

100 next slash 120 next 140 then the 160 mark, it practice follows next would be 180, then 200, then 212.

Again with out knowing if the gauge is accurate, the needle positioin is meaningless.

Suppose it 20 degress of in the other direction you 210 could be 190. Get a cheap kitchen thermometer and check the water temp in the radiator. Since the gauge sender is probably where the coolant is at its hottest, ther may be a 5 or 10 degree difference between the therm in the rad and the temp at the sensor bulb. check your kitchen therm by using ice water to determin if it accurate. then you will know what your gauge is telling you.

We have had threads about 1/4 tank being empty, 80 #'s of oil pressure being 30, and 220 being 170, so why guess what your 65 year old gauge is telling you????

[kbuhagiar]

We have had threads about 1/4 tank being empty, 80 #'s of oil pressure being 30, and 220 being 170, so why guess what your 65 year old gauge is telling you????

Touche - point well taken.

Now, where did I put that kitchen thermometer...?

[Reg Evans]

Touche - point well taken.

Now, where did I put that kitchen thermometer...?

This is your wife speaking. Kenneth.....stay out of the kitchen !

[Reg Evans]

Another test would be to remove the bulb from the block and dunk it in a small pan of boiling water.

[kbuhagiar]

OK, did some testing this weekend, using an infrared thermometer, with the car completely warmed up and idling in the driveway.

With heat gauge indicating 200 (yellow arrow), readings are as follows:

At cylinder head, where the heat gauge bulb is installed, 210-215 degrees.

At thermostat housing, direct center and top, 180 degrees.

Are the readings consistent with other similar applications, i.e. should there be a 30 degree discrepancy between the thermostat housing and cylinder head? Is this be normal for a SBC?

Which reading should be used to determine actual cooling system temperature - cylinder head or thermostat housing?

Thanks again.

[greg g]

sbc as in small block chevy???? If so that might have been some information to have been included in your original question.

What are you running for coolant, and is there a thermostat in the car.

[kbuhagiar]

sbc as in small block chevy???? If so that might have been some information to have been included in your original question.

Was included in post #5 - engine is a stock 1986 Corvette TPI.

What are you running for coolant, and is there a thermostat in the car.

50-50 mix regular Prestone - 195 degree thermostat.

Thanks.

[greg g]

system pressurized? Cap correct for pressure?

[kbuhagiar]

system pressurized?

Yes.

Cap correct for pressure?

Yup.