1967 Water Temperature Guage Test

[Timothy B.]

I have read the proper test for the temperature guage is a 40 Ohm resistor to ground which should put the pointing needle in the extreme hot (red) zone.

When I went to radio shack the resistors are 1/4, 1/2 and 1 watt resistors. Can someone tell me exactly which one to buy so I can test the guage.

Also, the guage reads low, something like 155*+- and has always been like that. I installed another original sender and it's exactly the same. It may be my eyes but the dash lights seem a little dim and I am wondering if there is any relation to that and the low reading guage..

Thanks for the suggestions, Tim

[Mike M.]

Re: 1967 Water Temperature Guage Test

Unless they are cross wired in some weird fashion, there would be no relativity between the Temp Gauge and the dash lights. If so, you would have other major issues. They are independent circuits.
HaND

[Jack H.]

Re: 1967 Water Temperature Guage Test

40 ohms is the ultimate 'plateau' for the temp sender... That should generate a gauge reading thats WAY off scale on the hot side!

I'd go with the test footnote GM put on the temp sender drawing. Essentially, 60 ohms = 230F.

On what power dissipation profile you need for a test setup, I use two 1/8 Watt resistors connected in parallel (because that's what I had on hand), and they work fine. Essentially, that's the equivalent of a 1/4 Watt resistor.

[Timothy B.]

Re: 1967 Water Temperature Guage Test

Thanks Mike and Jack,

There is nothing cross wired in the dash, I think the dash light issue is related to the reostat (sp) in the headlight switch. I am going to remove and clean this switch soon. I missed a auction on a 131 headlight switch some time ago and am looking for a NOS switch if anyone has one for sale.

I will use (2) 1/8 watt 30 ohm resistors parallel to check the 230 temp setting on the guage. The engine has 180 thermostat but the guage has always read lower. The car runs fine and system goes under pressure and it will sit and idle all day and never get hot.

If all checks out I will check the resistance on the sender to make sure it goes to 60 ohms with boiling water just to be sure all is working to spec..

[Rich P.]

Re: 1967 Water Temperature Guage Test

Tim,

about 15 or so years ago I restored a 66 L-79 with A/C. It had a brand new Dewitts radiator and I could not get that car to go over 160 degrees. What do you have for a radiator in the 67? I took my readings at the expansion tank to confirm the gauge and it was always correct.

Rich

[Wayne W.]

Re: 1967 Water Temperature Guage Test

Thanks Mike and Jack,

There is nothing cross wired in the dash, I think the dash light issue is related to the reostat (sp) in the headlight switch. I am going to remove and clean this switch soon. I missed a auction on a 131 headlight switch some time ago and am looking for a NOS switch if anyone has one for sale.

I will use (2) 1/8 watt 30 ohm resistors parallel to check the 230 temp setting on the guage. The engine has 180 thermostat but the guage has always read lower. The car runs fine and system goes under pressure and it will sit and idle all day and never get hot.

If all checks out I will check the resistance on the sender to make sure it goes to 60 ohms with boiling water just to be sure all is working to spec..

Better use those in series. You are making up 15 ohms in parallel.

[Timothy B.]

Re: 1967 Water Temperature Guage Test

Wayne,

Are you saying if I hook them one after another it's not 60 ohms? Can you explain better..

[Stephen L.]

Re: 1967 Water Temperature Guage Test

SERIES connection (one 30 ohm following the other) results in 60 ohms.
PARALLEL connection (2 30 ohm side by side) results in 15 ohms.

[Jack H.]

Re: 1967 Water Temperature Guage Test

SERIES connection (one 30 ohm following the other) results in 60 ohms.

And, if they're 1/8 W, then the combination of the two results in an 1/8 W rating...

PARALLEL connection (2 30 ohm side by side) results in 15 ohms.

And, if they're 1/8 W, then the parallel combination of the two results in a 1/4 W rating...

[Timothy B.]

Re: 1967 Water Temperature Guage Test

Thanks for the information, I understand now. I bought 1/2 watt 15 ohm resistors and will hook in series so it's a total of 1/2watt with all four in series..

Seems like a feed back pot would work better for this as the guage could be brought to 230* then resistance checked with ohm meter. I saw a good selection at radio shack, is this a good idea and which one should I look for, (range etc.)

Thanks again for the help.

[Wayne W.]

Re: 1967 Water Temperature Guage Test

Thanks for the information, I understand now. I bought 1/2 watt 15 ohm resistors and will hook in series so it's a total of 1/2watt with all four in series..

Seems like a feed back pot would work better for this as the guage could be brought to 230* then resistance checked with ohm meter. I saw a good selection at radio shack, is this a good idea and which one should I look for, (range etc.)

Thanks again for the help.

Anything that varies from 0 to 1000 ohms or so. In this case it makes no sense to worry about wattage. The draw is not significant.

[Jack H.]

Re: 1967 Water Temperature Guage Test

Seems like a feed back pot would work better for this as the guage could be brought to 230* then resistance checked with ohm meter. I saw a good selection at radio shack, is this a good idea and which one should I look for, (range etc.)

That approach will also work (use a pot). BUT, most of the pots you get from, say Radio Shack, have logarithmic taper. That means they 'jump' in resistance value pretty quickly compared to a linear tapered pot.

So, to dial up a specific resistance can be 'touchy'. Plus, once you get the setting you're after, when you remove the pot to go measure it's adjusted resistance any minor shock/vibration can move it slightly from where it was when you had it in circuit and made the original adjustment...

[Joe R.]

Re: 1967 Water Temperature Guage Test

When I went to radio shack the resistors are 1/4, 1/2 and 1 watt resistors. Can someone tell me exactly which one to buy so I can test the guage.

Tim,

I thought I would give you the more general solution should you need it for a different circuit.

P = Power [Watts]
V = Voltage [Volts]
I = Current [Amps]
R = Resistance [Ohms]

P = (V * V) / R

P = I * I * R

P = V * I

For example,

V = 3 Volts, R = 60 Ohms

P = (3 * 3) / 60 = 0.15 W (I would use a 1/2 watt resistor here.)

I like to use a resistor with twice the wattage rating of what it will see worst case in circuit. Mouser.com is a good website for choosing electronics parts. Just do a search on resistor, then choose resistance, wattage, etc. until it narrows it down to a few parts to choose from.

Joe

[Timothy B.]

Re: 1967 Water Temperature Guage Test

Thank you everyone for your help. I learned something new today..

[Joe R.]

Re: 1967 Water Temperature Guage Test

Thanks for the information, I understand now. I bought 1/2 watt 15 ohm resistors and will hook in series so it's a total of 1/2watt with all four in series..

Seems like a feed back pot would work better for this as the guage could be brought to 230* then resistance checked with ohm meter. I saw a good selection at radio shack, is this a good idea and which one should I look for, (range etc.)

Thanks again for the help.

Tim,

How much voltage will this resistor see? I would run the numbers another way as well. Here's how to determine how much maximum voltage the resistor can withstand before getting too hot once you know its wattage rating (let's use 60 Ohms 1/2 Watt):

P = (V * V) / R

V = SQUARE ROOT (PR)

V = SQUARE ROOT (0.5 * 60) = 5.5 V

The other reason not to use a pot is that it's easy to smoke it by dialing in too low a resistance. I would use resistor decade box instead (most I've seen are 1/2 Watt). I would first determine what the minimum resistance is at 1/2 Watt before substituting various values with the decade box. With the decade box you can see how much resistance you're putting in the circuit. Of course, I would still check the decade box by disconnecting it from the circuit, then measuring its resistance with an Ohmmeter before using that value as the gospel.

Joe