I've seen a few messages on this topic, but now I have some hard numbers to deal with.

I put a new radiator core in my '69 350/300 along with a 180 deg. thermostat and a new temperature sender. I also installed all new foam seal around the radiator. I got the temp unit, M112B from ZIP with a new pigtail (wire and clip). When driving, the temp guage has been showing between 212 and 225 deg. F consistently, depending on the outside temperature for some time.

Today, I clamped an A/C type thermometer (metal stem with a button dial) on the side of the top radiator hose next to the thermostst housing. The outside air temperature was in the low 70's. I drove the car for over an hour and it showed a steady reading -- did not move. The guage goes from 210 to 250 with only one hash mark in between, so I'm guessing it was reading about 212-215 deg. F.

I stopped the car with the engine running and popped the hood. The thermometer read 172 deg. F. I continued to drive the car and when I stopped and shut it off, the temp guage read about 205-208 deg. F and the thermometer read 175 deg. F.

Temp Guage Thermometer

at idle 212-215 172

stopped 205-208 175

Can anyone tell me if these temperatures make sense and how to correct the guage reading. I've heard you can put a resistor in-line from the temp sender, but how do you know how many ohms???

Thanks,

Jim

Jim,
This is probably the hardest way in the world to do it but it should work. If anyone can do this easier let Jim know.
Go to Radio Shack ( or someting similiar ) and buy a hmmmmm 500 to 1000 ohm variable resistor. Pull your sending unit out of your block and pull out your temp gauge. Connect the variable resistor between your sender and temp gauge in series. Put the sender in a pot of water and as it boils, adjust the restistor to make the gauge read 212. Disconnect everything and use an ohmeter to measure the adjusted value of the variable resistor and go buy a fixed resitor of that value and solder it in line between the sender and the gauge. Don't forget to use shrik tubing on the connections.
Like I said, not the easiest way but water not under pressure boils at 212 or close enough to do what you need to do.
Good luck, and let us know if it works.

Glenns:

Thanks for the reply. I posted this same message on Corvette Forum and the NCRS board to see what all I could learn.

I had another response about using a 1000 ohm 2 watt potentiometer as you suggested, but, I also had two replies that warned about putting a fixed resistor in-line at one temperature.

Jack Humphrey sent me an article tht he wrote for his club newsletter in which he went into great technical detail about how the temp sender works. Not being an engineer, the best way I can summarize the info is that even though the temp guage is supposed to have linear response to resistance, inserting a fixed resistor at one point shifts the scale. If you correct it for, say 180 degrees, it could be off and read low at 220 or higher.

I don't know if that makes sense, but with two different warnings, I'm going to hold off on a "quick fix".

Thanks,
Jim

Well, I am not an engineer either so I won't argue with him. If he is all that technical, did he give any answer as to how to fix the problem other than buy a new sender and gauge that has been "factory calibrated" ??
My understanding is an analog gauge is only supposed to give you a " representation " of what is happening ( like a temp reading ). A digital guage is supposed to be much more accurate but they have to be calibrated in some manner also.
Oh well ????? !!

You're right about analog versus digital. Basically, the article says GM was building an inexpensive sports car and they just wanted to provide a device that wuold warn of an impending "melt down" i.e. severe overheating problem.

There is a comment about one of the vendors catalogs having a note that their replacement temp senders may be off by as much as 30 degrees. I guesss I proved that right! Mine was off by 40 degrees.

As you said, no real fix is out there -- just be careful!

Try Vintage Automotive Research they will recalibrate your sending unit, or there are a few companys that will recalibrate the gauge using the proper trimming resistor. Jack Humphry is an electrical engingeer and is correct that the inline resistor is not the way to go (only accurate for the temp you set it to).

Mike

that is the most awesome 69 I've ever seen sir

Unless you can take your readings from the same place you will not get accurate results. I would suggest pulling the plug in the water jacket on the opposite head out and install a thermometer well in there for comparison readings. It also won't be exactly the same, but it will be a lot closer then the outside of the radiator hose. Seems like a lot of people (including me) have had temp gauge problems. It seems GM can't supply the correct sender. I finally bought a new sender from Lectric Limited and sent my sender and gauge to Jason at Vette Instruments for calibration. Please let me know if you want to go this route, I can get you his e-mail (there is no website).

Hey Big Fish,

I would like that info. I, like you, have purchased a new sender from Lectric limited. I now just need someone to recalibrate my functual, but inaccurate, gauge to it. Your help is appreciated!

Hinz-73 said: Hey Big Fish, I would like that info. I, like you, have purchased a new sender from Lectric limited. I now just need someone to recalibrate my functual, but inaccurate, gauge to it. Your help is appreciated!

Here you go: VetteInstruments@aol.com

The guy's name is Jason and you will need to copy the e-mail address to your mail program. Jason is a really nice guy to work with, good luck!

|UPDATED|5/2/2004 8:59:54 PM (AZT)|/UPDATED|