msk914

Hi all. I've been finally driving my '65 a fair amount. I notice that the temperature gauge is very erratic, it runs for most of the time at 180 degrees, but then pegs left, pegs (or mostly pegs) right, then back to 180, etc. It runs well and it is not overheating. I saw this a little before I started my refresh of the car, I though it might be the badly frayed green wire to the temp sender, but with that replaced, it still does it.

I tested the gauge by detaching the green wire, it goes to 100; then grounding it, it goes to pegged right, so I believe the problem is likely to be the temp sender. I read some of the excellent threads here, I bought a TU5 from Autozone.

My question: The sender is very hard to reach with any tools: Do I have to take the elbow out to replace the sender? If so, will a lot of antifreeze spew over the engine? (I could drain some in that case first; if so, where is the drain plug on the radiator?)

thanks,

Mark

Bluestripe67

The rad drain is at the bottom of the rad on the left side. Your description of removing the sender confuses me. Can you post a pic? It should be right up front of the manifold on the crossover water passage. After draining about 2 qts, carefully remove it and replace it. Refill via the tank and you are road ready. Dennis

Nowhere Man

If it's in the intake where it was originally why would you have to drain coolant

irwiny

Mark:

Did you recently replace your original gauge?

First get yourself a IR temperature testing device and compare the results between the gauge and good test point. I find the thermostat housing bolt a good location.

Also if use ohm meter to read the resistance of the sensor to ground at various temperatures.

There are some PDF's from previous post on this forum about the Ohms vs temperature of the sensor. After you have determined that the sensor is working fine, then you may need to calibrate the gauge and that's another project and I have seen a calibration inline kit. Sorry I don't have the PDF now to attach. Let me know if you can't find it on this forum's search.

Just make sure your car is running not running hot.

Irwin

msk914

The sender is in the center of the head, just below (and to the left of) the 'elbow' (the part that contains the thermostat). Hard to reach with tools without the elbow removed. I realized when I read your answer that taking out the sender will send fluid all over the engine anyway unless I drain it, and it is too cold for me to work now. [I think the job is to drain some fluid, take off the elbow (and thermostat within), replace the sender (now accessible) and put back the elbow and top off the fluid.] I'll give it to my mechanic, he needs the car for one other small job I cannot do comfortably in the cold.

Thanks Dennis.

msk914

Irwin,

The gauge shows 180 most of the time. The problem is that some of the time (when the car is warm) the gauge pegs left or right or swings wildly. When I ground it or open the circuit it shows correctly. So I am pretty sure the gauge is ok. Or at least the sender is by far the more likely culprit. (For instance, if it pegs to full right, I park it, pull the wire from the sender, the gauge goes right to 100; or if it is pegged to the left, I park it, ground the wire, the gauge correctly pegs to the right.)

I'll replace the sender (which cost me $8) see if the wild swings left and right and pegging left and right stop.

My mechanic first saw this by the way, he and I thought it was caused by the original green wire to the sender, which was horrible and frayed. I replaced that. He did put an IR on it, it always showed 180, even when the gauge was pegged left or right. I do not think I am overheating.

thanks,

Mark

Bluestripe67

Mark, I can't quite picture the location you describe, but I am sure it is far from being in a stock location on the top of the intake manifold. Good luck on getting all the bugs worked out. Dennis

jim lockwood

Even if the sender is in the intake manifold (which, based on other posts, it may not be) coolant needs to be drained because the manifold is not the high point of the system. Draining a half-gallon or so is sufficient.

msk914

It's now pretty obvious to me that I need to drain some fluid.

About location, i looked up pictures of 327/300 engines, and the sender in some pictures is where mine is, in other pictures it is not. Might depend on year, or on transmission type, mine is auto. But not a big deal, I will get this replaced shortly, I hope it is the problem.

Thanks all,

Mark

irwiny

Mark:

If I recall, as the resistance goes down when the car get's hotter. If you have a ground issue then the gauge would read low (pulled wire). For my brother that had the same issue with his replaced gauge reading high so I put a 33 Ohm resistor in line as a band aid since he was not overheating or running hot. It was easy because someone had already cut and replaced the wire from the firewall to the sensor.

Also if you can find the sensor, I heard having Teflon to seal it could cause a problem but that would only adding resistance (gauge reading low). I also read that if the original gauge had a resistor in parallel with the inputs.

If your near a good electronic store, try to find a 3 Watt/ 33ohm resister it should look like a miniature-wide ceramic shaped brick and when you see your gauge go far right, put it between the sensor and wire to see the results. We were not near good electronics store when I did this on my brothers car and so used a 33 ohm 1/2 watt resistor from Radio Shack. I haven't been able to finish the calibration with the suggest Thermo fuse / resistor combo (see below link feedback from expert members here). So far the RS resistor is working and if there is a issue with the lower watt resistor, normally it will open circuit and the gauge would go far left or short and go far right.

https://www.corvetteforum.com/forums...r-wattage.html

Let me know if you still need the PDF regarding the resistance vs gauge document.

I greatly enjoy this forum, but the bad thing is that the nice to fix items list continues to grow.

Happy Holidays

Irwin

msk914

Irwin,

Its not that the gauge shows high (or low), it shows both high and low, swinging wildly between them for a minute at a time; then going back to 180 for a lot of the time. That's why I think it has to be the sender. The gauge itself shows 180 most of the time, and when open it shows 100 and when grounded it pegs right. I'll let everyone know once I put the new sender in.

And you are right of course, the more you do, the more is left to do as new things come up. But that is part or most of the fun!

Mark

rongold

Guys,

What he's calling an elbow is the flat type of water outlet. Some small blocks have the temp sender located under the upper hose neck on the water outlet. I'm sure that with the proper sized socket and a universal joint, it can be removed without removing the water outlet.

msk914

Right, the water outlet is kind of flat, and the temp unit is under it. And I agree that with the right shallow socket and joint i could get it out, but the water needs draining anyway, so it would be easier to take the outlet off.

thanks,

Mark

0Willcox Corvette

Could the wire inside the dash be loose on the gauge? The sway you describe is usually caused by ohms input missing or being ground. (Missing ohms goes to zero, grounded pegged hot).

The sender (exploded below) creates variable ohms by the thermistor heating up and cooling down with the engine. Normally when the oil dissipates or leaks out of the sender, corrosion forms on top of the thermistor or between the spring and cap preventing the sender to function properly.

Always keep a original Delco sender, they can be rebuilt but boy what a PITA.

msk914

I thought it might be a bad connection to the gauge, except for two things:

a. The cluster was out, the connections seemed excellent.
b. The gauge itself works perfectly when the sender wire is grounded or opened at the sender side.

Also, the wire to the sender was changed, it had been old and brittle, I thought that was the problem initially, but it is new and shielded now.

Also, the way this is acting is consistent with a resistor being erratic, it goes from pegged left to pegged right, etc and then back to 180 within minutes once hot.

We will see soon, once I get a decent weather day to put in the new TU5.

thanks,

Mark

0Willcox Corvette

It probably is the sending unit... and that would be the easiest thing to check. If that fails then take a look at other items that could cause this issue. The nuts that hold the gauge to the pod, the nuts that hold the terminals in place.. and a very long shot, corrosion between the gauge mounting nuts and resistor.

msk914

A result: It seems that it was the temperature sender. I replaced it with a TU5, took it out today for about 10-15 miles. It sits at about 190 degrees while going 40+ mph. It goes up to 200-210 while sitting in traffic. But it does not peg left and peg right randomly. So I believe this is fixed. All instruments (and blower, heater, wipers and washers) now work. The car had all the usual stuff not working after 50 years when I got it: temp sender, odometer, clock, fuel sender in tank. Blower did not work, I replaced that. Washer did not work, I replaced that too, it comes out hard now (and correct direction too). Both headlight motors now work, the list goes on...but it is fun to complete this.

Mark