Temp Gauge Inaccurate, too high

[sunsprit]

I finally got the motor back together and installed in my '78 Spider; it's all stock, no mods.

Before the tear down, the temp gauge ran consistently at 180F indicated. Upon tear down, I found that it had a 160F single port T-stat installed, so I replaced it with an OE style 2 port 180F (with the lower plate to block out the by-pass on warming up).

Additionally, I put the car back together with an 880CFM electric fan in front of the radiator (which was sent out to a radiator shop to be cleaned and checked), controlled by an adjustable thermal probe switch. I located the temp probe bulb directly at the upper radiator tank seam, and sealed it with aluminum tape for tight contact to the radiator for temp sensing.

The car now runs at 212F indicated, but I used a Laser Temp gauge on the manifold by the engine temp sensor and it indicates an actual temp of 180-183F, and similarly at the t-stat housing. There's no signs of overheating, and the car goes through a normal SPICA cold start/warm up cycle of fast idle and fuel enrichment.

I've double checked the wiring to the temp gauge, and all the ground circuits. Several additional grounds had been installed on the car behind the dash, and from the motor and alternator to the chassis (12 gauge wire).

So, now I'm thinking that the gauge or the sender is faulty. Does anybody have a temp/resistance chart for the sender and/or gauge so that I can measure the sender resistance or use a substitution box to verify correct reading at the gauge?

Thanks in advance for your help.

[Kilgore Trout]

I've done several electric fan conversions on Spiders, and it always made better sense to me to place the fan sensor switch on the bottom tank of the radiator. The logic being that I'm measuring water temp after the cooling cycle has completed. I've also always used a threaded bung and immersible sensor, rather than an external, with the thought that the internal would provide more accuracy.

None of that is to say you've got any problems with your setup, other than perhaps with the gauge or sender, especially since your measurement at the t-stat housing sounds great. What does the gauge read if you ground the sender wire? What about taking the sender out of the manifold, wiring a temp ground to it, and dunking it into a pot of boiling water? Just a couple of thoughts . . .

[sunsprit]

Absent a temp/resistance chart for the temp sensor, it doesn't seem productive to dunk the sensor in boiling water (and we're at 6,000' elevation) to get a resistance reading or gauge reading. I already know that the gauge system reads 212F when the water jacket adjacent and the temp sensor body are at 180F (thanks to the laser temp probe, one of the best diagnostic tools ever invented for thermal readouts).

But I agree that an immersed temp sender for the electric cooling fan would be a more accurate way of controlling the fan. Where did you get the solderable bung and what sensor are you using?

As an alternative, what about placing a sensor in the bleeder port on the water rail?

[geh458]

Just a thought, but did you bleed all the air out of the cooling system? Perhaps the flow past the temp sensor is not correct.

[sunsprit]

The cooling system has been completely bled with a mix of 30% anti-freeze in distilled water, and a bottle of Redline Water Wetter additive.

You can tell that the radiator is dissipating heat on all the tubes by getting the readings with the laser probe ... as well as by placing your hand in the airflow stream coming out of the radiator. The thermal image presented by the probe throughout the cooling system is consistent with proper performance throughout.

So I don't believe there's a problem with the cooling system performance, but strictly an issue with the dash temperature reading.

I'm getting a new thermal sensor today, but my supplier tells me that the "Italians consider any termp reading plus/minus 10F of the correct temp reading is accurate". If only my gauge was cosistently that close and I could document it, none of this would be a concern.

Having spent all the dough and time to go completely through this motor with all new bearings, head work, pistons/rings .... I don't want to risk losing it all due to a simple gauge problem.

[Kilgore Trout]

Quote:

Originally Posted by sunsprit Absent a temp/resistance chart for the temp sensor, it doesn't seem productive to dunk the sensor in boiling water (and we're at 6,000' elevation) to get a resistance reading or gauge reading. I already know that the gauge system reads 212F when the water jacket adjacent and the temp sensor body are at 180F (thanks to the laser temp probe, one of the best diagnostic tools ever invented for thermal readouts). But I agree that an immersed temp sender for the electric cooling fan would be a more accurate way of controlling the fan. Where did you get the solderable bung and what sensor are you using? As an alternative, what about placing a sensor in the bleeder port on the water rail?

[Tifosi]

Quote:

Originally Posted by sunsprit You can tell that the radiator is dissipating heat on all the tubes by getting the readings with the laser probe ... as well as by placing your hand in the airflow stream coming out of the radiator.