Before tonight, my coolant temp gauge was pegged left giving no reading. I replaced the temperature sender (sensor) and now within a couple minutes of idling (didn’t check right away) my gauge is pegged to the right (hot). I changed the coolant this winter along with several gaskets, the heater core, and the thermostat. The car drove well (10-12 miles) after idling for quite a while (had an idle issue I had to resolve before I could take it out). There was no steam, smell, or noise while driving or after I was parked to indicate the car may be overheating. Any ideas on why the gauge would read full hot? Thanks for any advice

 

Does your fuel gauge read accurately? They share a voltage regulator, if both gauges are reading off, it may be that. You can also read up here:
https://forums.jag-lovers.com/t/temp...cooling/363938

 

My fuel gauge appears to be at least moderately close, but I’ll admit I don’t know if it’s 100% accurate. I put in 5 gallons on top of some other leftover gas that was in it from the last fill up (under 1/4 tank) and the gauge now reads 1/2 tank which seems reasonably close to correct. I read through the other thread and learned some things but not sure how to proceed still (not saying I expect instructions, just trying to decide what makes sense to do next).

based on the lack of steam, it seems unlikely I’m overheating to the extent that the gauge is reading, does it not? Should I refrain from driving until I can sort this out?

 

I’m also freaking myself out worrying about a cracked block from previously running with low coolant (rookie mistake I know, but did not drive it more than 10 miles like that, idled in the garage multiple times in that state). Is that a likely scenario?

 

I doubt it's overheating to that degree. Far more likely to be the guage, or how it's wired. If the engine is THAT hot, you would feel the heat as soon as you lifted the bonnet. The rad water does heat quite quickly when idling - did your fans cut in?

 

Do you have an infrared heat/temp gun?


A good investment on many levels. Aim the beam at the top radiator hose, get a reading, then at the bottom hose. Be sure it's always aimed at black hose. aiming it at 'silver/chrome' color will mess up the reading. If your fuel gauge is registering something, then the voltage regulator is probably ok. Do you know if the fans work? There may be an 'otter' switch that initiates the fans once the rad temp hits the turn-on point.

Here's a link to discussion of testing Otter switch:

https://forums.jag-lovers.com/t/e-ty...r-switch/80358

 

The gas gauge does have a reading, not sure if it’s entirely accurate but seems close

the fans did kick on, but I noticed that the temp gauge showed full hot prior to the fans even kicking on. Fans would turn on and off while idling, showing me the otter switch is at least functioning.

when you say “I’d feel the heat when I open the bonnet”, I mean, everything is hot when I open the bonnet, but so is my 2012 impala when I open it

 

even new sensors can be crap. I had similar issues after the original temp sender died. I ended up removing the sender and gauge and hooked them up with a spare battery, then put the sender into boiling water. It read 15 degrees too high. Another sender was ordered and I tested it before installing it to make sure it read properly.

 

Well I tried the spare sensor I ordered and had the same result. Took it for a 10.5 mile drive after idling for quite some time. My friend went along with me and he agreed he doesn’t think the car is overheating based on all of the signals it isn’t exhibiting. Will continue to troubleshoot. My IR sensor gun is on order and my friend is very interested in doing the detached sensor check with me. I’ll update as I find out more. Thank you all for the help so far!

 

Is the temperature wire grounding out?

 

If the sender wire is grounded it will peg the gauge. This is how you test the gauge.

 

Brief update:
drove 27 miles today (new record since not running for 4 years!)
IR temp gun reads top radiator hose at 163°F, bottom radiator hose at 147°F after the drive. Pretty near certain my issue really is in the gauging system rather than the car overheating. I was pretty sure before, but now I’m pretty close to fully confident.

side note: car drove great! It’s so exciting to take it out like this, it hasn’t run this well in a very long time

 

Exxcellent news, enjoy your driving!

 

So you have verified that your thermostat is working right and that the engine is running at about the right temperature. It's just that the gauge reading is too high. I had the same issue, and solved it by soldering a small resistor in series with the gauge. I think it might be in the 5 to 20 ohm range, and will no doubt require a bit of trial-and- error. You might start at about 20-30 ohms, which will probably make the gauge read too low. Almost any resistance can be achieved by putting resistors in parallel.

Bob Frisby
Boise, Idaho
E-type S2 FHC

 

Hi Bob and if you could post a photo of your resistor solder work for the gauge, I think it would be helpful to many, including myself (auto electric challenged :-)

 

I rewired my car’s coolant system. There is an otter switch and a fan relay and an ac relay that overrides both other switches when ac is on.

Also best to check temp above and below thermostats. Easy to see if they work . I don’t check rubber hoses I check metal parts like top and bottom metal inlets on radiator. More accurate.

 

Sorry for the late reply, I was OOT for a BritCar event in Pendleton, Oregon. Sadly, I can't post a a photo, because I don't know how.

With the swing-down gauge panel open, look at the gauge connections (not the red/white wires to the light bulb!). On the left is the green/black wire from the IVR (instrument voltage regulator), and on the right is the green/blue wire to the water temp sender (a thermistor, actually). Either one of these can be disconnected from the gauge, and the new resistor(s) inserted into the resulting gap. I added mine on the right side (green/blue wire).

My memory of 5-20 ohms was a little off. It actually measures about 32 ohms on my cheapie meter. You will probably have to experiment with different resistances to get the gauge reading you want. It may be hard to find the exact resistance you want with a single resistor, in which case you can use multiple resistors to get the value you'd like. These might be in series, in parallel, or some combination thereof. I did my experimenting with a combo like this as well as a resistance substitution box (which is handy, but not necessary), and then was able to find a single resistor that was about right. But a combination of resistors will work just fine as your permanent "fix".

Hope this all makes sense!

Bob Frisby
Boise, Idaho
E-type S2 FHC

 

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The gauge works on 10V full deflection. The car is nominally 12V (and goes well above that when running / charging).

10V was chosen in anticipation that at leas that much would be available most of the time. A voltage regulator was included to feed an average of 10V from the higher source. The stock regulator is mechanical and a very common and regular point of failure. It can fail open (0V, no meter reading) or closed (12+V, high or full deflection).

You can (easily and inexpensively) make an electronic regulator for a few $ and it will fit inside the original housing. Alternatively, replacements are available for purchase.

See the attached PDF for a full discussion