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TR2/3/3A Overheat issue, gauge adjustment

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Rick_Thompson

Rick_Thompson

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I have an electric fan conversion I did when installing my rack and pinion conversion.

A few weeks ago while out running, the car began to overheat and literally pegged the new to me temperature gauge I had installed. Original gauge had cracked the capillary tube and I did not send it off for a rebuild as I found this one on Ebay. The gauge stayed at 230 even after the car cooled down. Tapping on it a bit got it to drop back to a normal range. Since that time it doesnt seem to be working properly.

The overheat was from the fan blade disconnecting from the motor. It took a while to fix once I get the car home, but all is good now with a new nut and some red loctite.

Since then, the gauge will run up to around 200 degrees when in normal traffic, it will drop to around 185 when moving in 40-50 mph traffic. I have previously had the car run around 160-170 as I run a 160 degree thermostat.

When hot, I have pulled over, popped the bonnet and using an infrared heat gun pointed directly at the thermostat housing I get a reading close to 175-180. Pointing at the rad I get 170-175. Pointing at the water pump housing I see 175-180.

I feel the gauge was damaged when it was pegged full hot. I am about to pull the gaugeand make an adjustment.

What are your thoughts?
DSC04315.JPG
 
I think it's weird that it would read hot after being pegged -- I would think it would read low (based on nothing but my own imagination). But I agree 230% that you can and should have a go at recalibrating it.

I just saw an article on Hagerty about recalibrating a tire pressure gauge, which is essentially what you're going to do.
Calibrating a Mechanical Gauge
 
The needle does not look correct to me in the picture. The needle seems to tapper up to the top and I thought the needle was thicker at the top and then came down to a kinda point. Perhaps the eBay gauge is at fault. I would probably send the original out for repair.

The glass comes off by turning and that would give you access to the needle and you could probably calibrate it yourself in hot water with a good thermometer by bending the needle.
 
Thanks for the comments so far.

So, how accurate do you think the heat gun is? In all honesty, I can tell a hot motor when I am around one...and when this gauge is reading over 200 degrees and I pop the bonnet I sense it is lower than that. My heat gun seems to confirm my thoughts. When aiming at the thermostat housing where the temp bulb is screwed in, should I expect the temperature of the gauge and my infrared heat gun to be the same? Would that also be a good way of adjusting the gauge? By going by the reading of the gun?
 
These gages are simply a curved brass bellows that unwinds as the pressure from the capillary increases. When overheated, they get thrown out of wack and have to be re-calibrated. The brass extends too far and then takes a new "set". I had this happen on my initial engine run on my TR2. I calibrated the first time in the lab (kitchen), under very controlled conditions (Wife out of the house). Not wanting to remove the entire dash to remove the capillary, I did a field calibration with the gage installed in the car. Here is how it works...

1) Remove the capillary bulb from the engine. You will need to lower the coolant level before removing the bulb.

2) Remove the gage from the dash and pull it out about 4-5 inches, which is the range of the loose capillary tube under the dash.

3) Remove the chrome bezel, the glass, and the rubber o-ring. This will leave the face loose in the gage.

4) Mark the approximate location of 212 degrees from the face onto the front of the case (adjust for height above sea level for boiling water at your altitude). Then mark the approximate ambient temp of the day you are working...for me it was about 90 degrees. If it is cold outside, just put it under your jacket and mark 98.6 deg, (assuming you do not have the flu).

5) Now remove the face. You will have to very slightly bend the needle outward to do this. The internals are exposed, showing the bellows, the needle spring, and one of several ways to transmit the bellows motion to the needle.

6) Do whatever it takes to adjust the needle to the ambient mark you made on the case.

7) Boil some water in a pot. Take the pot to the car and dunk the capillary in the almost boiling water. Once the bellows settles, adjust the needle to the 212 deg mark you made on the case.

8) Repeat 6) and 7) until you get perfect readings on the gage when you go from boiling the capillary to letting it cool to ambient. You will quickly get a feel for how to move the bellows or mechanism to accomplish perfect readings.

9) Put the gage back together and in the dash...then reinstall your bulb and add you coolant back in.

I know this sounds complicated, but I accomplished the entire calibration in about 20 minutes. For free.

For pictures, look at this thread, starting at post #100

General TR - Instrument Faces

Here's a little side project Jesus and I have going: We are stuck on the proper part number. Judging from the picture, Smith's obviously produced KPH speedometers. Has anybody out there seen one, and possibly have a picture of one?
www.britishcarforum.com www.britishcarforum.com
 
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Awesome, just the info I was hoping for...

Will report back in a few days!
 
Done!

Went by the detailed post; thanks CJD....the gauge is cheap and made in China with a Made in England sticker on the face of the dial...
smiling smiley


Brought out a cup of boiling water, dunked the bulb into it and made a mark on the dial face where the trim ring goes. Using some needlenose pliers I was able to manipulate the needle down a little to the mark.

Here are before and after....the after gauge reading doesnt show that well as I was driving and couldnt get the phone farther away, but you can see it is vertical, the needle falling a little below the 185 degree mark...it is currently pretty warm here today, about 83 degrees out..

Once back home I let it idle for quite a while....the fan was on the whole time and the gauge is reading about 180 degrees.

Sorry about the pics...they loaded fine on another forum, not sure why they all flipped over??
 

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I was pretty sure it came off Ebay....but unfortunately we have moved and are renting where we are now and a lot of paperwork is in storage, so I cant find the seller until I get to those boxes! None of my searches on my Ebay acct showed anything however.
 
Rick_Thompson said:
So, how accurate do you think the heat gun is? ... When aiming at the thermostat housing where the temp bulb is screwed in, should I expect the temperature of the gauge and my infrared heat gun to be the same?
Click to expand...
FWIW (since you've already fixed the issue, yay you!)

The metal of the housing will insulate the heat somewhat, but OTOH after a long drive it will have heat-soaked to the point where it should be close to the temp of the water, maybe 10 degrees difference. Certainly not an error of 50F. I like to spot-check against the radiator tank and the upper rad hose. The three materials will insulate and radiate differently, but IIRC I usually get readings that don't vary by more than 5-10F.
 
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