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Old 05-25-2020, 10:25 AM   #1
Lon Jackson
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Default Temp gage acting stange

I have a 51 mercury flathead in my 36 ford pu. The temp gage shows off scale low when I start and drive for some time. After ample time for the engine to come to normal operating temperature, the gage still is off scale low. I return to the shop, and turn the ignition off. The temp gage almost immediately starts to register. The indicator shows a little below normal operating range within a minute or so. I have recently replaced the temp gage and capillary tube. I am wondering if maybe I have a stuck thermostat? of maybe the water pump is not working properly. Any ideas from any of you with experience like this would be greatly appreciated. Thanks, Lon.
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Old 05-25-2020, 10:42 AM   #2
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Default Re: Temp gage acting stange

Is your gauge an original or repo ?
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Old 05-25-2020, 11:34 AM   #3
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Default Re: Temp gage acting stange

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Originally Posted by Lon Jackson View Post
I have a 51 mercury flathead in my 36 ford pu. The temp gage shows off scale low when I start and drive for some time. After ample time for the engine to come to normal operating temperature, the gage still is off scale low. I return to the shop, and turn the ignition off. The temp gage almost immediately starts to register. The indicator shows a little below normal operating range within a minute or so. I have recently replaced the temp gage and capillary tube. I am wondering if maybe I have a stuck thermostat? of maybe the water pump is not working properly. Any ideas from any of you with experience like this would be greatly appreciated. Thanks, Lon.
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Last edited by Kube; 05-25-2020 at 12:46 PM. Reason: I goofed
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Old 05-25-2020, 11:46 AM   #4
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Default Re: Temp gage acting stange

Are we correct in assuming that you are using an original type (non-electric) gauge?
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Old 05-25-2020, 11:56 AM   #5
Lon Jackson
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Default Re: Temp gage acting stange

The gauge is reproduction unit from MAC's. Is a non electric capillary tube design.
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Old 05-25-2020, 12:04 PM   #6
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Default Re: Temp gage acting stange

Don't both the gauge and the sending unit have to operate on the same principle.

51 Merc gauges and sending units are King Sealy and both operate on the ohms principle.
Perhaps I am missing something but,
I doubt if a capillary tube set up will work properly.

Here's a great thread that explains it all.
https://www.fordbarn.com/forum/showt...auge+operation
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Old 05-25-2020, 12:22 PM   #7
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Default Re: Temp gage acting stange

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Originally Posted by 19Fordy View Post
51 Merc gauges and sending units are King Sealy and both operate on the ohms principle.
Maybe I'm just being picky here, but K-S gauges are not resistance gauges/senders, they work off of pulsing the current.
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Old 05-25-2020, 12:34 PM   #8
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Default Re: Temp gage acting stange

In reading his post he has a original mechanical gauge,not electric.So it has a probe with a capillary line and sight glass for the red fluid.
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Old 05-25-2020, 12:40 PM   #9
Lon Jackson
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Default Re: Temp gage acting stange

51 engine but installed in 36 PU with 36 gauges
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Old 05-25-2020, 12:43 PM   #10
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Default Re: Temp gage acting stange

Lon, Capillary tube type gauges have a calibration mark on the glass tube which indicates the boiling point 212 F. This glass tube is adjustable within the gauge face to align with the corresponding temp mark on the gauge face. Yours appears to be adjusted too low, indicating a rise in temp at shutdown from heat soak, which is normal in any system.
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Old 05-25-2020, 12:49 PM   #11
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Default Re: Temp gage acting stange

Take the tube out and put in in a pan of boiling water. It should read hot if it's not close then gauge is faulty. Borrow your wife's good sauce pan for the test.
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Old 05-25-2020, 12:50 PM   #12
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Default Re: Temp gage acting stange

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Originally Posted by ford38v8 View Post
Lon, Capillary tube type gauges have a calibration mark on the glass tube which indicates the boiling point 212 F. This glass tube is adjustable within the gauge face to align with the corresponding temp mark on the gauge face. Yours appears to be adjusted too low, indicating a rise in temp at shutdown from heat soak, which is normal in any system.
Wow! I think I really missed the mark on my earlier response.
Your response seems to make more sense. That is IF the repop sending unit is working properly.

I have heard the repop gauges are not worth spit so if it were my car to diagnose, I'd surely make certain that is working before attempting to adjust the glass tube.
Plus, it (glass tube) would have to be WAY off for the gauge to read all the way below the low mark. I don't see where that would be possible.
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Old 05-25-2020, 12:52 PM   #13
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Default Re: Temp gage acting stange

On the 35-39 temp gauges the glass tube is fixed (base is crimped in the frame at the bottom and non adjustable).The face has tabs that crimp over the frame and not adjustable.I have worked on many of them.
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Old 05-25-2020, 01:08 PM   #14
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Default Re: Temp gage acting stange

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On the 35-39 temp gauges the glass tube is fixed (base is crimped in the frame at the bottom and non adjustable).The face has tabs that crimp over the frame and not adjustable.I have worked on many of them.

Sorry to disagree with you. Look again.
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Old 05-25-2020, 01:29 PM   #15
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Default Re: Temp gage acting stange

I did and here is a close up pic of a 36 gauge.
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File Type: jpg 36 temp gauge.jpg (79.7 KB, 46 views)
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Old 05-25-2020, 03:15 PM   #16
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Default Re: Temp gage acting stange

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Originally Posted by deuce lover View Post
I did and here is a close up pic of a 36 gauge.

That's the head unit that mounts behind the gauge face. Looks a bit different than the '37-'39, but should still be adjustable as mounted on the gauge face. Look for a scribe mark on the glass tube, visible only from behind, to line up with the 212 mark on the gauge face. Also, as Marko39 says, the sending unit in boiling water should line up with the calibration mark.


EDIT: OK, I'll concede that the early units not only look different, but apparently are not able to calibrate as on the '37-'39, that feature perhaps an improvement for '37. There is one more issue involved here, that being that sending units are also specific to model, the depth being different than in other applications, which could affect the reading.
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Old 05-25-2020, 04:09 PM   #17
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Default Re: Temp gage acting stange

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I did and here is a close up pic of a 36 gauge.
I've got two NOS '36 gauges and upon close inspection find the tube is stationary - no allowance for adjustment. Also, the face plate fits in one registered spot.
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Old 05-25-2020, 04:28 PM   #18
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Default Re: Temp gage acting stange

Will the 35-39 temp gauge units interchange with each other?
(Not the year specific faces but the actual working parts & glass column with the red liquid)
I could use a working thermometer temp gauge to accompany the electric one hidden in the glove box

Last edited by johndee138; 05-25-2020 at 04:31 PM. Reason: added text
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Old 05-25-2020, 04:40 PM   #19
Lon Jackson
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Default Re: Temp gage acting stange

Now back to the problem.

Could I have introduced an air bubble into the system such that the temp probe is in air rather than water? Perhaps the the thermostat is also stuck closed so that circulation is preventing the air bubble from clearing
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Old 05-25-2020, 11:05 PM   #20
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Default Re: Temp gage acting stange

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Originally Posted by johndee138 View Post
Will the 35-39 temp gauge units interchange with each other?
(Not the year specific faces but the actual working parts & glass column with the red liquid)
I could use a working thermometer temp gauge to accompany the electric one hidden in the glove box

Not at all. Here is a 37-39,on the right.The frame is entirely different.I have put the 37-39 temp unit (glass tube w/capillary line) in a 35-36 gauge frame many times.The temp bulb on all the 35-39 temp gauges are identical.35-36 mount in the left w/pump and the 37-39 mount in the head with a NPT adapter.Neither are adjustable(meaning moving the glass tube up or down)
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File Type: jpg 36 temp gauge.jpg (79.7 KB, 5 views)
File Type: jpg 38 Temp gauge.jpg (30.8 KB, 5 views)

Last edited by deuce lover; 05-25-2020 at 11:14 PM.
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