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Calibration of Smiths temperature and fuel gauges

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Tonyfixit Avatar
Tonyfixit Tony M
Duncan, British Columbia, Canada   CAN
This could be considered my third post on Solving irregular temperature gauge readings.

Over the years many owners complain that their car is running hot and often go to extreme lengths and expence in an effort to just get a 'normal' reading on their temperature gauge.
I feel in many of theses cases the fault lays not with the cooling system, but with one or more of the three components that work together to give us a 'somewhat accurate ' gauge reading.

In this post I will be looking at the gauge itself, and hope to give members an idea of how it works and how to test the calibration, and how if necessary, how factory calibration may be restored.

I would very much like to thank Mike Leisner for donating two gauges for this project. I also wish to apologize for my delay in posting. This was due to my first set of photos turning out very poorly, and me having to re-do them.

I also want to point out that the Temperature gauge and the fuel gauge are internally exactly the same apart from the gauge face plate. So the information in this post can be used for either gauge, regardless of it being fuel or temp.

First off, if we look closely at the face of our gauge we will see 3 sets of 2 tiny dots on the dial (see photo 1)
These are the factory calibration marks and they correspond to each of three 'Test Voltages' that are applied to the rear terminals of the gauge.
If a voltage of 2v is applied to the terminals, we should see a gauge reading where the needle falls between the first set of calibration dots.
Likwise, if 4.8v is applied, we should get a reading at the second set or middle dots.
Finally, if 7.6v is applied, we should have a reading pointing at the third set. This is factory calibration.

Photo 3 shows my setup for applying these test voltages. A suitable power supply is regulated by a potentiometer
and regulated to each of the 'test voltages' In my case I am useing a Buck stepdown voltage stabilizer as a power adjustment, which is overkill. This would have been even better if I had bought the type with a didgital LED voltage readout (they cost about $2 on ebay) But I have had to add my multimeter into the circuit to get a voltage reading.

As you can see, this gauge reads just a little too high at the middle dots.

If we now look at the back of the gauge we see two holes Photo 2 (they may be covered by cork plugs) that allow for a calibration tool. DO NOT TRY TO ADJUST THESE WITH A SCREWDRIVER! to see how these work we must look inside the gauge.

First we remove the bezel. Typically this gauge has seen better days, so it was necessary to ease the tabs on the back of the bezel before it could rotate. When rotated the tabs will line up with slots and the bezel and glass can be removed.
Them we must carefully remove the faceplate which is in two sections. This one needed a little WD40 applied with an artists paint brush and a fine blade to pry it off.
Next we remove the two fasteners that secure the wire connectors to the back of the gauge. There are a number of washers an insulating tabs here, keep them in order.
Once removed the needle mechanism can be taken out fron the front of the gauge body.
Yes, it's a little rusty. A touch of WD40 on the adjustment arms and pivots MAY help to make calibration adjustment possible.

It will be seen the mechanism is quite delicate, and the adjustment arms do not allow for a simple rotation from the holes on the back of the gauge. Movment is more of a side to side action and I suspect a special tool was used at the factory. However, we are on the inside now, so a slight adjustment will be made from here and hopfully the rusted arms can be made free enough to make a final tweak possible once the gauge is back together again.

More photos later today. :-)



Edited 1 time(s). Last edit at 2016-10-10 10:33 AM by Tonyfixit.

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Tonyfixit Avatar
Tonyfixit Tony M
Duncan, British Columbia, Canada   CAN
I hope to restore this gauge to a half reasonable condition.

More later.....


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TR3driver Randall Y
Confusion, Los Angeles, USA   USA
Where did you find the information on the calibration points? I'd really like to see that document, if possible.

FWIW, this tool works quite well for moving the adjustments without disassembling the gauge:

The idea is that the larger turned portion engages with the hole in the back of the gauge, while the smaller round tip engages with the slot. Then when you turn the tool, it levers the slot sideways. Very controllable, much easier to make tiny adjustments.

Another important point: it takes a long time for the gauge to move to it's final reading when changing the applied voltage (or current). You need to wait several minutes for it to settle.

And finally, this information does not apply to all Smiths gauges. For example, the fuel gauge on a TR3 is a "balanced" movement that works (and is adjusted) completely differently.

Here is a document that some might find interesting, listing some of the different temperature transmitters used
https://drive.google.com/open?id=0B2H2NJt34OffeGxHNkhnWmFHWnc



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

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Tonyfixit Avatar
Tonyfixit Tony M
Duncan, British Columbia, Canada   CAN
Randall, I have no documentation for the calibration points, they were just pulled off several sites on the net.
But the voltages DO match calibration points on three 'known to be good' gauges I have tseted.

I will soon download more pictures I have taken while working on this gauge, including a tool I made from a 3/16" rod with an off centre peg for adjusting these gauges. But in this case, and in the other gauge I worked on, NO WAY would such a tool move the adjustment arms. I resorted to putting the gauge in a vice and using a srewdriver and hammer to free them. Final adjustment when they were free was with the tool.

No, these methods are not for all gauges.

I will eagerly study your info on senders/transmitters, as I feel this is key to getting logical readings on our cars.

Thank you.

PS, I have read your attatchment. Do those numbers accurately cross reference to any units readily available today?



Edited 1 time(s). Last edit at 2016-10-10 12:56 AM by Tonyfixit.

scardini1 Avatar
scardini1 Gold Member Jim Moscardini
Great Mills, MD, USA   USA
1968 Triumph GT6 "Rocinante"
2003 Jaguar XKR "Kitty"
I just read that Bulletin. Damn. So there were FIVE different 10v transmitters available in 1964. There may have even been a couple more by the time my car was built in 1968. How the hell am I supposed to figure out which is the correct transmitter for the MK1 GT6 Temp Gauge?

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
In reply to # 1405544 by scardini1 I just read that Bulletin. Damn. So there were FIVE different 10v transmitters available in 1964. There may have even been a couple more by the time my car was built in 1968. How the hell am I supposed to figure out which is the correct transmitter for the MK1 GT6 Temp Gauge?
FWIW, I have an original factory GT6 parts catalogue, first edition but with 4 amendments (last one dated 9/67), that lists P/N 137705. Also has TT4802/00 written in by hand.
And, a factory GT6 Mk-3 catalog that lists the same 137705.

Seems like pretty good evidence to me.

Sorry I don't have them scanned. My new scanner is NOT being cooperative!

Tony disagrees with me, and I don't mean to steal his thunder (or thread); but my opinion is the best route is to calibrate the gauge you have to the sender (and VS) that you have.

Also FWIW, I have a much later supersession list, supposedly produced from information supplied by British Leyland (but undated), that lists the 137705 as superceded by 150843, in turn by C40106 and then by GTR-108.

It also claims 131062 was superceded by 134435, which in turn was superceded by 131062. Make of that what you will! My take is that it was actually BL that did at least part of the "Oh, that's close enough, sell them that one." process.



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild



Edited 1 time(s). Last edit at 2016-10-10 05:20 PM by TR3driver.

scardini1 Avatar
scardini1 Gold Member Jim Moscardini
Great Mills, MD, USA   USA
1968 Triumph GT6 "Rocinante"
2003 Jaguar XKR "Kitty"
Wow... what a supersession. Talk about voo-doo. I might as well break out a Ouija Board. And I'd bet the BL "close enough" isn't far from the truth for the BL cars.

The bulletin mentioned a maroon insulator. Mine is black. I think I got it from Nigel at SpitBits. Anyone know the applicability and temperature vs. ohm specifications on that one?

Just to complete the picture, I'm also running an AC Delco alternator putting out 14v consistently and a solid state voltage stabilizer that seems to do a good job at holding 10v to the gauges.

Great info here guys. Thanks.

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Tonyfixit Avatar
Tonyfixit Tony M
Duncan, British Columbia, Canada   CAN
Quote: "Tony disagrees with me, and I don't mean to steal his thunder (or thread); but my opinion is the best route is to calibrate the gauge you have to the sender (and VS) that you have. "

Actually we are not necessarily in disagreement, as you may find when I sum up these series of threads.

What I have been trying to do is find a baseline for each of the three components (Voltage stabilizer, gauge and sender unit) Plus, the actual temperature of the coolant in our engine. If none of these components proove to be outright faulty, some form of adjustment to one or other components may be requires to give a logical, informative gauge reading.

I started this series looking at the effect two "suitable" sending units had on our gauge readings v actual coolant temperature. The differance was sufficiently significant to render the gauge virtually useless (other than you might notice the gauge needle rising higher than it's arbitrary normal)

I have tested a few sender units sold by various vendors, some of the senders were not even marked with a part #.
Often the sender units advertised list cross referance to often 12 or more part #'s. But I am pretty sure that many of those listed offer differant degrees of ohm resistance at any given temperature.

It may prove OK to adjust gauges the limited degree of adjustment to suit a mavrick sender unit. But I would want to make sure the 'adjusted gauge' still gives useful information throughout the often used range of the dial.

To my mind, other options MAY prove easier (than removing the gauge) and give more accurate (meaning informative) readings throughout the range of the gauge.

These options 'may' include: Fitting a variable resistance between the sender and the gauge.
Adjusting input voltage to the gauge. OR Finding a reliable, numbered, sender unit that actually works with the other two components as Triumph intended.

Tonyfixit Avatar
Tonyfixit Tony M
Duncan, British Columbia, Canada   CAN
Some more pictures. I decided to improve the gauge cosmetically while I had it apart.

A 21/4" 'O' ring replaced the crusty seal in the bezel.

The end of a pop bottle repaired the green bulb filter that was burned through.

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Tonyfixit Avatar
Tonyfixit Tony M
Duncan, British Columbia, Canada   CAN
My version of the adjustment too made from 3 1/6" rod.

The finished gauge in 'serviceable condition.


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tapkaJohnD Avatar
tapkaJohnD John Davies
Lancaster, Lancashire, UK   GBR
Thank you, Tony, a most informative thread!
I've posted a link to here on the UK Sideways Technologies site so other Triumpheros can enjoy.
JOhn

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