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MG MGA - water temp sender

The sensor on the end of my water temperature pipe with the curly wire round it which screws into the block has melted and therefore needs replacing. Moss & Brown & Gammons only sell complete instruments. Any one know who will sell me just the sensor end with or without pipe? Do I need a complete pipe with curly wire? Can one join bits of pipe together. (In that case I could rob my morris minor of one- a bit unkind)What's inside the pipe?. I can buy an electrical transmitter but I guess that won't do.
H L Davy

It's what's inside that counts, indeed!

In the pipe is ether (I think) and that acts as the capillary medium, transmitting the "reading" of the sender.

So unless you have the special vacuumchamber that reconditioners of these guages use, you must buy a complete unit or have it repaired professionally.

The curly wire protects the fragile pipe and may prevent it from buckling.

BTW: how did the pipe melt?
Willem vd Veer

Hi HL - I've heard of people damaging the sensor when removing it especially when it has become corroded - but not of it melting. You will need to have the sensor replaced professionally - I am sure someone on here will let us know of a reputable place to have it repaired in UK.
Cam Cunningham

Cam - a long story but i tried to nurse the car home when it was losing water and it overheated badly. Willem - guess you are right. Have read suggestion in the archives that one could freeze the bulb of the new sender but I am dubious about that, also don't know how it connects at the gauge which of course cannot see until I remove it anyway.
H L Davy

HL, Assuming you've still got the original bulb in the head, which is brass, it should be OK. The capillary tube is soldered into the brass bulb and also soldered into the dashboard gauge. Perhaps the temperature of the head went high enough to melt the solder although I'd be surprised because it would have to reach nearly 200 deg C.

Assuming you can remove the bulb from the head I think I would first test it by putting it in hot water to see if you have any deflection on the gauge. If you do have then the gauge will be alright. If not then you will have lost the fluid in the bulb. Almost certainly the leak will be at the head end so I think I would cut the bulb off about 4-6" from the end with a fine saw. Then heat up the brass bulb to melt the solder and remove the short piece of capillary which you can discard. Clean up the other end of the sawn through capillary, making sure you've not sealed over the tube during the sawing and solder this end into the bulb. To refill the bulb drill a small hole in the tip and inject 1 or 2 cc of diethyl ether into it. Finally seal over the hole using a soldering iron. Do a final boiling water test to check your work.

For a more detailed description check out the archives under 'temp/oil guage repair?' Good luck

PS I may be able to let you have some DEE if you can't get any assuming it hasn't all evaporated!..................Mike




m.j. moore

Unfortunately the bulb has indeed melted and disappeared! I guess I will have to get another gauge. I do have a capillary type gauge on a bracket in my minor which I either will fit to enable me temporarily to monitor the MGA's temp, or maybe cut the bulb off that unit and refit as you suggest but would one not need to get the DEE up the capillary as well as in the bulb?.
H L Davy

Hi Mike,

I read the 'temp/oil guage repair?' thread and found it fascinating to know that the repair can be a (competent) DIY job.

That had me thinking about the vacuumchamber I heard of and how it would be used.

I imagine that the tube with the guage attached is put in a vacuumchamber with the open end of the tube immersed in the DEE.
Once the vacuum is created by sucking out all the air, the tube will be empty of air which will have bubbled through the DEE.
When you then let in the air, the immersed pipe will "suck in" the DEE. After that you should be able to solder the bulb onto the pipe more easily?

I think I'm going to "lend and modify" the transparant salad bowl that's in the kitchen.......
Willem vd Veer

You do not need to completely fill the capillary. As long as the bulb is full. The DEE exerts a specific vapour pressure dependent only on temperature. A small amount of air in the capillary would have a negligible effect.
Art Pearse

I used to make thermometers with my students - just heat up the glass tube and then put the open end in the fluid and it is drawn in as tube cools. Then heat up the part of the tube with no fluid in it so that the air is driven out and then seal the tube - I guess that is how it is done with the gauge but with more precision - no vacuum needed.
Cam Cunningham

Art,

I was just musing how it could be done with a vacuum. In my "plan" I was hoping that the capillary would contain the required 2cc's DEE and that it would flow back in the empty bulb after soldering.
Willem vd Veer

The brass bulb will not have melted - you engine surely did not reach the temperature of 900-940C that would be necessary (if it did it must have been mighty hot in the passenger compartment)- it has simply fallen off and disappeared into the head passageways.

If you cut the one off your Minor you will have two gauges to repair. Just bite the bullet and get the original gauge repaired or buy a new one. The heathrow MG guys sell new ones outright
dominic clancy

Willem, It's possible that someone has used a pumping system to evacuate the gauge and capillary (after removing the bulb) and then backfill with a solvent to clean out the system. If a gauge has been left for some time with a ruptured capillary it's feasible that some oil may have found its way in. This would be dissolved by any ether subsequently used to refill and would almost certainly lower the vapour pressure giving a low reading on the gauge.

But thinking back to when I repaired my first one (an early 1950s model) I can remember that the bulb had a hole in the tip which had been loosely plugged with a brass pin and soldered over which was clearly used for filling.
However, the second one I did (post 1960) the brass bulb had no hole in the tip. So maybe later on Smiths decided to fill the tubes with ether from the gauge end. To do this they may have set up an evacuating system to get the ether in because the capillary is very small bore. It would be a lot easier finally soldering at this gauge end with it being so remote from the liquid.

As Art says the ether doesn't need to be in the Bourdon tube itself or the capillary and even the bulb doesn't need to be filled completely as long as there is sufficient liquid for there to be some left in the system at the maximum operating temperature.

HL, Before you decide to use the bulb from your MM gauge make sure it will fit into the MGA head because if I remember rightly the original MGA bulb was quite a close fit in mine........................Mike
m.j. moore

I agree with Dominic. A new gauge is $150 or yours can be restored both functionally and aesthetically. The sensor is not a separate part, the bulb is part of the gauge. It takes specialized tools and knowledge to repair it. Bite the bullet do it right buy the part.
R J Brown

Good discussion and fix on TD/TF board here:
Thread: Water Temp Gauge need repair 06 June

FRM
FR Millmore

This thread was discussed between 15/06/2011 and 17/06/2011

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This thread is from the archive. The Live MG MGA BBS is active now.