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MG TD TF 1500 - Moving the water temp sender unit on a TD

John, re. Infra red gauge. What temperatures did you get at various places?
Mike TD29330
M Lees

While waiting for the last parts to arrive (timing chain) I'm considering one last minute job.

My 51 TD has the water temp sender attached to the radiator (unlike the TF where it is on the thermostat elbow). Because my engine overheated due to blocked water passages in the block, I would like a better placement for the sender unit - all it really tells you is the temp of the steam of the radiator; the block temperature, if blocked, could be very different.

It seems like the most likely place to move the sender would be to the plate at the back of the head. Mine has already a fitting brazed to it (half inch, used originally for the heater pipe, but I've switched it and blocked it off).

According to the archives, "The thread for the temperature gauge sending unit in the head is 5/8-11 (American thread)." (I think Dave Dubois posted this in the archives)

I'd just like to confirm this fitting size and ask if anyone else has done this? The only downside I see is the need to carefully coil up the extra tubing I will have...
Geoffrey M Baker

I don't think that is a good spot for accurate measurement. Scroll through this: http://www.mg-tabc.org/library/the_xpag_cooling_system.pdf The water there would be the coolest in the entire system, as it goes from the bottom of the radiator, to the water pump, then along the outside of the block and then upward. You need to measure at the front of the head and/or below the thermostat. George
George Butz

Can anyone tell me if a mgtf thermostat elbow will also fit the MGTD block and the TD thermostat? If so, that would allow for better temp readings.
It might be the coolest part of the engine, George, but measuring from the back would still be more accurate than the current placement, where it is measured at the top of the radiator, and not any part of the block itself.
Geoffrey M Baker

I drilled and tapped the elbow below the thermostat for a pipe nipple. I used some sealant on the threads. Did this about 5 years ago.
Mort


Mort 50 TD

On my MGB the temperature sensor is a thermocouple or a thermistor or some type of electrical doodad located just below the thermostat housing (functionally in the same place as Mort's) with a single conductor wire running to the temp gauge at the dash. I suspect that the gauge is simply a voltmeter calibrated to reflect in degrees the voltage output of the sender. It would seem that one could drill and tap (as Mort did) for such an electrical doodad and either mount a second temp gauge so that one could compare the readings above and below the thermostat or just put in a warning device (a buzzer or a light - maybe use the rarely functional low fuel light) to let one know that the water temp in the block has exceeded an acceptable level.

Perhaps even better, have one of the instrument restoration guys replace the innards of the combo oil and temp gauge with workings that sense voltage instead of the temperature of ether in a long tube. In fact, on the MGB, the oil pressure tube also goes to an electrical doodad that converts pressure to voltage so getting to the instruments in the dash is simply fishing wires instead of trying to route stiff and unforgiving tubes. Perhaps the instrument restoration guys could replace both the temp gauge and the oil pressure gauge with electrical innards, save the face and needles so that the combo gauge still looks original and no one (i.e., the originality police)would be the wiser but the whole thing would be much easier to work with. Just musing but I may actually do this since the temp part of my combo gauge is non-functional. Jud
J. K. Chapin

Geoff, you're still just measuring the temperature of water. It you are trying to indicate the temperature of a solid material, at a specific location, you're going to need a different type of system. Bud
Bud Krueger

I've thought about that, Bud; you can buy systems that use a thermocouple to give you a direct reading on the head, which isn't a bad idea.
My point, though, is that measuring water temperature from any place except the block itself is a method prone to failure, because as your block fills with crud and the water stops effectively cooling it (as happened to mine) the water sensor on the radiator is going to keep telling you everything is fine... at the radiator. As the water is no longer moving into the block enough to cool it, the block is getting hotter than the water system, but your meter won't tell you that.
A sensor at the thermostat elbow, while better than the current stock TD placement, still isn't giving you information from inside the block. So it's subject to the same failure. A thermostat at the back of the block might give you a better reading, the reason being that if water is moving through the block effectively, it will give you a reading the same as the thermostat elbow, less a couple of degrees lost to cooling; but if the water passages inside the block start accumulating crud and the water stops moving as efficiently, the sender at the back of the block will start reading higher and higher temperatures as the block overheats and the head temperatures increase.
A sender located on the thermostat elbow would still be in the siphon loop at the front of the engine, and the existing bypass mechanism could keep that sender reading cooler temps even as the block heats up when water isn't getting to it. In short, the water pump would keep moving water in a smaller loop from radiator to pump to bypass pipe, without pushing water through the (blocked) head, and giving a false reading.
Just my two cents.
Tapping the existing thermostat elbow for a sender is a good idea; it's just that I see some small possible advantages to putting it at the back of the head, too.
Geoffrey M Baker

Experimental testing and comparison of temperatures at different locations could be done with one of the relatively inexpensive, and common, infrared handheld temperature measurement devices. I recently got one and have been having all sorts of fun measuring temperatures here and there and everywhere.
John Quilter (TD8986)

Mort, Your relocation of the temp sender to the elbow interests me. What fittings did you need to do this and where did you obtain them. The thickness of the elbow seem marginal to support the threads. Did you weld in a boss and a fitting? And what did you use to plug the radiator hole?

John Quilter
jquilter[at]peoplepc.com
John Quilter (TD8986)

John,
Since I did this 5 years ago you are really taxing my memory.
Alas, I don't know how to weld.
The thermocouple fitting was a standard pipe fitting size. I used a brass plug to cap the radiator with some thread sealant. I bought a coupling to fit the thermocouple. I used a brass nipple that fit the coupling and drilled and tapped the elbow to fit. I used some thread sealing compound on all the joints and have never had a problem.
The elbow wall is thick enough for this application. There is virtually no load on the joint.
That's the best my memory can do.
Good Luck,
Mort




Mort 50 TD

Another view


Mort 50 TD

Mort,

Thanks for the additional detail and photos. That may become a winter project for my car. What temperature thermostat are you running?

John
John Quilter (TD8986)

Now you are really stretching me beyond the scope of my memory. I will have to check old invoices.
I found the invoice where I bought 434-178 "Housing & Modern Thermostat" but no temperature info.
Moss sales says it should be 160 degree????
Mort 50 TD

John, re. Infra red gauge. What temperatures did you get at various places?
Mike TD29330
M Lees

Will have to check the next time I fully warm up my TD, but it is running a 180F thermostat which usually only shows about 70C on the gauge which is connected to the stock location in the radiator upper tank.

John Quilter (TD8986)

Someone (now deceased-Gordan?) showed a series of temperature readings a while ago. Bud
Bud Krueger

Reactivated for John Quilter
Mort 50 TD

This thread was discussed on 09/09/2015

MG TD TF 1500 index

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