Thermal switch (URP1126) not playing properly

Technical MGB discussion
Vic Butler
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Re: Thermal switch (URP1126) not playing properly

Post by Vic Butler »

Evening Charles
I checked my BL manual wiring diagram and there is the voltage stabilizer with a green wire and a light green wire.
Obviously I missed it, thanks. I thought it odd there wasn't one.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
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Charles Farran
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Re: Thermal switch (URP1126) not playing properly

Post by Charles Farran »

Phil,
I note that you have a 1979 MGB. There was a change in the otter switch for the fan from URP1126 to KTP9003 sometime during the 1979 model year (from GHN/D5 511291). I don't know if this would have any implications for you as to when the respective ones were designed to cut in. Certainly the way they are fitted are slightly different . Is the correct one fitted to your car?
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Charles
1980 Roadster
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Charles Farran
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Re: Thermal switch (URP1126) not playing properly

Post by Charles Farran »

Hi Vic,
Apologies - i hadn't seen your last post when dealing with mine.
Hope all good with you.
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Charles
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Re: Thermal switch (URP1126) not playing properly

Post by Chrome or rubber »

Many thanks to all.
I have checked the wiring from start to finish and all is good, I will today be checking the switch in boiling water. Once I establish if any of the switches are good/bad I can progress to coolant issues.
As far as I can tell the coolant is where it should be and flows around the circuit as it should. However, it is tricky to determine if there is air trapped in the area around the 'otter switch....
With regard the switch itself, I have now ordered the OEM product made by Lucas from a different company to the last unit which came without manufacturer's details. The Lucas version states on the website it is a later version to supercede the URP1126 switch, it didn't explain why the change but confirms closure at 92°C and reopen at 87°C.
I shall inform the results via this thread when the task is complete... fingers crossed we will have a result. At that time I shall inform the group where I obtained the switches so that you are aware of those vendors.
Thanks again, most helpful.
Vic Butler
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Re: Thermal switch (URP1126) not playing properly

Post by Vic Butler »

The otter switch on my 77;BGT is pushed into the radiator and I believe the later ones had a clip to hold them in, hence the different part number.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
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Charles Farran
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Re: Thermal switch (URP1126) not playing properly

Post by Charles Farran »

Yes you are correct Vic,

If the later one to URP1126 means a KTP9003 type then Phil, will also need a stepped sealing ring KTP9005 & Clip KTP9006.
Incidental note re possible further air pockets - ensure the heater control valve is open when filling the block via the thermostat hole with coolant.

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Charles
1980 Roadster
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Re: Thermal switch (URP1126) not playing properly

Post by Chrome or rubber »

This is all great information and most useful.
Charles the info on heater valve position is also something I hadn't noted. I have owned the car for only a few weeks and although I've done quite a few miles in that time this is the only known issue on the car. But, I have no history and while some parts (radiator its 'otter switch and other items such as hoses etc.) are new (ish) i know nothing about what has previously been done or why these parts had been replaced.
Already I am learning fast...
The new switch is the Lucas SNB712 and it replaces the URP1126, the BHM7287 and a 71276009, according to the documentation. It will come with a seal... I do not believe it comes with and retainer clip.
I will keep everyone informed as I get to the bottom of this, so that the thread is closed and completed properly... with any luck in a day or 2.
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Charles Farran
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Re: Thermal switch (URP1126) not playing properly

Post by Charles Farran »

Phil,

If the car didn't come with the A5 sized handbook , it is worth getting hold of one. It covers all the main routine service areas of the car. The later ones (covering the last couple of years) are harder to come by as i think only the ones previously are available as faithful copies. The MGB Repair Operation 1978 AKM4070 (Originally a BL production) is invaluable if you intend to do some serious work on the car. (You can freely download this from the internet rather than buying a hard copy). I personally prefer it to Haynes , but refer to both etc.

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Charles
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Re: Thermal switch (URP1126) not playing properly

Post by Chrome or rubber »

Thanks Charles,
I plan to do as much work myself as possible and essentially if that doesn't involve heavy lifting gear, welding or painting then I shall be doing it myself, so any documents that help me on the way will be invaluable.
I love the free download too, seems the best option.
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Re: Thermal switch (URP1126) not playing properly

Post by Chrome or rubber »

Thanks to everyone i believe I have now isolated the problem. All my switches have now been tested and work at the prescribed temperatures plus or minus 2 degrees (92C to close 87C to reopen).
So I went in hunt of air and found it to be trapped within the heater, having cleared the air the system seems to flow better.
I left the car to idle and watch the temperature climb worryingly close to the red line limit and just as the needle touched the edge of the red mark, boom the fan sprung into life. It took a few minutes to show an indicated reduction and the fan cut. The fan cycles very close to the red mark limit, that seems awfully hot to me, but I have become so used to modern vehicles where temperature doesn't move from a middle scale point, I have lost touch with watching games.
I'm now happy that the fan will cut in, I can't say I'm happy it waits so long, as the amount of indication time remaining to let you know all is well or disaster is about to occur is insufficient.
I may place a jewel light in the cockpit to let me know when the switch closes so that I am reminded to keep an eye on the temperature, or to place a different switch that will cut in much earlier....
Do other members find this steep climb in temperature when stationary a worrying situation?
Thanks again everyone it seems fixed, even if I'm not 100% happy at the temperature indication situation
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