Thermal switch (URP1126) not playing properly

Technical MGB discussion
User avatar
Chrome or rubber
Posts: 9
Joined: Sun May 09, 2021 4:21 pm
Forename: Phil
Surname: King

Thermal switch (URP1126) not playing properly

Post by Chrome or rubber »

Helo, I am very new to the forum and also to MGB's. I recently purchased a 1979 B, which seemed on the face of it to be working on all systems.
Under the bonnet there appears to be a new radiator, complete with new thermostat switch (pt No: URP1126), however, when stuck in traffic the temperature climbed to near the red and thankfully the queue began to move before I had a need to shutdown the engine.
I disconnected the cable from the switch and shorted it out, to find the fan does indeed run and in the correct dirrection.
I checked that the coolant level was correct at the thermostat housing, which was close enough.
I found on other forum questions that these switches were prone to failure due to the high current, so with a freshly purchased original spec URP1126 switch and a suitable relay, I rewired the connections so that switch simply acted as a trigger and power was vdirected via the relay to the fan, which was bench tested as OK and the fan would run, should the thermal switch close.
However, after warming the engine to near the redline, the fan still failed to spring into life.
Currently I have the switch by-passed and the fan runs all the time the ignition is on, which works to keep the engine cool (too cold in fact) but is clearly not good for the longevity of the electric motor.

Are these switches so unreliable that a brand new one may exhibit failure straight out of the box?
Is there a better way to wire this up so that my fan works as required but without the unreliability (if that is the problem) of the URP1126 switch?

Thanks
Phil
Vic Butler
Posts: 606
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: Thermal switch (URP1126) not playing properly

Post by Vic Butler »

The switch on my 1977 BGT is still working.
Before you get a new switch, , check the contacts in the old switch. Spray an electrical switch cleaner in them then try it.
The switch is called an otter switch.
You could find another sensor that fits in the top hose but I've not heard of the original switches being unreliable but I don't know where the new ones are made. It would be worth replacing it with a new one if cleaning the contacts doesn't work.
When the Kenlow electric fan on my Land Rover stopped working it was down to a dirty contact in the inline fuse.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
User avatar
Chrome or rubber
Posts: 9
Joined: Sun May 09, 2021 4:21 pm
Forename: Phil
Surname: King

Re: Thermal switch (URP1126) not playing properly

Post by Chrome or rubber »

Thanks Vic,
But that's the rub, I have installed a new switch obtained from MGBHive but even the one removed is as new.
My understanding was that they fail internally particularly if the motor is sticky on initial spin-up. By installing a relay into the circuit the current flow is unlikely to harm the 'otter switch. So, I have done that and fitted a new switch... I can't see what else can be done.
I only allow the temp gauge to enter the very edge of the red zone, never hotter as I would have thought the fan should cut in way before that... 3/4 scale on the pointer would seem a sensible place to start the fan going. I may be wrong of course.
User avatar
Chrome or rubber
Posts: 9
Joined: Sun May 09, 2021 4:21 pm
Forename: Phil
Surname: King

Re: Thermal switch (URP1126) not playing properly

Post by Chrome or rubber »

I should add i have no history for the car, I only purchased it a month back and I don't think the cooling fan has run since I bought it other than when I have bypassed the otter switch and it spins well and fast... currently I leave it set to run whenever the ignition is on.
Ian Fozzard
Posts: 615
Joined: Fri Mar 27, 2015 8:18 am
Forename: Ian
Surname: Fozzard

Re: Thermal switch (URP1126) not playing properly

Post by Ian Fozzard »

Phil, autoelectrics are not my strongest suite, but could it be that you are by-passing something else as well as the switch when you get the fan to operate? Perhaps a fuse or a broken wire somewhere?
Can you test the otter switch in a bucket of very hot water?
How close is the otter switch to the sensor which is indicating the overheating? They may be reading very different temperatures?
Finally, could it be that your temperature sensor and/or gauge are reading incorrectly and misleading you?

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
User avatar
Chrome or rubber
Posts: 9
Joined: Sun May 09, 2021 4:21 pm
Forename: Phil
Surname: King

Re: Thermal switch (URP1126) not playing properly

Post by Chrome or rubber »

Ian, you raise some good points.
When I suggest by-passing the otter switch I base that on the following: The switch is nothing more than an on/off gate... I removed the connector that links the 2 sides of the switch and shorted across the connections to simulate closing of the otter switch and the fan runs. I am confident that the switch is therefore not closing.
What I have not looked into as thoroughly is where the guage gets it's information from and as you say, the 2 temperstures could be quite different. that said, if I drive the car with the otter switch by-passed forcing the fan to run constantly, the guage never goes above the midway point. So clearly somewhere the fan is not being asked to run, when it should.
It has to be something simple... I just cannot find it.
All that said, I have forgotten the absolute basics of running a simple check of dropping the otter switch into some boiling water to see if it then actually closes... so thank you very much for offering me that simple option, I will get at it as soon as time permits and I cannot believe I didn't think of that myself.
So cool, I feel i'm getting somewhere now :-)
Vic Butler
Posts: 606
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: Thermal switch (URP1126) not playing properly

Post by Vic Butler »

I've had a look at the wiring diagram. The radiator fan is fused to the in line fuse that has a brown and white wire going in from the rear and a green wire coming out to the front of the carhis wire goes to the otter switch and the black and green wire to the fan itself.
As the new switch doesn't work you should go back to the supplier.
You could bridge the wires going into the otter switch with wires going to an override switch on the dash. This could be switched on if the temperature rises too much. I don't know what temperature the otter switch cuts in. On mine it doesn't quite reach the red so I presume the set temperature is common to them all.
As the fan runs if the switch is bridged then the fault appears to be the switch but what if the temperature gauge is reading incorrectly. The only way to try is to see if the fan cuts in when the gauge reads high . It does seem odd that 2 switches don't work. It could be that at some time the temperature gauge sender had been replaced with one that doesn't match the gauge.
Normally I would suspect the voltage stabilizer that feeds a constant 10 volts to both fuel and temperature gauges but there isn't one.
Now, this will probably appear pointless but check the radiator cap on the expansion tank. These cars have a higher pressure than earlier ones and the radiator cap should be a 15psi. If it's not then the water will boil at a lower temperature which could be why the switch isn't cutting in. Just a thought and if the cap rubber looks worn then replace the cap with another 15 psi.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
Ian Fozzard
Posts: 615
Joined: Fri Mar 27, 2015 8:18 am
Forename: Ian
Surname: Fozzard

Re: Thermal switch (URP1126) not playing properly

Post by Ian Fozzard »

My BGT, being '72, doesn't have an otter switch, in fact I hadn't heard of them before this thread.
Does anyone know why they have this name?

Perhaps it's because the switch operates as the coolant gets 'otter😁?

Or is it a reference to that aquatic mustelid we have in this country?

It could also be because the switch was invented by Dr Otter, that well known German automotive engineer?

Answers on a postcard please,

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
User avatar
Charles Farran
Posts: 359
Joined: Fri Oct 31, 2014 11:08 am
Forename: Charles
Surname: Farran
Location: Warwickshire

Re: Thermal switch (URP1126) not playing properly

Post by Charles Farran »

Vic,
There is a voltage stabiliser within the wiring. I know because i have checked the wiring to mine when i had a problem with the fuel gauge & checked out the contacts there before identifying the problem as a corroded contact at the fuel tank sender end. Easily seen on the late Haynes wiring diagram for late UK MGBs (sheet 33 i think) of the Auto- Wire set of wiring diagrams freely available on the internet. (Also item 19 within the BL wiring diagram of the ROM 1978 AKM 4070). The voltage stabiliser is used for both the water temp & fuel gauges.
Phil,
My fan doesn't cut in until the pointer gets close to the red & usually only cuts in if i am stuck in traffic for a long time.

I appreciate you need to check that the otter switch itself works ok but consider the following:-

When the engine / coolant is cold have you also checked that the expansion tank is approximately half full. You can get some funny readings on the temp gauge if the overall coolant level is too low which may also affect how the otter switch works & when?
Incidentally the coolant temp sensor is on the side of the block just below the thermostat housing - always worth checking that the wire contact there is clean.
I'm not sure how long you have had the car & whether or not the cooling system has been flushed out including both the block & the radiator. Is the thermostat operating correctly & consistently at the appropriate temperature?

Cheers,

Charles
1980 Roadster
User avatar
Paul Scott
Posts: 224
Joined: Mon Jan 08, 2018 8:48 pm
Forename: Paul
Surname: Scott
Location: Milton Keynes

Re: Thermal switch (URP1126) not playing properly

Post by Paul Scott »

Hi Phil,
Welcome to the MGB Register forums.

I'm going to throw this out there. I don't own a 77B but it's sort of in my line of work ethic.

Looking at a 77 Rad the fan sensor is located up the top close to the top hose. I don't know if an airlock could effect how the stat works. Looking it is triggered 87c to 92c and guessing this would usually be immered in the coolant. If by joining the two wires that go onto the stat brings on the fan it is unlikely to be a wiring issue.
You could as suggested hang in boiling water it should switch to a closed circuit. If it does then maybe for some reason its not touching the coolant. I could suggest a different style of "cutin" unit fitted in a different place but I'm not quite sure how to fix it to rad or hose.
We use thermal 'cut in' stats that click on at 65c or 100c and probably somewhere in between.

Unsure if I am being helpful.

Stay Safe

Regards
Paul
1975 MGB Roadster
Webguru for MGB Register
Post Reply