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MG MGB Technical - Overheating again

Hi all,
I posted a thread just before Christmas after a catastrophic overheating problem on my 78 B rubbernose. Catastrophic because of my own stupidity. The short version is the temperature switch plugged in at the top of the radiator came out and I lost all my coolant. Refilled it while the engine was still hot. QED stupidity.
After getting a new cylinder head and back on the road just in time for the holidays, the car was running better than ever before. One would hope so with a brand new cylinder head. Even on days where the weather has been over 40C heatwave, the temperature gauge has only risen to the middle and no further.
Now in the last week it has started overheating again. It seems to happen when I am stuck in traffic and falls once I start moving. After a short diagnosis I found the fan is not switching on.
I disconnected the fan at the temperature switch and short-circuited the connection to manually switch it on. Fan works. So I guess the temperature switch is the problem.
Before I race out and buy a replacement, is there a way to test the temperature switch on the bench just to make sure it really is the switch and not something else? It certainly looks like the switch is the problem after shorting the connection and testing the fan. Is that all I need to confirm it?
Thanks in advance.
Dave
D O'Brien

Dave - If you know the temperature the fan is supposed to come on, you can attach a continuity tester or a multimeter set to ohms and dangle the switch in a pan of water with a thermometer in it over a stove. watch the continuity tester and when it indicates that the switch closes, check the water temperature. If the fan is supposed to come on at a temperature over the normal boiling point, mix antifreeze in the proper proportion to raise the boiling point to something above the temperature that the switch should come on. Cheers - Dave
Daved DuBois

Great idea. I knew that being a switch the only states I could measure were on/off but I couldn't think of a way to activate it with sufficient heat, short of using a blow torch.
I'm pretty sure it's dud anyway, but ruling it out will help me sleep better.
D O'Brien

Switch checks out fine on the bench. Very strange. Unless I'm missing something, it definitely wasn't working in the car.

When I start the engine in the morning the temperature gauge is cold. It takes the usual amount of time to reach normal. Then after a short while in traffic it starts to climb slowly all the way to the red line, falling again (more slowly than it climbs) when I start moving. After stopping the engine and turning the ignition on without starting, the fan does not turn on.

Is it possible the thermostat isn't opening and the water in the radiator isn't mixing with the hot engine, therefore not reaching temperature to activate the switch?
D O'Brien

If the gauge is 'stable' when driving then changes are your thermostat is fine. When you "I disconnected the fan at the temperature switch and short-circuited the connection to manually switch it on" did you just short the thermo switch or feed 12V direct to the fan? If latter do you have power to the switch? (Brown/White from fuse block(5) to switch via 'in line' fuse.
Bernie
B Anderson

"Is it possible the thermostat isn't opening and the water in the radiator isn't mixing with the hot engine, therefore not reaching temperature to activate the switch?"

I am less familiar with the "electrical" temp gauge, but I think you should be able to tell if the thermostat is opening. From cold the gauge climbs to about the usual position. It then drops back briefly(as the thermostat opens) and then climbs back to the usual position. All this can be done at idle. If the thermostat was stuck shut you would overheat very quickly.

If the fan does not turn on with the ignition just after stopping from hot, then something is wrong with the switch / fan circuit that would appear heat related. Did your "pan" test show the switch turned on and off at the same temp or did it turn on at one temp but not turn off to well below that temp?

Replacing the switch will eliminate one item (at a cost) but worth checking both supply and earth connections. One way to test is to make up a jumper lead, wait till the car is getting to "overheating", (and the switch has not operated) fit the jumper lead across the switch terminals, and the fan should work. If not it's something other than the switch.

BUT- are you actually overheating? Are you losing coolant? Or is the gauge just reading high(er) than it used to? We are so used to immobile temp gauges on modern cars we forget how less damped these old ones are.

As the problem is when stuck in traffic, you can replicate this when stationary "at home". An infra red thermometer would be helpful.
Michael Beswick

Don't forget it's normal for the temp gauge on MGBs with electric cooling fans to move up from N to about 1/3rd to 1/2 the way to H before the fan(s) come on. If it gets more than half way without the fan coming on then there is a problem.

If you linked the two wires that go to the fan switch and the fan ran - and more importantly brought the temperature back down again, then it is the switch that is the problem. But ...

Is the radiator around the fan switch hot? If not then the thermostat is stuck closed or you have insufficient coolant to reach the switch. In fact feel the radiator all over and it should be hot all over, when the fan isn't coming on and the temp gauge goes up. By that time the whole rad should be hot, including the outlet to the bottom hose. Cold spots but a hot bottom hose indicates a partially blocked rad. A cool bottom hose indicates lack of flow through the rad.
PaulH Solihull

Hi Dave,

All above is good advice.

Check the switch in situ, i.e. short it out with a jumper lead, whilst connected in the car. This will confirm whether the whole fan circuit is OK or not.

I wonder if a continuity test is accurate enough. If the switch has burnt contacts, with an ohm or two resistance, a continuity check will show it as OK, but when trying to switch the fan current it fails. Do the in car test.

Regarding thermostats, it is my understanding that they are designed to fail open, causing a cool running condition.

Heritage, and probably other vendors, have a blanking sleeve, that goes into the thermostat housing and blocks off a bypass hole. This hole is designed to allow coolant to bypass the radiator, for more rapid warm up. However, it was designed with the original thermostat in mind, which, when it opened, had a part that blocked this hole. Modern replacements don't have this function, so there is always coolant bypassing the radiator. If all else fails you could try fitting this blanking sleeve. I have done this, but have not been stuck in traffic on a hot day to determine its effectiveness. It does, however, increase the time for the engine to warm up.

Herb
Herb Adler

You also want to be sure that your cooling system is completely filled with no air pockets.

The fan sensor is mounted at the top of the radiator where air pockets may exist. If the sensor is not immersed in coolant it will never operate.
Steven Rechter

If the temp is normal when you are on the move, and the system is full then one must assume water must be circulating. Wire an over-ride switch across the the temp switch terminals, if you don't want to buy a new switch. Also check the timing.
Allan Reeling

Yea the air pockets would keep the sensor from sensing anything. Make sure to purge the system nice and good.
JRB Mr

Thanks for all the advice. As always I'm learning a lot here.

Well it looks like it might be a simple problem with my gauge.

I was attempting to set up a fuel warning light last year and replaced the standard voltage regulator with a solid state device. I understand this causes the gauges to respond a bit differently to the on/off pulse they usually get from the standard regulator, but otherwise I was able to get the fuel gauge (all I was worried about) reading accurately at all levels. I didn't think about the temperature gauge at the time.

Anyway, I bought one of those infrared thermometers today with a laser pointer. I ran the engine until the temp was at halfway. Measuring right next to the sender on the cylinder head (just beneath the thermostat housing) it was reading approx. 78C. Measuring the top of the radiator where the fan switch is inserted, roughly the same reading.
I allowed it to heat up until the temperature gauge was around 90% (a little short of the red mark). Cylinder head reading was 90C and radiator was 89C. A few seconds later the fan came on. Whew! At least that problem is "solved". I have a feeling the coolant was just a bit low when that problem was occuring and I've since topped it up during all my other diagnostics.

I measured all around the radiator and found it a uniform temperature all over. No cold spots (or hot spots) that I could find. Fortunately the infrared device has a "scan" mode to help me to this.

As far as I can tell, the fuel gauge is still accurate. I just drained the tank and it showed empty. Half filled showed halfway on the gauge. Full tank showed full on the gauge. I assume that means my solid state regulator is still doing the right thing.

Is it possibly my temperature gauge is somehow reading too high? What temperature should I expect at the cylinder head when the gauge is at normal, etc?
D O'Brien

AFAIK 'Fan on' is about 90 deg C May be worth checking you sensor. (Resistance varies with temp) No idea what the resistance is at various temps though.
Bernie
B Anderson

Sounds like your fan switch is spot on. When the needle is positioned over the 'N' that should be about thermostat temp i.e. nominally 82C. Someone else has measured this as 82 ohms, on a pre-77 sender. There were three gauge and sender combinations over the years and it is possible to get the wrong combination for two of them as the gauge and sender changed at the same times, different times for the third combination from what I have been able to find out - http://www.mgb-stuff.org.uk/electricstext1.htm#tempgauge

The electro-mechanical stabiliser outputs an average of 10v, so if you replace it with a 10v electronic stabiliser the result should be the same (barring a slower rise time with the electronic).

Swapping over the green/black and green/blue wires between the two gauges should show whether it is the gauge or the sender that is out. I've not had experience of the electric temp gauge but I have had to recalibrate the fuel gauges a couple of times on replacing the senders, can't see why the temp gauge should be any different - http://www.mgb-stuff.org.uk/electricstext1.htm#adjust That might be enough to bring the gauge back into calibration - for N at any rate, then see where it gets to when the fans cut it, which should be about 1/3rd to 1/2 the way from N to H.
PaulH Solihull

This thread was discussed between 15/01/2013 and 17/01/2013

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