cooling problem

MGYTF 135 03 Reg

Temp gauge usually rock steady just below normal. Yesterday on Motorway, steady 60 mph; after half an hour or so temp gauge climbed to about 3/4 - couldn't tell if rad cooling fan operating (traffic too noisy!). Turned heater onto max; temp gauge fell quite quickly back to usual. Gauge rose again after 5/10 mins - repeated same for next hour. When stopped, checked coolant level - all fine

This morning on driveway, ran engine for 15 mins; fine. Just giving up when suddenly rad fan kicked in even though temp gauge still at usual. After a minute or two temp gauge started to rise up to about 3/4. After several mins gauge back to normal but rad fan still running and did so for next 10 mins.....

I don't know if the sensor for the temp gauge is separate from the sensor which controls the rad fan? Something a bit odd here.....

So, any ideas, please?

Thanks

There are two independent temperature sensors, one for the gauge, the other tells the ECU what the coolant (and thus engine) temperature is. You can thus have the symptoms you have described. The temperature gauge is notoriously inaccurate, and as your cooling fan DID cut - in after 15min of "ran engine for 15mins", it sounds as if the sensor connected to the ECU is O.K.

If you have not already added a low-coolant alarm, I would recommend that you DO. It has added much more peace-of-mind to al my longer journeys. I fitted a cheap system which uses a sensor attached to the outside of the header tank. Easy to fit and an LED (connected in parallel to the warning buzzer) behind the unused automatic transmission failure symbol in the instrument display is an easy upgrade. P.M. me if you want more detail. I am in Devon (near Honiton) if you want to see my installation.

Thanks, Ellisojo. I do have a low coolant alarm - agree that it does give peace of mind - even on short journeys in heavy traffic.

So, you suspect the sensor/gauge may be faulty? Where is the sensor for the temp gauge situated, please?

Most gauges aren’t accurate, which is why they don’t display values, just vague ranges, to be accurate they’d need regular recalibration……… that’s not to say that they’re not useful, indeed they offer the driver an indication of what’s going on as a comparator.
They give the opportunity to learn what is normal and then notice any difference to that normality.
Much better that silly idiot lights,

As with most things, with the exception of our opinions on Christmas & Easter, I’m with Cobber. Unless you have a serious sports car where the driver needs to know how everything is going on & where the slightest drift in an indicated value might spell trouble, the gauges supplied in cars such as ours are as Cobber says comparative indicators. They are indicators of things being about quite right, or Oh shit somethings adrift. If you consider the processes in your temperature indicator; current flows through the temperature sender, which is on the left hand side of the engine block right at the back & this is a thermistor, a resistor which changed value as the temperature changes. They can be positive temperature coefficient or negative, one increases in value as it warms & the other decreases. If the current then flows through a meter movement the meter will indicate that change in value as the thermistor changed in value. QED
If your temperature gauge is showing an increase in value then more current is flowing through the network. I have never come across a gauge which implies more current is flowing than there actually is. I can think of a mechanism which would allow this to occur but the chances of this are vanishingly small. The other device within the network is the sender; for this to give you an erroneous reading it would have to drop in value* passing more current than normal & thus increasing the deflection on the meter. This type of device when on the point of failure usually go high resistance, you then get a reduction in indicated temperature. A similar effect to this can occur when PTFE tape is wrapped around the sender thread to prevent leaks. This tape can prevent the system working all together. Corrosion will have the same effect.
So I’m sorry to have to piss on your chips but I think your going to have to look further & in a different area to resolve your problem.
* depends on positive or negative temp coefficient.
Have you given the coolant system a good old bit of blood letting, it might just want a good burp.
M

John in Devon wrote:

Thanks, Ellisojo. I do have a low coolant alarm - agree that it does give peace of mind - even on short journeys in heavy traffic.

So, you suspect the sensor/gauge may be faulty? Where is the sensor for the temp gauge situated, please?

Hopefully, below is a picture of my engine bay, with the ECU sensor circled in red. The sensor for the water temp gauge is below, and extremely difficult to reach with engine in the car!


I have tried to insert a picture from MG-Rover site (Credit to "CJJ") with engine out which might help explain locations better.

Thanks again - very helpful.

My son asked me if I was confident the thermostat is OK? All I know is it's 20 years old!!

SO, last question (I think) - could you please explain where the thermostat is located? (My engine knowledge only runs to the A series (Mini)!

If you go on the download page you will find copies of the workshop manual for your car (we’ll not specifically yours you understand) download that & it will tell you exactly how to change it. My guess is that you will have a pressure relief thermostat, in which case you will be working underneath & probably getting a bit wet in the process.
I changed over from the block mounted thermostat & fitted a PRT which now requires changing so I’ll also be treated to a cascade of OAT.
There’s Probably a how to in the how to section, where there will be no shortage of advice.
M

Anyone know how to test the engine bay fan on a Mk2 TF ?
On all earlier cars it can be handily tested by unplugging the ambient temperate sensor that's located right by the oil filler & dipstick. The later Mk2 TFs still have an ambient temperature sensor there but due to logic differences it wont start the fan by default when disconnected.

Do you want to make sure the fan is functioning or do you want to verify the control electronics?
As you correctly point out the Mk1f fan network is so straightforward to test you could introduce a switch to test it whilst doing your general checks.
Check the oil, check the coolant, flick the switch to check the fan.
I’m away & am several hundred miles from my circuit diagrams, if your still struggling by weekend send me a PM & I’ll have a look.
There was a fad a few years ago for fitting override switches on the fans. Not to be outdone I did the same, along with the switch there’s a feedback LED & so I can turn them on & off with the ignition on aux & check the fan by hearing them run & by the LED, know that they’re running whilst on the move.
M

Notanumber wrote:

Anyone know how to test the engine bay fan on a Mk2 TF ?
On all earlier cars it can be handily tested by unplugging the ambient temperate sensor that's located right by the oil filler & dipstick. The later Mk2 TFs still have an ambient temperature sensor there but due to logic differences it wont start the fan by default when disconnected.

When I was installing the engine on my 2004 Sunstorm the fan started running, at the time the two coolant temperature sensors that Ellisojo circled were unplugged and so was the ambient temperature sensor, plugging the latter in stopped the fan running. Whether it was due to the coolant sensor plugs being disconnected (unlikely) or the K Maps ECU (equally unlikely) or some oddity in the wiring is unknown. If I remember I will disconnect the plugs tomorrow evening to test it.

Thanks.I like the idea of introducing an led and an override switch at some point.

The idea for now is to find a quick way to test the engine bay fan without removing the engine cover (time and weather not being on my side). If I can prove the engine bay fan had caused fuse 6 to blow i can source a replacement ready to change once weather permits.
As disconnecting the ambient sensor is just ignored by the Mk2 electrics as being out or range im wondering what value resistor to apply to enable the fan (or at least blow the fuse if the fan is indeed buggered as i suspect. )