Car ECU, replace or repair

[Simon]

Right following on from a very informative discussion about air flow rates into a car engine I have a new problem. My Fiesta MK4 ECU has decided to no longer turn the radiator fan on. I have been to a mechanic and we have checked. He was able to make the fan run by instructing it to via his diagnostics machine and he as I is getting temperature readings from the diagnostics bus. So somewhere in the ECU it has decided to no longer turn the fan on. I am rather perturbed by this as clearly it is not just a blown output transistor as he got it to work via the ecu with manual instruction and the requuired information is coming in. It's like the things programming has changed.

I don't know if it is relevant but a few miles prior to this problem being discovered (on being stuck in traffic) my engine suddenly started running more smoothly and with less petrol (I had a few miles back accelerated to 3700 rpm which I rarely do so might of shook some rubbish out somewhere ?). Is it possible that the ecu reprogrammed to account for a change in the engine condition and forgot it has to activate the fan ? or has something burnt out. I am puzzled as the information is going in and it can be made to actuate the fan but it chooses not to.

He said I can have it sent away and fixed for about £200, but as this point I wonder, should I just get a new one, it's 12 years old, will anything else in it fail. Or is it a program fault. Educate me on ECU's 

[PeterG]

Are you 100% sure the ECU controls the cooling fan?
In some setups the fan has its own temp switch. If that switch fails the fan wont engage.

Regards

[Simon]

yes it does, checked the diagrams and we instructed it to turn the fan on so it definitely controls the fan.

[PeterG]

One trick that can fix errors like this is to disconnect the battery for about 24hours. This clears the memory, when you reconnect the battery and start the car it will reset to default settings.

Regards

[Simon]

hm worth a try, 24 hours ? that's going to be difficult.

[PeterG]

Then again, if the car is running better, just rig up a fan controller and leave the ECU alone......

Regards

[Simon]

my concern is that if it is a physical problem more might go wrong. A good reset should do no more harm. Yes the mechanic suggested I make my own controller.

[Baliszoft]

At first, i would try to "manually trigger" the the fan. If you already got the schematics, you should know which wire has to be (usually) pulled to ground in order to activate things. BTW: usually it is not the engine ECU which is controlling the fan(s), but if you say so, it may be.

[Simon]

The ecu controls the fan relay. We plugged a diagnostics machine in and told it to run the fan and it did. So I conclude the ecu can actuate the fan if it wants to.

[Baliszoft]

Ok. Try to activate the relay somehow (without the ecu), if it is working you should check the harness and if that is ok too, it may be the ecu. Also check for the (positive) supply for the fan/relay too (wiring, fuse, etc).

[Simon]

your not listening, the whole fan actuation circuit works, from the fan right back to the ecu because we made the fan run manually, and whats more we did it through on the diagnostics line of the ecu so even the output stage that actuates the relay works in the ecu.

[Baliszoft]

your not listening, the whole fan actuation circuit works, from the fan right back to the ecu because we made the fan run manually, and whats more we did it through on the diagnostics line of the ecu so even the output stage that actuates the relay works in the ecu.

Ah sorry, you're right. I missed that you could start it manually. If the hw is working, no idea then.

[PeterG]

Simon,
 these people may be able to help more.

Fiestaguides.co.uk

Some info here as well.

http://uk.answers.yahoo.com/question/index?qid=20120323060644AAzcEE2

Regards

[Baliszoft]

How about the engine coolant temp sensor?

[aargee]

Sometimes, that fan is controlled by the ECU as well as by a standard temp control loop. The ECU control is for aircon, with the A/C on the ECU triggers the radiator fan at certain (near idle) engine speeds to improve cooling. Not saying this is the case here, but there could be a side by side control for the fan.

[Simon]

How about the engine coolant temp sensor?

that works fine, my permanent diagnostics panel is reading the temp fine. so info is going in, fan can work, but it wont.

[PeterG]

Sometimes, that fan is controlled by the ECU as well as by a standard temp control loop. The ECU control is for aircon, with the A/C on the ECU triggers the radiator fan at certain (near idle) engine speeds to improve cooling. Not saying this is the case here, but there could be a side by side control for the fan.

This makes more sense to me.

Regards

[Simon]

Sometimes, that fan is controlled by the ECU as well as by a standard temp control loop. The ECU control is for aircon, with the A/C on the ECU triggers the radiator fan at certain (near idle) engine speeds to improve cooling. Not saying this is the case here, but there could be a side by side control for the fan.

according to the schematics the ecu directly controls the fan relay, nothing else. There is no air con on this car and it's the basic model.

[Simon]

Sometimes, that fan is controlled by the ECU as well as by a standard temp control loop. The ECU control is for aircon, with the A/C on the ECU triggers the radiator fan at certain (near idle) engine speeds to improve cooling. Not saying this is the case here, but there could be a side by side control for the fan.

This makes more sense to me.

Regards

This makes sense if you have air con as the air condenser also needs air throught it from the same fan.

[PeterG]

All i can say is check out http://fiestaguides.co.uk/ .Someone else must have had this issue at some stage in the last 12 years.

Regards

[G7PSK]

I had a similar problem on a Vauxhall Combo van one hot summers day I got stuck in traffic and found the radiator fan would not work and had to drive home with the heater on to stop the engine boiling. I checked the fan it worked I replaced the sensor switch still no fan when the engine got hot I put both the old and new sensors into boiling water both worked, I heated the sensor while in circuit on the van but not screwed into the radiator the fan came on, so I cleaned out the radiator still no joy so I went the main dealer. I was told that this is a common problem most people don't notice it , It is a design fault the sensor is at the bottom of the radiator not the top and the coolant stratifies  leaving hot at the top and cold at the bottom particulary a problem at low revs, slow speeds and with a water pump that is not working correctly. I fitted a new water pump and the fan would come on.

[Simon]

my sensor is in the block, sounds like the radiator was a bad design, should have been a mutipass

[amyk]

Not so sure how easy it would be to reflash the firmware (or even obtain it) but that would be one way of ruling out firmware corruption, provided it is a flash MCU and not mask ROM/OTP (which is a lot less prone to corruption).

The ECU probably has analog circuitry and electrolytic caps, the latter are known to fail.

Or it could be something as trivial as a solder joint that's cracked from vibration.

Unless the fan output is really directly controlled by the MCU there are other things to go wrong.

[G7PSK]

my sensor is in the block, sounds like the radiator was a bad design, should have been a mutipass

Are you sure the sensor for the radiator fan is in the block, the one in the block is usually the engine temp. sensor with another in the radiator for fan control as the engine is often hotter than the radiator and with the fan controlled from the engine block you can have it turning on when not required in the winter when the thermostat is keeping the engine hot but the radiator is cold.

[Simon]

my sensor is in the block, sounds like the radiator was a bad design, should have been a mutipass

Are you sure the sensor for the radiator fan is in the block, the one in the block is usually the engine temp. sensor with another in the radiator for fan control as the engine is often hotter than the radiator and with the fan controlled from the engine block you can have it turning on when not required in the winter when the thermostat is keeping the engine hot but the radiator is cold.

I expect it is near a water channel or pokes into one. the fan control comes from the ecu we know that for certain, there is no other circuitry that I know of that controls it.