St150 Radiator Fan Not Engaging.

[SpontaniousCombustion]

Hello all, thanks for taking the time to read this post.

Whilst sitting in traffic the other day, the temperature gauge raised above where it usually sits. Within the normal brackets still but on the limit.

I pulled over, popped the bonnet to check if anything looked amiss. The coolant was bubbling like kettle. My instant thought was why hasn't the radiator fan engaged even when the temperature gauge is showing hot. Stuck the A/C on thinking the fan would engage almost immediately but still nothing. No sign of life from the fan.

Got home, and started testing thing. Stuck a new Relay in it I had laying around still no luck. Tested the 40amp fuse behind the relay box and that had continuity so that's not blown. Then I removed the radiator fan resistor and it looked a bit worse for wear to be honest. Took the shroud out from the radiator and ran 12v through it via some crocodile clips to see if the fan was at fault here. The fan ran fine no problems.

Could the fan resistor be responsible for the fan not working at all?

Anymore information needed please let me know. Would like to get this sorted asap, can't risk it having a 2nd die on me.

 

[Handy Andy]

Did you try the AC mode - see if the fan kicked on then?

When I find myself in stop start traffic and long idle times waiting for lights and the traffic to move - I oftentimes put the AC on, put the climate fan on 2 - then also turn the control to HEAT (Warm) setting. This forces the climate control to work and use the FAN in the front on high - might be uncomfortable for some - but the "boiling bubbling" that the tank has to endure - is also a condition that can make the coolant tank fail from the pressure build and subsequently unable to vent until it hits 21psi to finally move the vent to allow the vapors to escape.

You have to pull the heat out of the motor, so running the AC fan should work and the HEAT setting allows the heater core to take on some effort of what the radiator is waiting on - it's fan.

Little comfort does that provide you when the seam fails before the fan turns on or the venting occurs - you can lose all your coolant from this type of event. This site has plenty of examples.

But that doesn't solve the original condition of having this thermal runaway.

When it goes hot like that - the resistor can be removed and checked - see if it's shows ohmic Ω resistance

?Usually about 4.7 to 5.6 Ω on the meters 1X setting?

Not sure of the year of your vehicle - I'm curious to know where you found the resistor - was it in the shroud (Controller) or in the wiring harness?

Why?

Some shrouds had a controller and sensed the temperature from the airflow - so if it got above a given temp - the fan used the low-speed resistor to start the cooling process. Not sure if your vehicle has such a shroud/controller unit.

 

[SpontaniousCombustion]

Did you try the AC mode - see if the fan kicked on then?

When I find myself in stop start traffic and long idle times waiting for lights and the traffic to move - I oftentimes put the AC on, put the climate fan on 2 - then also turn the control to HEAT (Warm) setting. This forces the climate control to work and use the FAN in the front on high - might be uncomfortable for some - but the "boiling bubbling" that the tank has to endure - is also a condition that can make the coolant tank fail from the pressure build and subsequently unable to vent until it hits 21psi to finally move the vent to allow the vapors to escape.

You have to pull the heat out of the motor, so running the AC fan should work and the HEAT setting allows the heater core to take on some effort of what the radiator is waiting on - it's fan.

Little comfort does that provide you when the seam fails before the fan turns on or the venting occurs - you can lose all your coolant from this type of event. This site has plenty of examples.

But that doesn't solve the original condition of having this thermal runaway.

When it goes hot like that - the resistor can be removed and checked - see if it's shows ohmic Ω resistance

?Usually about 4.7 to 5.6 Ω on the meters 1X setting?

Not sure of the year of your vehicle - I'm curious to know where you found the resistor - was it in the shroud (Controller) or in the wiring harness?

Why?

Some shrouds had a controller and sensed the temperature from the airflow - so if it got above a given temp - the fan used the low-speed resistor to start the cooling process. Not sure if your vehicle has such a shroud/controller unit.

Yeah, I've tried engaging the A/C and still the fan is refusing to kick on. I've left the car to idle for a good 10mins and still no fan is kicking on. Best thing I've been doing at the moment is as you stated but instead I'm cranking the heat all the way up and sticking the blowers of number 4 to keep the engine temps down. I'm literally cooking myself while driving.

I'll take the resistor out again today and test it for that. When I tested the relay "R2" for a voltage to understand if there is voltage reaching the relay box. I tested terminals 3 & 1 on relay R2 and it was giving me a strange reading of 32... I was expecting something more along the lines of 12 so I've not a clue what's going on there.

And yes, the resistor is is built into the bottom left of the shroud. It's an 05 Fiesta ST150.

I did a quick test for voltage at the relays this morning before I had to go out and the readings don't seem right to me. Testing on Relay R2 terminals 1 & 3 should read about 12v if I'm thinking rightly. I'm getting more in the range of 20V for each terminal. This doesn't seem right to me.

22.0 for terminal 1
24.8 for terminal 3.

I have also attached pictures of the resistor.

 

[SpontaniousCombustion]

I'm a sausage. I had the Multimeter on the wrong setting which is why it was giving me them strange readings. I can confirm the relay box is receiving 12v and there is no break in the wire between the battery and the fan itself.

I started the car up, ran it around the block and got them temps up. Got home, allowed the car to idle for another 5-10mins and bam, the fan engaged and came on! I haven't a clue what I did differently but this time the fan is working.

Hopefully that's the overheating done with and the relay I changed was the problem! I'll keep an eye on it over the next week or so and see how I get on.

 

[Handy Andy]

Then the coolant sensor is faulty- but with all the messing around (alerts) going on, I'm thinking - some lean to, corroded wiring issues so be aware of that as you progress. The resistor is toast - the case is showing some cracks - even if it's mud, when the wiring in it - being plated - is rusted like this, time to change this out.

The socket plug that the Resistor and the Coolant Temp sensor goes into - might need some TLC so see if you can work in some means to clean it.

You also need to check the thermostat - when you have a sudden moment like this, something (which means everything) needs to be inspected. The thermostat itself may not be bad, but if there's debris in there - it can resist opening up fully due to the blockage.

There should be a bubble (bell) valve on the thermostat. At least, a hole to burp air caught in the block/jacket.

IF that had a piece of itself fall away, it may not "jam" the thermostat, but it can affect its operation if it's left to itself - it can cause gremlins to show up.

 

[SpontaniousCombustion]

Then the coolant sensor is faulty- but with all the messing around (alerts) going on, I'm thinking - some lean to, corroded wiring issues so be aware of that as you progress. The resistor is toast - the case is showing some cracks - even if it's mud, when the wiring in it - being plated - is rusted like this, time to change this out.

The socket plug that the Resistor and the Coolant Temp sensor goes into - might need some TLC so see if you can work in some means to clean it.

You also need to check the thermostat - when you have a sudden moment like this, something (which means everything) needs to be inspected. The thermostat itself may not be bad, but if there's debris in there - it can resist opening up fully due to the blockage.

There should be a bubble (bell) valve on the thermostat. At least, a hole to burp air caught in the block/jacket.

IF that had a piece of itself fall away, it may not "jam" the thermostat, but it can affect its operation if it's left to itself - it can cause gremlins to show up. View attachment 6930

It may be worth me changing the thermostat regardless just because it's probably as old as the car is. It's one of the thing I neglected to change when I swapped the engine, I just completely forgot to do it.

At least I'll know it's working correctly and completely rule it out of any of the cooling problem.

 

[Handy Andy]

This is for the reader that comes across a similar issue - as a Legacy post,
Wanted to help others in finding theirs...

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To me this may be more of a limit switch - kind of like those you'd find in a Xmas-tree string of lights. The one bulb lights up the whole string - but if its bi-metallic arm touching the filament making the circuit complete - warms and gets hot, it lifts off the filament and interrupts the power flow for that string.

Since it bi-metallic - it can cycle the lights on and off for quite a while as long as the string is not subjected to abuse.

​

These limit switches work in a opposite way, its OPEN when cool or cold, but when the system needs or demands more cooling - this limit switch then "clicks on" and bypasses the resistor - it acts like a dead short taking in all the current thru it to allow the fan circuit to be complete and run at full maximum speed.

These are not lifelong units in high current loads - they can only do so many cycles of this - then they are blown open like a fuse and won't work anymore.

You may be on borrowed time...