Overheating 2003 Mustang GT
 

Yesterday we started having a problem with our 2003 Mustang GT overheating. The car is all stock, with about 275,000 miles on the 4.6L engine.

The overheating seems to only occur with the A/C is on. The coolant will wind up boiling as the car gets very hot.

I read some things about the fan may not be working, but I cannot find a procedure to test it. The fan comes on while sitting in the driveway while running the A/C. The temp is right in the middle of the gauge - the car seems fine. But take it out, and a few miles from home it is overheating.

Any help would be appreciated - thank you.

I found some Hayns manual info. I disconnected the 3-wire connection that goes to the fan. There are 3 wires - black (ground), orange stripe red wire (low speed) and blue stripe red wire (high speed). If I connect ground and high, the fan comes on. If I connect ground and low, I get nothing. I'm thinking I have a fan with a bad low speed - does that make sense to cause overheating on the highway?

Thanks!

The fan should turn on low as soon as you turn the AC on. If the low speed circuit fails the PCM will command the high speed fan to switch on and off repeatedly. So if your fan comes on low speed when the AC is switched on then that circuit is ok.

Leave car idle in drive way with AC on high,, monitor fan speed. Should come on low and stay that way unless temp gets above 238 then high will come on.

When it boils out where does it come from? Maybe just a new cap?

Given the fact that it over heats on the highway, and it has 275k miles on it, i suspect a mechanical failure of either the thermostat or water pump or the reservoir cap. Possibly even faulty data from a failing ECT.

The low speed never comes on - even if I connect 12V and ground directly to the fan's 3-wire connector. The 4.6L engine had an oil pump failure around 250K and got a whole bunch of new parts including a new water pump and thermostat. The boiling antifreeze comes out around the cap.

You say the fan comes on when the AC is turned on so I cant see how its not working at all. Or is it the high speed that works when you turn the AC on?

Maybe it just needs a new cap ,, maybe its not overheating untill the cap looses pressure and boils.

Thanks Dawson1112.... I don't know how this fan is supposed to work. If we sit in the driveway idling, with A/C off, the fan never comes on. If we turn on the A/C, the high speed fan will come on once the temp gets high enough. However, if we take the car out on the road, using the A/C, the car will boil over. Leave the A/C off, and the car does not boil over.

I unplugged the 3-wire connector at the fan. I then jumpered 12V to the high speed side - the fan came on. I jumped 12V to the low speed side, and got nothing. That makes me think the fan low-speed side is shot. Maybe I need a new cap too. I'm still in the early stages of learning this Mustang but I've been working on the 4.6L in my Town Car for years.

Thanks!

Hook a test wire up to that low side wire,, make sure nothing can get tangled up in the belt or engine,,

Start the car turn on the AC test for voltage at the low side wire using your test wire and a multimeter.

If no power when AC is on then CCRM is shot,, if you get voltage when AC is on then most likely fan motor is shot.

"If we sit in the driveway idling, with A/C off, the fan never comes on."


If the temperature outdoors is anywhere higher than severe winter cold, this is an impossibility. With virtually no air movement through the radiator to carry away the heat of the idling engine, it is bound to soon overheat. imp

Okay, I have done the testing with a multimeter. I am getting 12V on the low speed (orange stripe) fan connector when the A/C is started, but the fan does not move. Then when the car gets warm enough and 12V goes to the high speed (blue stripe) fan connector, the fan comes on at high speed. I checked the voltage before and after the resistor that clips into the fan shroud, and that seems good as well... 12V going in, and like 9V coming out.

So I think I have a bad fan assembly.

Thanks everyone - Walter

Agree with bad fan diagnosis. You have power at the male connector, and feeding 12V directly to the fan does not cause it to come on, so yeah that pretty much confirms it.