Engine Fan

j70Stang

New Member
Sep 17, 2019
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Lakeville, MN
Got a 97 GT that we have been restoring for a few years. Finally got it painted and out driving it but not it runs very hot. Started with the basics, new thermostat, Flush the radiator. It the process of doing that we discovered that the fan is not coming on when it warms up. It will come on when the AC is on, so the fan is working and the relay that turns on the fan must be working. We have tested the original ECT sensor and tried 2 different ones including a Ford one. The Ohms read right about where they should when cold and hot. The fan will also come on anytime the ECT sensor is unplugged from its harness.

So it seems that the fan works, The ECT works, and the fan relay works, Only thing I can think of is the engine computer is not telling it to come on?

One last little tidbit. In the process of welding patches in the quarter panels when we were doing body work we managed to nuke the engine computer to the point we had a no start / no crank. Hauled it to the dealer. They didn't say exactly what they did, but it sounded like the re-flashed the computer to get it running again.

Any suggestions would be greatly appreciated.
 
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wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
So it seems that the fan works, The ECT works, and the fan relay works, Only thing I can think of is the engine computer is not telling it to come on?
The Mustang GT uses a two speed cooling fan. In addition, the 1996-1998 model year uses two different engine temperature sensors. One for the PCM and another for the cluster dash.

It seems to me that:
  • you only know that the fan works on the low speed (yes this is correct. The LOW speed fan is the default AC on fan speed).
  • we know that the PCM is able to "see" when then ECT is disconnected.
  • you believe that the ECT resistance value is correct. NOTE, the two different ECT sensors are different and are not interchangeable to each other.
Have you confirmed that the cooling fan works on BOTH high speed and low speed when powered directly from battery?

Do you have an ODB2 scanner? Hands down. The best and easiest way to trouble shoot this is with a bi-directional ODB2 scanner. We can:
  • Find out what the PCM "thinks" the engine coolant temperature is.
  • Find out what speed fan the PCM is calling for.
  • COMMAND the PCM to turn on the low speed fan in the bay/driveway.
  • COMMAND the PCM to trun on the high speed fan in the bay/driveway.
Here's some information on a very affordable ($30) bi-directional capable ODB2 scanner.

ForScan ODB2 scanner w ELM327 USB

Finally. Are you sure that the motor is actually over heating? I have helped more that one person where the problem actually turned out to be system voltage related. That is that the alternator could not keep up with the load. The usual cause is poor grounding related. This caused the system voltage to drop. The makes the ECT read high. So I encourage you to double check the battery and charging system. Plus cross check the PCM reported temperature with an infrared thermometer scan of the T-stat housing.

If a ton of work has been done on this car, double check that the motor's grounding strap is in place from the left hand motor mount to the car's frame rail.

Howto perform charging system voltage drop test
 

j70Stang

New Member
Sep 17, 2019
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Lakeville, MN
WM
Thanks for the reply and things to check. We will be tackling it today. Couple of things come to mind in from your reply. I forgot to mention that the battery was relocated to the trunk as it saw drag duty at some point in its life making some of these test hard to do unless I use very long wire. Will that effect the test? We have long wondered it If the ground was good enough. It is grounded to the chassis right out of the battery box. We ground off the paint where the short neg. battery cable is connected. Not sure what else to do for that.

We did try a different battery that was known to be good. But we didn't do any load testing.

We did know that there is a different between the 2 sensors and verified that the fan sensor seems to be working. We will try to direct wire the fan. I am assuming that we should still incorporate some kind of fuse ?

We have an Innova 3120d scan tool, but I must admit I am not all that adept at using it. I don't know how to turn on the fan with it so Ill have to do some research on that.

I'll post again after we do more testing.
 

wmburns

SN Certified Technician
Aug 14, 2009
5,438
433
164
Houston Texas
I forgot to mention that the battery was relocated to the trunk as it saw drag duty at some point in its life making some of these test hard to do unless I use very long wire. Will that effect the test? We have long wondered it If the ground was good enough. It is grounded to the chassis right out of the battery box. We ground off the paint where the short neg. battery cable is connected. Not sure what else to do for that.
The purpose of the test is to find out IF the fan works or not. It does not matter WHERE the power comes from. Use another battery. Or an old school battery charger. Or what ever you have handy that can supply some amps.

And yes it's a good idea to use a fused jumper. Especially if you are unsure about your abilities to work with electrical.

Regarding your question about the quality of the ground on a relocated battery. To answer this question use the "voltage drop" test method. And yes you will need a long test lead. Any scrap piece of wire will do.

Set the VOM on DC volts. Put one test lead on the battery negative. Put the other on the alternator case (or radiator core support). Read the voltage drop with various loads turned on. The voltage drop should be a very low value. Usually less than 0.25 volts.
We have an Innova 3120d scan tool, but I must admit I am not all that adept at using it. I don't know how to turn on the fan with it so Ill have to do some research on that.
From looking at the web site your scanner does not have bi-directional capabilities. However the scanner can still provide very useful information. It should be able to tell you what the PCM "thinks" the ECT value is as well was what speed fan the PCM is calling for.
 

j70Stang

New Member
Sep 17, 2019
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Lakeville, MN
So over the last couple of weekends we have been working on this issue. We have long suspected the battery was week and so it was replaced. In the process we found that the positive cable end of the battery was loose on the cable. Not sure if that caused any issue or not.

We took it to a friend of a friend who had a snap on scan tool and found out a couple of things. He was able to turn on the fan with that tool in both low ans high speeds. Ever since then the fan seems to have been working. Maybe the relay was stuck?

We also found that the gauge is off and reads high when in reality the actual temp is 200 degrees when which is also when the fan will come on low. We vereified the temp with a infa red temp gauge.

We replaced the gauge sending unit and it still reads high, same as before. Looks like a gauge issue. From what I have found the gauge is not available or serviceable. If it is reading wrong it should be replaced along with a voltage regulator for the instrument cluster?????
 

wmburns

SN Certified Technician
Aug 14, 2009
5,438
433
164
Houston Texas
We replaced the gauge sending unit and it still reads high, same as before. Looks like a gauge issue. From what I have found the gauge is not available or serviceable. If it is reading wrong it should be replaced along with a voltage regulator for the instrument cluster?????
I have helped more than one person with a similar problem. In many cased the reason why the ECT reads higher than normal was CAUSED by low system voltage. The ECT is calibrated assuming a certain operational system voltage. The calibrated value will be incorrect if the operational voltage is also wrong.

What are some "possible" causes for low system voltage?
  • poor motor ground, alternator ground or ground at the radiator core support.
  • Missing motor grounding strap from the left hand motor mount to the car's frame rail. This strap is frequently over looked during a motor swap (ripped out during motor removal).
  • low positive voltage at the alternator. IE poor alternator output.
  • low positive voltage at the battery.
  • weak or corroded connection at the ECT itself.
I have already given the link on how to perform voltage drop testing. If understood how the test works it will positively narrow down IF there's a problem as where it is.

Did you see the value of using a bi-directional ODB2 scanner?

The other possible cause for ECT sensor error is air trapped inside the cooling jacket. The ECT sensor can not read correctly unless it's in direct contact with coolant.
 
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