Mercury mystique


i have a 1998 mercury mystique with the 2.0 4cyl 16 valve DOHC and the fan wont turn on. i tried some new relays and nothing. i also replaced the (ECT) engine coolant temperature sensor and nothing. but when you disconnect the plug for the ECT and wait a few seconds the fan turns on. im going to need to start driving this car sooooooooon and i cant figure it out. why wont it work???



Does the fan come on when you engage the Max AC setting?


The engine has to get HOT before the computer turns the engine cooling fan on. Otherwise, the fan isn’t SUPPOSED to come on, except, when the a/c is on! HAS the engine coolant gotten hot enough to turn on the fan? How do you know? When you disconnect the electrical connector to the coolant sensor, the engine computer is, probably, DESIGNED to turn on the engine cooling fan. See if turning on the a/c will cause the engine cooling fan to come on.


ill try the A/C thing tomorrow. if it does work what does that mean?? also the temp has gotten hot enough because on the temp guage its spelt out NORMAL then there is a red line after the L. the needle has gotten to the L. my BRO’s contour only goes to the M before it kicks on.



Cheap car may equal cheap solution. On an 83 Camry that was like your car, I wired up a toggle switch, then installed it up-side down so that the fan would come on with the switch up. Saved a lot of time. I drilled two holes straight through the floor by the gas pedal where I had lots of room.


The fan relays are controlled by the PCM and a dual pressure switch. Without knowing the limits of your electrical expertise it would kind of difficult trying to go through all of the testing procedures on-line.
AutoZone has a schematic for the cooling fan circuitry but it’s up to you to analyze it. Note the PCM provides grounds for the fan relays based on a number of inputs.

Since Haynes manuals are near worthless and Chiltons are totally worthless, you might consider a subcription to ALLDATA for 25 a year.
It should lay out most of the details anyway. ALLDATA is not perfect but it’s better than the other two by far.

Make sure the coolant level is full in the radiator.  If the coolant isn't high enough to touch the sensor, it won't read correctly. Click on this web site url to take you to the Auto Zone "How-to" section: It will show you how to check the engine coolant temperature sensor, and how to replace it, if you choose. The price for the ect is $18.


On my first Diesel VW I owned it for years before that fan came on the first time. I remember that day well, it was in the desert in Nevada or California when I heard the strange sound. I stopped at a rest area to check it out. Most of the other cars were stopping due to overheating and the locals were warning them not to run their A/C I was not having any problems.

The point I am trying to make the hard way, is are you sure it should be on??


The fan does kick on when the Max a/c is on. what does that mean??



It means the engine cooling fan motor, and the relay, and the a/c switch are ok. Go back to the previous replies for checking other stuff, like the coolant sensor and wiring, and ensuring that the engine is actually getting hot enough to turn the cooling fan on.


I think TSB# 98166 might be relevant to your car. Maybe someone can give you more details, but it sounds like the temp gauge reads high on these cars.


i used the anti-freeze that you have to mix 50/50 yourself. if i didnt mix it right “too much anti-freeze and not enough water” could this be part of the cooling problem? if there was too much straight maybe its too thick… right?? then maybe the anti-freeze isnt getting hot enough. even tho the guage says it is. if this could be the problem. should i drain the stuff i did and buy the stuff already mixed?



I think if you really want to figure out the problem you’re going to need a Helms electrical manual and the factory diagnostic manual or possibly subscribe to ALLDATA and hope the cooling fan diagnostics are on there.

AutoZone provides a few cheesey schematics but it’s always debateable how accurate they are. Figure 17 shows it as a simple circuit, but…

The dual pressure switch is providing a ground for the relays when the AC is on, but note that this simple diagram does not go near that far.
About all I can recommend at this point is that you use a test light and verify that power is provided to the relays from TWO separate points - the 60 amp fuse (no doubt a fusible link) and the ECM power supply lead.
When the engine gets hot enough for the fans to cycle the black/white wires at the relay should be grounded through the PCM. (Verifiable with a VOM)

My memory is very fuzzy on this and the Ford diagrams I have here do not cover a Mystique, but it seems to me the PCM controls the fans through info obtained from the Engine Coolant Temp sensor and some models do not use a “normal” coolant temp switch to cycle the fans.
I also think that some models use the ECT to control the dashboard temp gauge also.

It could very well be an internal PCM problem and it’s going to require some very good information to trace the problem down since a PCM can be a fairly pricy guess.
I do not think that coolant mix has anything to do with it.
Sorry I can’t be of more help.

(And if the road dead ends, one could always wire up a simple toggle switch arrangement. Take a lead off the high speed relay black/white wire to the switch and then from switch to ground. This should energize the relay and provide high speed cooling.)


Here is a wiring diagram. Hope it helps.


THIS wiring diagram shows the engine cooling temperature sensor ONLY feeds the engine computer (PCM). Click of FIG 9. As you can see, the brown/green wire carries the power (12 volts) to the ects. It, also, powers the knock sensor. The W/Pk (white/pink) wire goes to PCM terminal 38. The PCM will ground this wire when it wants to turn on the cooling fan.
ANOTHER engine coolant temperature sensor feeds the dash gauge. An infra-red theromometer will read the engine temperature directly. Radiator repair shops have them.


I agree that the ECT (engine coolant temp) sensor provides a signal to the ECM.
However, on most late model Ford vehicles the ECM uses this input to operate the AC and the cooling fans.
The same ECT that provides input for the emissions/fuel injection also provides the ECM with a means of determining when to power the fans up.

From memory only, I think the early Topaz/Tempo cars used a separate coolant switch for the fans but everything else was ECM controlled only.
My '87 Sable used the ECT as a means of controlling the fans through the ICRM module (later called the CCRM), and my Lincoln Mark, as all others. operate on the same system.

I have several Helms manuals here that state the system works like this and Helms manuals are the electrical books that are provided to the Ford dealers for diagnosis and repair.
The exact words from the manual are: (in regards to the ECT used in the engine controls section)
“Measures engine coolant temperature. Used by the PCM to determine coolant fan and A/C operation, etc.”


One wiring diagram (different Ford model) shows the temperature sensor wire splitting, one to the gage, and one to the pcm. /// There is a factory TSB (Technical Service Bulletin) Article 98-21-3, on high gage reading. Ford considers a temperature to be “high normal” to a temperature of 239F degrees. An “overheat” would be a temperature of over 239F degrees, with the temperature gage in the red zone. Ford does not make any repairs for temperatures below 240F degrees. The OP has been given adequate information for trouble-shooting, evaluation, and corrective action.


You could have put the toggle switch in by now instead of doing all this reading. Just joking around. Good luck with the better ideas.


when i tried to bipass the relay it kept blowing a fuse that would shut the car down. but when it idles the guage will climb. but when i drive it the only way the guage will move is down and it goes to the middle… could this be a thermos stat?? why would it go down when i drive it but go up when it idles.



The temp goes down when you’re moving simply because air is now flowing through the radiator. At idle you have no air movement unless the fan is operating.

As to the fuse blowing, exactly which fuse is blowing and which color wire leads are you jumping?