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Old 09-22-2007, 11:02 PM   #1
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Default Cooling fan Ambiguity in Service Manual


The text of the 94 service manual says that:


The PCM will complete the ground path for the winding of the low speed fans ("Fan 1") when it sees Engine Temperature above 100C (212F), or when A/C is requested.
The A/C head pressure switch signals the PCM to turn ON the high speed fans ("Fan 2") relays when A/C pressure exceeds 210 psi (1448 kPa). "Fan 2" is also turned on when engine coolant temperature is above 108C (226F).


A page or two further on, the schematic shows the temperature-sensor and the A/C head pressure switch signalling the PCM.... but nowhere does it show anything about the A/C "on" switch which is what I would assume is meant by "when A/C is requested".

Is it, in fact, supposed to turn on the fans low-speed, when A/C is simply switched on???

In my particular situation, the fans appear to be coming on a few degrees late... about 104C on the dash gauge for low-speed, and about 114C for high speed. But they do not come on at all when the A/C is switched on (if the coolant is cool). Of course the A/C has leaked out a couple of years ago and is not working, so the head pressure will never be high enough to trigger the High fan-speed condition, but should the LOW speed come on anyway???? And if so, does this automatically mean a PCM problem??

Oh... and is it a reasonable assumption that replacing the temp sensor will solve the late fans? Or are there other possibilities for the late fans?

Coincidentally, a "low coolant" light comes on randomly, even though the coolant is right up and appears to be in good condition (glycol).

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Old 09-22-2007, 11:07 PM   #2
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Regardless of the amount of refrigerant in the system, the fans should turn on low. Unless of course, its governed by the PCM seeing ANY compressor pressure and thereby turning the fans on since it did mention a higher pressure reading turning on the high speed setting. And actually that would make more sense since I'm pretty sure that, at least with MCC, there is nothing monitoring the position of the *****.
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