If you wire the relay grounds to a switch or switches, if you have 2 fans,  the car will not set a code and they will switch off with the key.  I have had mine wired like this for a year and have never set a code.  I located my switches on the knee bolster. they are out of sight and reachable.  If you want to learn how to do this send me a message.  For l-98's.  Mine is a 90. 

Thanks Anthony -- the job is now done and seems to be ok -- tho it was explained that a code would be set but no CEL. So I'll see how it works out as it is.

Speaking of codes: I'm very interested in obtained a reader. I understand that my '94 is somewhat of a 'bastard' system being somewhere in between OBD1 and OBD2. My installer says there is a shop reader available but I need one that more affordable.

A reader for me would be more convenient that reading these codes thru the ALDL panel.

If anyone has any info on an available reader, I'd appreciate hearing about it.

BTW is my '94 a standard 12 pin connector such as this:

12-pin ALDL connector pinout:
F E D C B A
G H J K L M

Thanks,

 Colin.

Yes the PCM can 'see' the MFC but it does not throw a CEL. The PCM stores a 77 and 78 code but that's no biggie.

Love the manual control for when things appear to be getting too warm.

C.

How about unplug the A/C high pressure switch located adjacent to the RF shock absorber? One grn/wht, one blk/wht wire. The fans will run whenever the engine is running, but still turn off at 45MPH.

Would be a good compromise for summer weather. Fans will run all the time, except in excess of 45 MPH when they're not needed, but set no codes, nor is any wiring cut or modified.  During the winter, or cooler weather, plug it back in. Fans will operate normally.

Sometimes the easiest solutions are the most elegant, because they cost nothing, are effective, do no permanent damage, and are simple to reverse.

This is all old news. The OP modified his car months ago.

Cooling fan control circuit  DTC's 77 and 78 do not exist prior to Model Year 1992. So no, your 90 will not set a code, because it "isn't in there".

DTC 77 and 78 set when the voltage on the control wire to the relay is opposite of what the ECM is commanding.
When the fan is off,  the control is High. When the ECM commands fan ON, the control goes Low.

Grounding the control like you did in a 92-up for manual ON will set a code if the ECM sees the control Low, but is not commanding a fan-on.

There are several ways to wire the fans so codes aren't set. The method I came up with is to connect a 200 Ohm 1w resistors connected to 12volts to trick the ECM when I've run the control to ground to command manual-On. The ECM will see the dummy load as a relay, and not detect the difference. It can still pull the control voltage low, if it commands the fan on, and the voltage goes high when it turns the control off.

Again, this is old news. The OP modified his car months ago. I would have done it differently than he did, but he's happy, so that's what matters.

Sorry to bump an old thread, but I'm experiencing the problem. My fans work (if I turn on AC or Defroster) but with those off they turn on much later than I would like, I've just replaced waterpump, thermostat, and all new gaskets put in.

Next will be new radiator and lower cooling fan switch, but for a temp solution I was intrigued by unplugging the high pressure switch. Part of this could be my gauge is off (possibility) but it nears the 260 mark before the fans come on.

I have a 1991, and there are 2 plugs near the right shock absorber, one as described here with green/white wire but unplugging it did not start the fans. I tried the smaller plug next to it and still nothing. I then unplugged both and still nothing would get the fans running. Both of these plugs on are the small metal line that run into the air conditioning unit.

There is another larger plug up near the firewall, but unplugging that one did nothing also.

If someone knows how to make this work on a 91 I would really appreciate it.


|UPDATED|3/27/2015 11:38:18 AM (AZT)|/UPDATED|

I fitted a manual fan override myself. switch is under the dash, poking through the dash carpet thing. thus, it is invisible until you really look for it (probably climbing into the footwell ;) as I didn t want to disturb the original plastic dash or center console.

One thing I did for my 1988 was purchased from corvette central a lower temp fan switch. I am in AZ and temp gets hot here in the summertime after changing the fan switch the temps run 210-220 and does climb to 240 in stop and go