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Aux fan will not engage when supposed to

basso4735

NAXJA Forum User
Location
ma
My auxillary electric fan does not come on when my engine temp gets above 210. it does come on for the a/c, so i know it works. what do i need to check/test and what can i do to fix it?

its a 96, auto, 4.0

Thanks in advance.
 
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What year? Mine doesn't kick in until my temp. gage shows 220 or a little above (my temp. gage is a little stingy).
 
sorry, 96 classic. my temp gauge was almost at the 3/4 line then i shut it off. a good deal above 210. i belive it *should* have come on and it did not.
 
I've never had any trouble with mine in the 96. But from what I understand the sensor in the thermostat housing tells the computer when to turn the fan on. And the computer switches the fan relay on. Fuse? relay? sensor? I don't really know if the fan is wired separately in the 96 for the AC mode, it is in the RENIX.
The easiest way to test the sensor is with the motor cold, first thing in the morning do an ohm test on the MAT sensor in the intake and the coolant sensor in the thermostat housing. The resistance changes with temperature, but with the motor cold, the temperature of both should be fairly close ( the ohm/temperature table to test them both is identical). They should read nearly the same resistance.
 
basso4735 said:
ok, ill try that tomorrow morning. anyone else want to chime in?
Probably the Temperature sensor in the thermostat housing is bad or the connection is loose or corroded. The signal to engage the A/C clutch typically is used to turn on the electric fan as well, thus bypassing the PCM, computer. Probably the same relay and fuse is used for both on signals, so I would suspect the T sensor in the Thermostat housing or the connection to it.
 
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ok, after searching all morning, i think i figured out the problem. while running i unplugged the temp sender in the thermostat housing and the aux fan came on. (yay!) So i assume i need a new temp sending unit?

my check engine light is now on, is that ok? i tried the key trick and i think i got codes 12 and 22, which mean computer was detached and coolant temp sensor, respectively.

also, anyone know how much one of the sending units run? at dealer or auto parts store?

thanks
 
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I had this same problem on my '92. I changed the sensor in the t-stat housing with no avail. I ended up twisting the prongs on the relay a little bit and it started working.
With the sensor unpugged it wanted to run rich so, I'd expect that if you leave it unplugged. You could also try some electrical contact cleaner on the relay and socket if ya want also. Chris
 
i just checked out the relay for the aux fan and i dont have one......

the spot where it should be doesnt have anything in it, so even if i put one in it wouldnt do anything. its just an open slot for some reason. hrmm.
 
basso4735 said:
i just checked out the relay for the aux fan and i dont have one......

the spot where it should be doesnt have anything in it, so even if i put one in it wouldnt do anything. its just an open slot for some reason. hrmm.

Try the A/C and see if you can find the relay that the A/C uses to power up the fan, the 96 probably uses the same relay to turn the fan on but uses two different paths, or signals to turn the relay on?

Mine is 87, so I don't know where your is. There were various changes in 91 and 96.
 
Ecomike said:
Probably the Temperature sensor in the thermostat housing is bad or the connection is loose or corroded. The signal to engage the A/C clutch typically is used to turn on the electric fan as well, thus bypassing the PCM, computer. Probably the same relay and fuse is used for both on signals, so I would suspect the T sensor in the Thermostat housing or the connection to it.

Now that I think more on this, doesn't the 96 use just one temperature sensor for the gauge and the computer? If so your sensor sounds OK, but the signal or wire from the computer to the relay is not working? I still think (guess, based on logic) that it uses the same relay to operate the fan, but two different power circuits run to it to power the relay.

The older models (91 to 95) had 2 sensors one for the gauge and one for the computer. The 90 and older models (Renix) had two sensors and a temperature switch in the radiator for operating the electric fan.

Either way you need find a wiring diagram for the 96 and find out if the ECU powers up the relay in both cases or if the A/C has a seperate signal to the relay since the A/C powers up the fan.
 
Ecomike said:
Now that I think more on this, doesn't the 96 use just one temperature sensor for the gauge and the computer? If so your sensor sounds OK, but the signal or wire from the computer to the relay is not working? I still think (guess, based on logic) that it uses the same relay to operate the fan, but two different power circuits run to it to power the relay.

The older models (91 to 95) had 2 sensors one for the gauge and one for the computer. The 90 and older models (Renix) had two sensors and a temperature switch in the radiator for operating the electric fan.

Either way you need find a wiring diagram for the 96 and find out if the ECU powers up the relay in both cases or if the A/C has a seperate signal to the relay since the A/C powers up the fan.
i have 2. i also have the one on the drivers side at the back of the block near the firewall.
 
heres pics of the 2

dcp1685wt8.jpg


dcp1687dm4.jpg
 
basso4735 said:
i have 2. i also have the one on the drivers side at the back of the block near the firewall.

ok, after searching all morning, i think i figured out the problem. while running i unplugged the temp sender in the thermostat housing and the aux fan came on. (yay!) So i assume i need a new temp sending unit?

In that case it probably is the sensor in the thermostat housing. I would just buy one and replace it with that much data!

That is interesting that unpluging it turned the fan on, the older renix sensors get more conductive as they heat up, the newer ones (like yours) must increase in resistance as the coolant heats (the oposite of my older renix, 87 model) since the fan came on with infinte resistance (when you pulled the sensor wire the resistance became infinite and the computered on the fan).

I do not have any sensor test data for your year Temperature sensor, maybe some else else does (1996), they can be tetsed with a simple digital ohm meter if you know what the readings should be.

8Muds procedure below of comparing the two temperature sensors sounds like a good idea if you want to run a test on them before buying one. Might not be a bad idea as the gauge sensor could reading too high, like 20 or 30 degrees higher than it really is!

Might try searching here, and asking in a new thread if anyone knows the correct resistance readings at various temperatures for those two sensors. The more I think about it, it could either one, or both could be off about 10 to 15 degrees each in opposite directions!:doh:
 
Before buying a new one (a lthough they are fair ly cheap, 17.00 or so new from Advance or the Zone) consider going to a junkyard and pu l ling a few used ones to test.
 
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