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Another Auxilary (electric) Fan Issue

S

sflier

NAXJA Forum User
Location
Danbury, CT
My auxiliary fan only comes on when turn on the A/C, it won’t come on when the engine temperature is above 200 degrees like the FSM says it should. It’s a ’94 4.0L with idiot lights, no gauges. I checked the ohms on the coolant temperature sensor (on the thermostat housing) and the readings were all in spec for the various temperatures I checked so I know that’s working. Best as I could tell from the wiring diagrams is that if the fan comes on when I turn on the A/C, the fan relay is okay. I’m stumped. Can I just plug a 300 ohms resister into the sensor connector to trick it into thinking engine is running over 200 degrees? I’m assuming the PCM could be at fault but I have no other PCM symptoms (i.e. engine runs great otherwise). Any ideas?
 
My XJ has a sensor for fan in rad, which operates fan, the sensor on the back of the engine is for gauge or light.
 
thermostat, would be my guess, im having cooling probs and mine only comes on when AC is on and sometimes on its own. a few ppl told me it was the thermostat going bad or it is bad. i dont know if that will help any
 
'94 has its coolant temperature sensor on the thermostat housing and the temperature of the coolant is getting above 200 degrees. The fan is just not coming on like the FSM says it should. Again, anyone know if I can just plug a 300 ohms resistor into the connection without harming the PCM? That will trick the PCM into thinking the coolant temp is over 212 degrees.
 
yes! you certainly can substitute a resistor for the CTS! That is a valid and acceptable diagnostic procedure.
 
You can also just find the relay (RENIX models had it on the driver's side fenderwell, I think OBD-I should be similar,) and wire up an override switch to that, allowing you manually "force" the fan on.

Since you say the fan's coming on with AC, I think we can assume that the relay and fan are good. I'd check anyhow, and I'd need to get an FSM handy to check out wiring diagrams, but that sounds like your best bet.

To check to see if you've got the fan relay, have the engine and AC on (electric fan should be running.) Pull out the relay you think drives the fan - if the fan goes off, you've got it. But, don't leave it out too long, the engine will appreciate having that fan on more than not...

5-90
 
If you jump the temp sensor, don't leave it jumpered.
The ECU needs to know the correct coolant temp for other things beside the fan.
My guess it it's just not getting hot enough to turn the fan on.
 
Thanks guys, I'll give all the suggestions a try and report back. I started looking at the coolant temp sensor after I noticed that she seems to take a bit longer to start when the outside temperature is over 70 degrees. I had read a good thread suggesting looking at the coolant temp sensor as a potential source of longer crank times as the PCM gets its initial injector pulse setting from that sensor. When I looked into testing the sensor with a multi-meter, I noticed this fan-not-turning-on issue. The sensor seems good ... I let the engine run a while then shut it down and read ~400 ohms of resistence. Based on my read of the FSM specs, that should have turned the fan on, but it didn't. I'll try plugging a (temporary) 300 ohm resistor in place of the sensor and see if it turns the fan on. If not, I'll test the relay per 5-90's suggestion.
 
Well I plugged a 280 Ohms resistor into the temperature sensor plug and the fan came on as expected. I ended up replacing the sensor which fixed the problem. I'm assuming it was just getting slightly inaccurate at the higher engine temperature range (i.e. low resistence side).

Flip94ta, I think you're right about the fan coming on at 220 and off at 200. I was sure I read in the FSM that it should come on at 200 but I agree that it seems too low given the 195 thermostat.
 
I just had my AC converted/recharged and now the electric fan is kicking in.

However since my 90 XJ normally runs around 100C (212F) when warmed up the fan is cycling on and off every 10-17 seconds and clicking of the relay is driving me nuts.

Is there any way to adjust the sensor or should I look into replacing it?
 
===================================
Is there any way to adjust the sensor or should I look into replacing it?
===================================
If you AC is on and/or your setting in hot weather? I would say she is operating normally. On the out side your clutch fan MAY be a bit weak. As long as it is keeping engine temp normal. I would not be in a rush to change things.
 
FRAM said:
I just had my AC converted/recharged and now the electric fan is kicking in.

However since my 90 XJ normally runs around 100C (212F) when warmed up the fan is cycling on and off every 10-17 seconds and clicking of the relay is driving me nuts.

Is there any way to adjust the sensor or should I look into replacing it?
Click to expand...

What sensor? The difference in R12 and 134a can cause that. If the pressure in your AC drop low when it's running it will turn the Compressor and Fan off. the cure is a better conversion.
 
Last edited:
badron said:
===================================
Is there any way to adjust the sensor or should I look into replacing it?
===================================
If you AC is on and/or your setting in hot weather? I would say she is operating normally. On the out side your clutch fan MAY be a bit weak. As long as it is keeping engine temp normal. I would not be in a rush to change things.
Click to expand...

26C yesterday when this was happening. It happened again in the late evening last night and this morning when the temp was lower but yes the cycling was spread out more. It happens whenever I turn on the A/C. When the electric fan kicks in it drops the engine temp fast. When it's off it the temp returns to just slightly over 100 when the electric fan kicks in again and the cycle repeats.

langer1 said:
What sensor? The difference in R12 and 134a can cause that. If the pressure in your AC drop low when it's running it will turn the Compressor and Fan off. the cure is a better conversion.
Click to expand...

I assume there was a sensor to dictate when the electric fan needs to come on versus running whenever the AC was running. So you think the AC compressor is shutting down at the same time too?
 
Last edited:
It is completely normal for the electric fan to cycle with the a/c compressor..its designed that way to force air across the condenser that is located in front of the radiator when the compressor is running.
Quick cycles of the compressor indicate 1 of 2 things...slightly low on freon, or, low blower fan speed causing cold temp cycle of the system.

If you have your a/c system on max, blower fan high and engine rpm at ~2000 rpm, the clutch should not cycle. if it does, the system is low on freon and either was undercharged or there is a leak.
 
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