Temp guage question

[Highmiles]

I've searched, but can't seem to find any leads on this one...

Driving home in stop & go heavy traffic in 90* heat with the AC on... The temp guage jumps from its normal 210* up to 250* (just under the red). I turn off the AC and within 2 to 3 minutes the temp drops back to 210*

I flushed the system in the spring - no issues, no leaks. The mechanical fan works and is tight when the Jeep is off. I'll wire up the electrical fan tonight to check that.

Is it possible to gain / loose 40* in a matter of 5 minutes or is the guage suspect?

 

[langer1]

You know that's backwards from what normally happens, usually turning the A/C on forces the electric fan on, and lowering temp. I would look at water flow through the radiator
. It sounds like your cooling system is just barely keeping up, when the A/C is running it heats the air going through the radiator causing your temperature to rise.

 

[Highmiles]

I guess my confusion is still can you see a 40* temp. change? I mean, if my electric fan is failing, sometimes running and sometimes not, would you see that much of a change?

 

[Eagle]

I guess my confusion is still can you see a 40* temp. change? I mean, if my electric fan is failing, sometimes running and sometimes not, would you see that much of a change?

Yes, you would. (a) The a/c compressor is a drag, making the engine work harder; and (b) the a/c system more or less by definition is removing heat from the interior of the cabin. Where do you think that heat is going? Into the condenser, which is located directly in front of the radiator. That's why the aux fan is supposed to come on whenever the a/c compressor kicks on -- if it's not working, overheating is the expected result.

 

[Highmiles]

Given my situation - 30 + minutes in heavy traffic in 90+ heat with AC blasting - temp is normal (210*). Suddenly, the temp jumps to 250*, then, within a minute of turning off the AC (literally, I timed it) it drops back to 210*.

Should I suspect a faulty fan relay or connection that is stopping the fan? I figure the temp would've jumped much earlier if the fan wasn't working at all.

 

[langer1]

Given my situation - 30 + minutes in heavy traffic in 90+ heat with AC blasting - temp is normal (210*). Suddenly, the temp jumps to 250*, then, within a minute of turning off the AC (literally, I timed it) it drops back to 210*.

Should I suspect a faulty fan relay or connection that is stopping the fan? I figure the temp would've jumped much earlier if the fan wasn't working at all.

The A/C will not work without the fan.
Is the A/C working?

 

[Highmiles]

Yes, the AC is working like a champ.

 

[langer1]

Yes, the AC is working like a champ.

Then it's not the fan or relay, Dirty cooling system low on water would be next.

 

[Highmiles]

I know it doesn't mean anything, but it was flushed this spring. The fluid is new from the flush and looks - according to the bottle - to be full. I've also checked the top hose coming off the rad to make sure it heats up - it does.

My thought that with poor flow thru the rad the temp would be constantly high. It isn't - it jumps from 210 to 250, then drops just as quickly back to 210.

My FSM gives suggestions for constantly high temps, but I think I could be chasing this one for a while. I'd also hate to be chasing nothing but a faulty gauge.

 

[Rattman]

Do you have coolant in the cooling system or just water?

 

[langer1]

. The engine driven clutch fan could be weak then
, it’s working well enough for just the engine heat but not good enough for the added heat from the A/C.



The clutch fan is suppose to slip at, lets say 210 but lock up at higher temperature, sounds like yours is not doing that.

 

[Highmiles]

The best coolant that Jiffy Lube offers... They're the guys that did the rad flush this spring, but I've had hot days and traffic without incident before this...

I had this thought - about 3 weeks ago I had the dealer replace the AC clutch and serp belt. Clutch was bad and belt was replaced because I figured it was time. Is it possible / likely that the water pump is slipping on the belt, not forcing coolant through the motor and causing the temp rise?

I am hearing no squeal form a slipping belt, but I just had the thought.

 

[langer1]

The best coolant that Jiffy Lube offers... They're the guys that did the rad flush this spring, but I've had hot days and traffic without incident before this...

I had this thought - about 3 weeks ago I had the dealer replace the AC clutch and serp belt. Clutch was bad and belt was replaced because I figured it was time. Is it possible / likely that the water pump is slipping on the belt, not forcing coolant through the motor and causing the temp rise?

I am hearing no squeal form a slipping belt, but I just had the thought.

It's possible, it's not uncommon for a new belt to stretch a little, the water pump just runs off the backside of the belt, so it could be the first to slip.

 

[Highmiles]

OK - yesterday was 90+, traffic sucked, and the AC worked fine with no overheating. My only thought is that something is killing the Aux fan intermitantly.

I was thinking of starting with replacing the relay, but it's an $80 part! Is there any way to test this before I spend $80 on a guess? If not, is there a temp sensor or switch that might be easier to test and help me narrow this down?

 

[langer1]

OK - yesterday was 90+, traffic sucked, and the AC worked fine with no overheating. My only thought is that something is killing the Aux fan intermitantly.

I was thinking of starting with replacing the relay, but it's an $80 part! Is there any way to test this before I spend $80 on a guess? If not, is there a temp sensor or switch that might be easier to test and help me narrow this down?

As soon a the Aux fan stopped so would the A/C, if your AUX fan stopped the A/C would stop also and the water temp would rise.
A lose belt don't always squeal.

 

[Eagle]

Langer1, can you explain how the a/c compressor knows when the aux fan isn't working? The a/c circuit also activates the fan relay, but if the relay doesn't work or the fan motor itself doesn't work, how does the a/c know this and not function?

I don't think it's that smart.

 

[Highmiles]

Here's what my FSM says...

Electric cooling fan 4.0L: With or without A/C
request the electric fan will come on only when the
coolant temperature is at least 106° C (223° F), and
will remain on until the coolant temperature drops to
103° C (217° F) or below. Regardless of coolant temperature,
When air conditioning system pressures
reach 2068.5 +/- 138 kPa (300 +/- 20 psi) the electric
fan will engage and continue to run until the A/C
system pressure drops to 1620.3 kPa (235 psi) minimum,
then the electric fan will shut off.

So, it seems there are two triggers for the Aux fan, one being temp and the second being pressure.

 

[langer1]

Langer1, can you explain how the a/c compressor knows when the aux fan isn't working? The a/c circuit also activates the fan relay, but if the relay doesn't work or the fan motor itself doesn't work, how does the a/c know this and not function?

I don't think it's that smart.

If the AUX fan stops, the condenser will stop working causing pressure to rise and High pressure switch to trip. You will also feel the air temp rise because no liquid is being returned to the evaporator

 

[Highmiles]

As I read further in the ol' FSM....

TEMPERATURE GAUGE READS
HIGH OR THE COOLANT
WARNING LAMP ILLUMINATES.
COOLANT MAY OR MAY NOT BE
LOST OR LEAKING FROM THE
COOLING SYSTEM
1. Trailer is being towed, a steep
hill is being climbed, vehicle is
operated in slow moving traffic, or
engine is being idled with very high
ambient (outside) temperatures and
the air conditioning is on. Higher
altitudes could aggravate these
conditions.
1. This may be a temporary
condition and repair is not
necessary. Turn off the air
conditioning and attempt to drive the
vehicle without any of the previous
conditions. Observe the temperature
gauge. The gauge should return to
the normal range. If the gauge does
not return to the normal range,
determine the cause for overheating
and repair. Refer to Possible
Causes (2-20).

I guess this is just normal?! I love the phrase "repair is not necessary"!