1999 Jeep Cherokee temperature anomaly

The SO has been driving me 1999 Jeep Cherokee because it has air conditioning. She’s run into a situation where she’s using the a/c and the temperature gauge indicates overheating. There’s apparently a warning chime as well. Note that we’re in Northern Washington and it doesn’t get that hot here. When she turns the a/c off, the temperature gauge drops to normal immediately. Were the engine really overheating, the gauge would not drop like that, but drop gradually. I have not witnessed this problem, as I usually drive (the Prius) when we go out.

Any clues as to what’s happening?

I’m not sure I can help, but unless the changed it in the 95 redesign (I had a 1993 for a number of years) the 4.0 Cherokee had two fans, the main fan was driven by the serpentine belt and there was an auxillary electric fan that only engaged when the A/C was turned or it reached a certain temperature. When my mechanic accidently left it unplugged it would redline if the AC was on while idling, but would be OK if the AC was off while idling or was either on or off while driving.

I’d start by checking the coolant level. A low coolant level, if it’s below the temperature sensor, will make the gauge do some really goofy things. It’ll read hot when the water hits it (since it’s over heating, being low on coolant), but when the level falls back down below the sensor, it’ll read cold again.

If you’re not mechanically inclined, you could see if you get heat in the cabin. No heat usually means no coolant. Even if the car claims to be overheating.

As for it matching with the AC, my only thought is that with the AC on, everything is working harder and faster and maybe the pump is keeping the water level pushed up past the sensor. But that’s just a WAG.

Like I said, I’d check the water level just to start.

Out of curiosity, and I assume you would have mentioned it if it was the case, but does anything else odd happen when you turn your AC on or off? Do any lights dim? low battery light? Any of the other warning lights?

I was thinking about the possibility of the temp sensor being bad (which it still could be) but it seemed odd to me that it would correlate with the AC. Then I started to wonder if the AC is dragging on the alternator/battery. If that was the case, it might case other problems as well. But that seems like a long shot.

I wonder if the AC clutch is bad and making it hard for the belts to turn. How are the belts set up in that car and could it manifest it self like that? Maybe slowing down the water pump? Does the steering get difficult when the AC is on?

I think Joey might be on the right track. Some cars use a voltage limiter to operate the gages. The a/c voltage draw may somehow be interering with it. My first suspect would be a defective ground somewhere, possibly to the engine. Has the starter been changed lately? Or the alternator?

Would the gauge drop immediately to normal in this case?

I don’t drive the Jeep anymore, unless it’s snowing. (The Prius gets slightly better fuel mileage.) After having sat for a long time, I took it to the shop last December. New battery, tune-up, oil change and filter, cooling system flush and refill, brakes and fluid, wheel cylinders, belts, and of course the ignition switch that caused the lack of accessories that promted the trip to the shop in the first place.

I’ll go out and check the coolant level after I wake up a little.

Low coolant can (IME) cause the gauge to rapidly swing. If the coolant is low, the engine will overheat, when the hot coolant surges past the temp sensor, it sense it and push the gauge into the red, but when a pocket of air passes by the sensor cools off (even though the engine is still hot) and the gauge drops back to normal.

The correlation with the AC makes me think this probably isn’t it, but like I said, this is the first thing I’d check.

I have never though this made sense. If the coolant is say 225f the air in the system is going to be either 225f or very damn close to that. It not like the sensor is going to ignore any temp unless it is from coolant. It is a temp sensor, it detects temp it does not check the temps passport before reporting the temp.
If I understand the OP correctly the temp gauge spikes as soon as the AC is turned on and comes right down as soon as it is turned off.
If this is the case I would get a wiring diagram and look for a common ground circuit inside the car between the gauge cluster and AC control unit and possibly the heater fan circuit.
The symptoms are just like a bad ground.

Actually, the temperature is usually normal while the a/c is on. But sometimes it will spike, and then come right down when the a/c is turned off.

I checked the radiator, and it’s green to the top.

In that case I wod suspect either an electric fan that is not operating or a partially plugged radiator.

Thanks, Rick. I’m guessing the radiator isn’t partially plugged, since I had it flushed less than a year ago. It’s an intermittent problem, so maybe a fan plug isn’t secure? I’ll have to see what I can see.

If the o/heating is around town or at idle = fan issue. If it happens on highway at speed it’s the radiator.

She’s driving it tomorrow, and it’s supposed to be warm. I’ll ask her to keep an eye on it.

[Not arguing, just learning…]
You never understood it AND you’ve never seen it happen or you’ve never understood it BUT you’ve seen it happen?

Also, I was thinking about it. I wonder if it’s the opposite. Instead of the gauge running hot and then going back to normal when a pocket of air hits it, maybe it runs normal (or warm) and then red lines when a pocket of steam hits it. That might make more sense. Of if there’s not much water in it, it might stay red lined if there’s just steam running around, but come down a but when some water comes by and cools it off a bit.

Either way, would you say that temp gauges act funny when the coolant level is low? All I have to go on is the one time I had it happen to me. A 95 (or so) Ford Bronco, the coolant was probably about half way down, the temp needle was all over the place and I was confused as hell as to what was going on until I turned on the heat and didn’t get any.

I’ve never seen it but I have heard it described by others.

I know of cases where engines overheated but the temp gauge reading didn’t show it, even though the temp gauge system was working. Temperature sending units and sensors are designed to be immersed in liquid, and some if not all of them will not read accurately when surrounded by vapor rather than liquid.

I have a digital pyrometer (electric thermometer) that has three different sensing probes. One is for use in air (or an airstream), another is for immersion in liquid, and the third for physical contact on a solid surface. They may all read the same when first plugged in after they’ve been sitting for some time, but while each one responds well in its intended medium, they respond poorly if at all in the other media.

Anytime the temp gauge reading is squirrelly, a logical first step is to make sure the system is full with no air pockets.

Some more information.

SO reports that she’s been watching the temperature to see what it’s doing. Straight up on the gauge is 210 degrees, and the temperature runs ‘one tick’ above that. I haven’t driven the Jeep recently, but IIRC that’s its normal operating temperature.

With the a/c on high, she heard the chime and immediately looked at the gauge. It went from ‘normal’ to like 250ºF. She turned off the a/c. In about a minute the gauge went back to normal. But it did not move back to that indication gradually. It stayed high, and then took one second to go back to normal. When she turned the a/c back on, all was well.