ganger

I just bought a Cherokee and the temperature gauge does not work (yet). I squeezed the radiator hose to see how well it was pumping and it didn't feel like it was at all. I took out the thermostat & boiled some water & tested it, and there's nothing wrong with it. While the thermostat housing was off i cranked the truck up and had someone watch the hole to see if coolant shot out of it, to see if the water pump was pumping, and it was. Do these 4.0s not pump coolant that hard, or what? Also, does the electric fan kick on when the engine is getting hot, or when it warms up to normal temp? I've noticed it kicks on with the A/C.

5-90

A few things (in some points I could tell you more if you'd give with your model year - it's helpful.)

The thermostat, as you have seen, remains closed until a specific temperature is reached (typically 160*, 180*, or 192-195* - for the AMC 242ci, it wants to run at 210*F or so, requiring a 192-195* thermostat.) Before it opens in engine operation, coolant flow will be negligible. (There's a bypass cast into the cylinder head that gets covered by the thermostat housing, but it's there mainly to help prevent the formation of hot spots that would normally occur of coolant flow was entirely restricted.)

Pressure in the cooling system is not developed by the pump - the pump is an impeller type, and servies only to generate flow potential. The pump must be an impeller type - it's always running with the engine, and a positive displacement pump would blow out if it's deadheaded (by, say, a closed thermostat.) An impeller pump may spin without signficant resistance to "no flow" conditions. Caps are rated for 13-16 psig on our XJs (closer to 16psig, usually,) and you may not notice that if your grip is strong.

Coolant does not, effectively, circulate until the thermostat opens. This means that you'll have simply a nominal flow (by way of the bypass passage) until the thermostat opens and allows main flow.

Flow, when it's generated by the pump and allowed by an open thermostat, is a little more than enough to circulate the coolant through the system and the radiator. You do not need a high flow rate - and it's possible to have the coolant running through the system "too fast" anyhow (heat is rejected from a system at a fixed rate. The coolant must stay in the radiator long enough for this rate to allow maximum heat rejection - move it too fast, and you won't throw off all the heat you need to.)

The electric fan (the auxiliary on the driver's side) takes its reading either from a switch - on the 1987-1990 "RENIX" models - or from the ECU - 1991-2001 "HO" models. The fan typically comes ON sometime around 220*F, and turns OFF down around 205*. The fan will also turn ON if there is an "A/C REQ" (Air Conditioning Request) signal from the ECU, to account for the extra thermal load of the aircon (due to the condenser being mounted in front of the radiator.) A quick check of the fan circuit proper is to have the engine running while cold, turn the aircon on, and see if the fan comes on.

I could tell you more about the electrical setup if you tell me what year yours is - it changed a few times. I suppose I could look at your profile - but I want to get you in the habit of helping others help you. Recall that there s no such thing as "useless information" when someone is helping you troubleshoot - either you haven't figured out what to do with it, or you don't know what it means (but little details could be the ones that crack the problem wide open. Start with year/powertrain combo, and we can ask you more data if we need it. But, if you're doing your job properly, we won't have to ask for anything obvious - more like, we ask you to check something out and report back.)

SeriousOffroad

What year?
Older model XJ's (95 an older, IIRC) have a sending unit located in the top driverside corner of the ctlinder head. If the sender goes out, your temp gauge won't work.
The water temp sensor is located in the water neck where the thermostat is aat. If this sensor fails (open) the Aux. Fan will be on continuously.

The fan comes on under the following normal operating conditions:
Engine temp. (on at 218Deg F., off at 209 Deg F.)
A/C is turned on (Cycles on and off based on compressor clutch cycling).

The Fan Relay is activated when the PCM completes the ground circuit. This energizes the relay coil which closes the Normally Open (N/O) contacts in the relay and sends battery power to the fan motor.

SeriousOffroad

D'Oh! Seems someone beat me to the punch!!


How ya doin' 5-90?

ganger

My Jeep is a '92, 4.0 HO, auto. I've noticed on other vehicles i've owned, you could feel a surge in the radiator hose when you squeezed and released it, after the vehicle had warmed up. Also, i bought a Haynes manual and it says to check the water pump by squeezing the hose. I suppose it needs a new coolant sensor, as i believe the fan is on all the time.

5-90

Haynes? Buy a Chilton's, and use the Haynes to soak up your next oil spill.

The sensor for the gage is at the driver's side rear of the cylinder head, as mentioned. That one has nothing to do with the fan.

The sensor in the thermostat housing reports to the ECU on engine coolant temperature, and the ECU uses that information to decide when to turn the fan on WRT temperature. If the ECU gets an AC REQ signal, the fan goes on.

The ECU trips the fan by either energising or grounding a relay coil (I'd have to check,) but if your fan is on all the time, it's possible that your relay is "frozen shut" - it's rare, but it happens. (I think I've seen it 3-4 times in the last thirty years.)

However, if your gage isn't working, that doesn't have anything to do with the fan. If the fan isn't working, that doesn't have anything to do with the gage (those sensors were finally merged in 1996, when OBD-II rolled out. There's one CTS on 1996-up rigs, and it reports to the ECU. The ECU drives the gage over the CAN bus, drives the fan relay, and that sort of thing - just from that single sensor.)

Be very careful when removing that sensor - it's been known to have the head twist off, then you're in for a workday to get the body out! I've found it helpful to try to tighten it slightly - about 1/4-turn, max - before trying to back it out. This can give you a running start over the crud that is holding it in place.

Clean the hole out (a wire bore brush will serve - about a .44 or .45 cal or so) and put some never-seez on the new sensor. Tighten to 2-3 "flats past finger tight" to seal it, check it at first run to make sure the seal has taken. Tighten only if the coolant is leaking! (It should not be - 2FPFT is actually just a bit much for that size of fitting - 1/8"NPT likes 1 to 1-1/2 FPFT to seal.)

Wrench size is 7/16", I find a deep socket is easiest to get in there.