MoparFirstTimer

Hello. I just bought my 99 Jeep Cherokee with a 4.0 a couple of days ago. It had been doing well. Today however, temperature went a little over 210, and the vehicle shut off. It would crank and crank, but not start. Assuming it was a bad fuel pump, I left the vehicle alone for a while. A good 20 to 30 minutes later, I go by the Jeep, and it started. I seized the opportunity, and drove it back home. When I got home the coolant sounded like it was gurgling. I already have my suspicions as to why it is overheating. Does anyone have any ideas as to why it shut down and would not start though? Could it have had something to do with the overheating?

MoparFirstTimer

Quick update:
I went to Autozone today today to run the diagnostic scanner, and it picked up three codes. The first two do not really sound relevant to the vehicle shutting off, although may be. The third, I am not sure what it means.

P1491- Radiator control fan relay circuit malfunction
P1494- EVAP leak detection pump switch or mechanical fault
P1694- No PCM Bus messages.

Waterluvr

Replace the crank position sensor and remove the battery terminals to allow the PCM time to reset while that is being changed, faulty CPS will create issues on the PCI BUS and trigger code P1694.

Obviously, with the engine off your scanner is able to communicate over the PCI BUS so the circuit itself is intact.

MoparFirstTimer

Thank you. I think if anything, I will replace the crank sensor just in case. Yesterday, I replaced the E- Fan relay, and reconnected the little elbow that goes to the intake manifold. After that, I cleared the codes, and it has been working better since. I have driven a good 70 miles with no shutting off, or check engine light turning back on. I think the jeep indeed turned off due to overheating. Just as a precaution though, I will replace the crank sensor

Waterluvr

How many miles are on your Jeep? Assuming your relay was showing signs it had gotten hot? Burnt relay contacts are a pretty solid indicator the efan motor is tired and drawing to many amps.

dave1123

The fan relay on the 99 is a pulse modulated unit mounted underneath the right front headlight support and requires cutting an access hole it get to it. It's controlled by the PCM and gets quite hot because it pulses very rapidly to reduce the voltage to control fan speed. Here's a pic. Worn bearings in the fan motor can cause it to draw too much current and make it overheat.

Waterluvr

That's a great picture Dave lol! When the engineers didn't think to make an access hole and it's almost 20 years later

dave1123

The funny part is I got that picture off this forum. When I needed to do mine, I pulled out my headlight and wella! Someone had already cut a hole! The reason it's mounted there is for airflow cooling. Incidentally, if you're looking for the vacuum reservoir, it's just in front of the relay.

MoparFirstTimer

My Jeep has about 144k. The old relay did not seem burnt. What I did notice though, is since I installed the relay, the e-fan is always on after starting the vehicle. I am guessing the previous owner may have rigged it.

dave1123

I think it was in 2002 they switched to 2 relays in the PDC (underhood fuse/relay box) and a 2-speed fan. It took them that long to realize they had a weak spot in the old system. Same thing with the Teves to Akebono brake calipers. Maybe it took them that long to use up the old parts! OH....2002 also saw the "TUPY" head with the 0331.

One thing you've got to understand. In 99, jeep started with a total electronic data system where all the modules were interconnected and shared data. The PCM was the Big Boss and "put out the instructions" as it were to everything else. There is no connection from the sensors to the gauges. That information is sent to the PCM, interpreted, then sent to the gauges. The body control module handles all the interior lighting, power windows, door locks, security key identification, even the windshield wipers, but takes it's instructions from the PCM. This makes diagnosis of problems very complicated.

Waterluvr

You are one of the very few people I have come across on forums that has a legitimate working knowledge of how the PCM controls EVERYTHING on post 98 models, well done!

I cringe reading some of the threads where people are just lost diagnosing problems and throwing their wallet randomly at sensors etc. hoping for a positive outcome that never happens.

MoparFirstTimer

My mechanic would call that "playing witch mechanic"

But I have got to agree. I work at an automotive parts store, and it amazes me to see how many customers will come in asking for random sensors or parts without having even used the diagnostic tool on it yet.

dave1123

Actually I bought the Full Service Manual for my WJ from pacificcoastmanuals.com and actually READ how the systems function! Having a little knowledge about computer systems helped me understand. Do you know what a "driver" is? It's a solid state switch that uses a 5 volt input to switch higher voltages or loads. For instance, the fuel injectors are supplied with a constant 12 volts and are grounded thru drivers in the PCM to actuate. The PCM controls injector "on-time" to control mixture. The 3 coils in the coil pack are also energized constantly building magnetic fields and are UNgrounded by the PCM thru drivers to create the high voltage spark. The PCM can "read" the 02 sensors quickly enough to adjust the mixture for the next cylinder to fire! Nothing is analog anymore! You can thank the FEDS for their higher fuel mileage standards for making engine control systems this complicated. BTW, the 4.0 uses 3 coils to fire all 6 plugs, 2 at a time. It's called "waste spark" and the plugs fire at the top of the exhaust stroke in an effort to reduce emissions.

The FSM can be downloaded for around $15 and is about the best source of info there is about your specific year jeep. It's a compressed pdf copy of the Factory Service Manual used by dealerships. It explains how each system works, how to diagnose problems, where to find components, how to remove them, and complete wiring diagrams with pin-outs for the connectors. There is also a COMPLETE list of all trouble codes in numerical order.

Waterluvr

I program and repair some common issues with ECU's/PCM's so I'm very familiar with how it all works but that's a great outline of how the critical driver circuits function, far to often it seems people just assume computers distribute power to control functions.

We started using "switchboxes" bedded in electronics potting compound back in the late 70's on outboard motors to control all the critical ground functions on marine ignitions and to eliminate old school points/condenser type ignitions, interesting what that technology evolved into with automobiles and emissions related functions.