GRohr

I got my dad a 97 ZJ 4.0L a yr ago. Today he called and said it cranks but wont start. I told him how and where to check the CPS. The only part that I am unsure of is that the fuel symbol stays on apparently and does not turn off like the rest of the lights. I am going to have him check the ASD and pump relays as well. Is there anything else that I should have him check?

Thanks

dave1123

Yeah, have him check that there's ACTUALLY fuel in it. I think that gas pump symbol is actually a low fuel warning light. It's difficult to do, but someone may have siphoned gas out of it.

GRohr

Has gas, has an alarm system on it and I guess the light is on but not flashing. It normally flashes when it is alarmed/activated. Just found out that when he turns it to run but doesnt try cranking it, the guages dont work. I believe that this can still be from the CPS (am I wrong?). He is dealing with the CPS now, its a bit hard when I am 3 states away.....

Thanks again guys

dave1123

In start mode, the CPS reads the slots in the flexplate to tell the PCM that the crankshaft is turning. When the PCM gets this signal, it turns on the ASD relay and the fuel pump relay. The ASD relay turns on the ignition and fuel injector circuits. Once it's running, the CPS signal tells the PCM when a cylinder is at top center. The CAM sensor tells the PCM which cylinder to fire. The PCM decides spark timing depending on rpm, throttle position, vacuum and load. It reads the 02 sensor and engine temp for fuel trim. It's a bit more complicated than that, but those are the basics.

Any interruption in the CPS signal and the ASD relay drops out. It will also drop out if the security system tells it to. I think the problem isn't the security system because the gauges don't turn on. If the security system was set, the engine would start and run for 2 seconds, then die. It may be in the ignition switch itself.

GRohr

Yup exactly it. The alarm was still triggered but i think that was from him just trying to start it so much. He switched out the CPS and it started right up. The fuel symbol was a bit confusing but once i read about the gauges trick to test the CPS it was pretty clear that it was the problem.

thanks guys

97grand4.0

"once i read about the gauges trick to test the CPS it was pretty clear that it was the problem."

New one on me, what is the gauges trick to test the cps?

GRohr

I found a post that OleBlue put up from cj7-tim. There is a much longer version but I just copied what I read that was super helpful about the gauges. In our case, it wasn't just the voltage gauge and fuel gauge but all of them except for the fuel light indicator. Once my dad unplugged the CPS and turned the ignition to run, they all worked correctly.

Diagnostic steps to confirm the CPS is the cause of your no-start
1) You should be able to verify a bad cps, by unplugging it, and turning the ignition key to on. If the voltage gauge and/or the fuel gauge now display correctly and/or the No-Bus is gone, replace the CPS.
2) If the CEL does not come on when the key is turned to the RUN position... then the CPS/CKP is bad.
3) Unplugging and reconnecting the CPS sensor where it connect to the main harness near the back of the intake manifold usually resets the ECU and if the jeep fires right up after doing this you can bet that the CPS is faulty and needs to be replaced.

The 2000 and 2001 will have the CPS in the same location on the bell housing, but the wire connector is near the Transfer Case, not as shown in the diagram below. Simply follow the wire from the sensor to the connector.

CPS Testing

Crankshaft Position Sensor Connector (CPS/CKP)

97grand4.0

"I got my dad a 97 ZJ 4.0L a yr ago. Today he called and said it cranks but wont start. I told him how and where to check the CPS."

The OP says he had a 97. This works on a 97?