97 wont start

My 97 4.0 cranks but does not fire. There is no power getting to the coil and none at the fuse. also the fuel pump is not kicking on. I have changed the relays, cps, checked the fuel pump and now I am stuck and on the verge of replacing the ecm. What am I missing? What else can I check to help narrow it down to the computer? Is the computer directly responsible for activating the relays and is there something else that will prevent them from switching on. I bridged the auto shutdown relay which is responsible for sending power to the coil and still no fire to the distributor. I understand that the computer is responsible for sending intermitant ground to activate the coil. How can I test my computer?

Ok, no power to the primary side of the coil, no ground to the ASD relay from the PCM--start with the system grounds, then use an ohm meter to check the harness from the PCM to the ASD relay.

Do your gauges work?

I have checked the grounds below the coil and the grounds to the pcm...are there any more that I should check?

ok...I checked the guages and they don't work. The only one that shows any life is the oil pressure. It goes from dead to zero. The fuel and rpm and the rest don't do anything. I'm not sure what this means but apparently its important.

I'm suspecting a shorted CPS/CKP crank sensor.

Unplug the crank sensor and see if the dash gauges come alive. If they do, then replace the crank sensor.

I replaced the crank sensor.....would the cam sensor cause the same symptoms?

I have never heard of the synch sensor in the distributor causing a "no gauges" issue. The synch sensor in the Chryco XJs can prevent starting (it won't stop a Renix), but I believe that would just eliminate the firing pulse to the injectors, unless the PCM kills spark as part of a pre-programmed fault response. Maybe one of the later XJ owners can clarify that point for you.

Did you unplug the crank sensor, and did that return the gauges to operation?

Ok.....I unpluged the cps and nothing changed on my instrument cluster, its still dead. The cps is new....it was the first thing that I replaced.

Check all the fuses with a test lamp. there are little openings in the plastic to test the fuse while it is in place. a good fuse on a live circuit will light the lamp on both sides. a blown fuse will only light on one side.

A shorted sensor will drag down the 5v reference signal the PCM generates and cause the PCM to act dead. Unplugging sensors until the gauges come alive will lead you to the bad sensor. the CPS is most common to cause this but a bad MAP, CMP or TPS has the potential to do the same.
If you are familiar with a DVOM, all 3 wire sensor connectors (harness side) can be tested the same.
For example:connect the DVOM ground lead to a good ground ...the battery is best... and switch it to DC volts. turn on the key, unplug the TPS and probing carefully, test the 3 leads in the TPS connector. You should see 5v, 5v, and 0v.. not necessarily in that order, and not usually a perfect 5v.. like 4.8 to 5.2 is ok, but you get the idea.
If the 5v ref signal is being shorted you might see something like 2.1v 5v and 0v
If the PCM is dead, you might see 0v 0v 0v
If you have a weak or poor ground, you might see 5v 5v 3v

have I confused you enough yet ?

Thanks for the clues....I'll give it another shot tomorrow and let you know what happens...

oh and, if you decide to try testing with a DVOM, the PCM has 2 different 5v generators.... one feeds one set of sensors and one feeds another set. so unplug, test and reconnect each sensor, one at a time.

Roger that.....thanks again and I'll let you know the results!

Thanks again for your help. I replaced my pcm yesterday and everything came to life. Now I have a new issue. The part numbers on the pcm's don't match. The one that was on my cherokee is P56041 277AD and the one that I purchased from a salvage yard is P56041 280AD. The yard says it should interchange with no problem. I called my local dealer and they said that the 280AD had been discontinued and that is all they could tell me. I do know that both part numbers belong to a 97 4.0 automatic which is what mine is.

if it starts, runs and there is no CEL... you are golden... IMO

Digger87xj said:

if it starts, runs and there is no CEL... you are golden... IMO

Click to expand...

Agree, run it.