GrayTshirt

The vehicle in question is a 96 Grand Cherokee Laredo with a 4.0L engine. My girlfriend got in it a few weeks ago and started it up, just when she got out of the drive way it died. Since then I have replaced the CPS (as that fixed it before), the PCM (it has been saying coolant sensor bad for over a year after I replaced the coolant sensor), the distributor cap (needed to be done), and ignition pickup sensor. I have checked the Auto shut down relay (ASD) and it checks out.
The fuel pump comes on when I turn on the key and the fuel pressure at the fuel rail is approx 50 psi. I'm getting 11 amps at the coil and there is a spark, but it doesn't seem to be extremely strong by any means. Aside from that the engine cranks over strong and there are no lights on the dash. The PCM is a refurb that was programmed with the necessary specs from the VIN #. Any assistance you all can offer would be greatly appreciated. I am stumped...

Ray

zedpapa

you replaced the crank sensor. have you checked the cam sensor in the distributor? might be what you're calling the ignition pickup sensor. i would verify that you have spark and that the fuel injectors are getting a signal form the ecu.

zedpapa

GrayTshirt

Yes, I replaced the cam sensor in the distributer, sorry for the mix up on the name. I do have spark, but haven't checked to see if I have a signal from the ECU. Do you have any suggestions for a best practice on how I can check this?

Thanks for the response by the way...

GrayTshirt

Disregard, the question about checking the signal at the injectors, I did a search and found a method of checking for power on the injectors, I'll try to check tonight if I can get over to the garage. I checked the coil lastnight to get a voltage reading while cranking and I'm getting 9.8v when the engine is cranking. Does that sound correct?

zedpapa

sounds a little low. you should have at least 11 volts while cranking. i would check the battery connections and make sure they are clean and tight and that the battery is fully charged. next time try spaying some starter fluid in the intake to see if will run on that.

zedpapa

GrayTshirt

I thought that seemed low as well. The battery is new, fully charged and the connections are clean and tight. I tried spraying starter fluid in the intake before, but I'll give it another go. I just read somewhere that the O2 sensor could cause grounding which could result in a "no start" scenario. Does that seem likely?

SpooledupRacing

not the O2.

I would pull up all your grounds and sand them down make sure they are clean and put them back down. check your starter and alternator connections.

Also another thing to do is to crank it for say 10 seconds then pull the plugs out and see if they are wet or if they look like they are getting and spark to them...

maybe pull a wire and plug and crank it and see what the spark looks like coming out of the plug...

zedpapa

i just reread your first post. you seem to be questioning the spark. if you pull a plug and put in the plug wire and then set it somewhere where it will ground, you can crank it and watch the spark. the spark should be bright blue and loud. if it isn't, then you could have a weak coil. it may spark fine outside the combustion chamber, but not be strong enough to jump the gap when under pressure.

zedpapa

GrayTshirt

That is exactly what I was thinking. I'm heading over to the garage now, busy week. I have a lot of test procedures to follow, so I'll take what you have suggested along with the other tips I got and get back to you. Thanks again...

GrayTshirt

All right, so going on the weak spark symptom. I found some steps for finding a faulty sensor that could have been causing a grounding or signal loss preventing the PCM from sending the correct voltage out to the coil, thus causing the weak spark. These are the steps I did:
Test powers and grounds at the Power train Control Module (PCM) on the C1 connector. Pins 2 and 22 are power, and then check pins 31 and 32 for ground.

Next, test the 5-volt power supplies at pin 17 of the C1 connector and pin 31 of the C2 or middle connector for shorts to ground. Sensors on the circuit are the Camshaft Angle (CMP) sensor, Crankshaft Angle (CKP) sensor, Throttle Position Sensor (TPS), Manifold Absolute Pressure (MAP) sensor and Vehicle Speed Sensor (VSS).
Unplug the sensors on the 5-volt power supply, one at a time, while monitoring the 5-volt supply.

When the shorted sensor is unplugged, the PCM will wake up and scan tool communication will return.

The wiring harness could also be shorted to ground on one of the 5-volt power supply wires. Cut the wires at the PCM on pins C1-17 and C2-31, then check for the 5 volts on the PCM of the connector or checking for any continuity to ground with the sensors unplugged at the PCM connector on C1-17 and C2-31 to check the wiring harness.

Read more: http://www.justanswer.com/questions/2t1pc-i-have-a-96-grand-cherokee-laredo-straight-six-4-0-i-had-a#ixzz0fnvBNW9T

I checked each and every sensor and they all checked out. (I hate diagnosing electrical issues.) The next thing I started looking in to was the coil. I checked it with an ohmmeter before and it checked out, I even took it to AutoZone to them check it and they said it was good. I pulled the lead going from the coil to the distributer and checked the spark on a single plug coming directly out of the coil and the spark looked like it should normally from the distributer. Since I had already placed the distributor cap and rotor button, I figured it was worth a shot to replace the coil. I looked at the options and the only thing the local auto part store had was an AC Delco coil (which is what was in the vehicle and had only been there for about 16 months). I found a thread discussing weak OEM coils and AC Delco coils and found a large amount of people recommending the Accel high voltage coil, so I pulled the trigger. Last night I placed the new coil in the vehicle and was checking the gap on the plugs when I decided to check the spark with the new coil. I had the #1 and #6 plugs out of the vehicle and cranked it over to see if there was a noticeable difference in the spark and the vehicle started.
I quickly shut it down and buttoned it back up and today the vehicle is running good as new. Thanks for your assistance and guidance in this matter.

JosefCarias

Check your body grounds under the hood. I have ran into a lot of chrysler vehicles with that problem. Turns over but no start, no lights at dash. Motor to body grounds have been my solution to that. Even if they are bolted down, take them off and clean the real good with sandpaper or anything you have to bare metal. Try that.