Megadomeus

I got a real humdinger here. Crank, no start, good spark, NO injector pulse.


I did a valve job for a friend who purchased a 92 Cherokee 4.0 Sport 4x4. Apparently the only thing that was ever purchased for it was gasoline. The list of horrors to kill is far too long to list here but, that's getting off the subject.


I installed the head and associated parts and when I went to fire it up for the first time I got no start. Went through the usual suspects, it puttered when gas was poured in the TB. Cornered it to the Cam Position Sensor but I'm thinking it's good and the problems upstream.


The wires from sensor to the computer are fine, no cuts or even deformation, and no resistance in any of the three. No resistance (less than 1 ohm) from sensor to battery ground. 5v reference wire shows a steady 5v. Now here's the kicker, I've 10 v on the signal circuit. Sensor plugged in or not, there's 10v. Over about 20 mins I watched it slowly move from 9.5v to 10.5v and begin to drift back down again. It has voltage if the sensor is plugged into it or not. I've unwrapped the harness, wiggled giggled and ticked every wire in the compartment, no changes in voltage were noticed. Passing metal to get the hall effect unit to trigger produced no fluctuation whatsoever. Pulling the second big fuse from the front cut it in half to 5v but alas no triggering when attempted. 5v with sensor plugged in or unplugged. IE coming from the ECU. Been through all grounds and cleaned their contact areas, as well as cleaning the battery cables connections well. Tests were done with a new battery fully charged. I hate to say it's the ECU as it drove in here and we'll that would mean I somehow broke it or just got that lucky. Anyone have an idea of where I might look next?

Dave51

Well this sounds like an interesting problem. Can't be too many things that could go wrong:



That "big fuse" you referred to is the one for the PCM?

It could also mean if that were the case whatever broke it is still underfoot and will blow the next one so arbitrary "let's see if this works" could get expensive.

One thought, is the ASD relay working correctly? I am thinking there may be more than one issue underfoot.

Like the signal circuit has developed it's own power source, kinda like Arnold in The Terminator...

Megadomeus

Ok now that has to happen to me first for sure. Lol Big fuse is starter relay circuit. Agreed, I smell 2 issues contributing, minimum. I'm a hippie, throwing money at problems isn't an option for me. Lol Checking out the starter circuit in a few. Noticed something looks like a knot between the alternator and coil worth examining, and ASD is now on the list. Thank you. Time to greet the sun and get another one to run.

lawsoncl

On a 92, the crank sensor produces a/c pulses as the windows in the flexplate go by. The voltage of the pulses gets stronger for faster revolutions (somewhere over on Naxja I have a o-scope trace posted where we were discussing adding a cap across it to boost the voltage at lower cranking rpm). Just waving a piece of metal by it probably wouldn't be noticeable with a meter. 10-volts bias voltage sounds familiar though? Check the autoshutdown relay, as that will kill fuel if it's bad. All the grounds reattached, including the one where all the sensor wires converge to the wire under the dipstick bolt and the ground strap from the head to the firewall? TPS isn't stuck wide open, telling the computer to do flood clearing?