I have 1997 F150 and the original 4.2 went bad (gasket of course). Purchased and installed another 4.2 longblock. Went in OK, no big problems, but now no spark...

I have replaced and tested crank and cam sensors, along with coil pack. Still nothing. I have ran up and down trying to figure this out.

Ran codes, come back with upper throttle positioning sensor, and upper coolant temp sensor. These shouldn't have any bearing on the ignition system though? Should they?

Speculating on the PCM but it worked fine before replacement. Engine came with all new sensors, so I don't quite understand what is going on...

Any suggestions?

I have checked ground, but am still speculating that is the problem. I only have one groundstap from block to frame, but some else said to check ground from block to body?

I am at a loss and a bit frustrated. Anyone with any experience/suggestions let me know.

Thanks.

-Trent

 

No experience working on your particular engine, but I can read schematics and work on any electronic control system. My specialyty happens to be the older Gen 1 & II SHO engines, but most of the systems are similar enough that previous experince is applicable.

You will need a multimeter. If you don't have one, go to Radio Shack or Sears and get a $20 digital meter. It will be adequate.

First verify Vbatt (battery voltage) at the coil's R/LG wire with the ignition switch in RUN or START. Without this voltage, there can be no ignition. If it is present, it will be necessary to trace circuits back to the PCM to verify integrity, then to check on the inputs to the PCM that are prerequisites to getting a spark.

Not sure how you tested the named components. Please advise.

Do you have an inductive timing light available? They are very helpful in detecting spark without endangering yourself.

Steve

 

What were the codes? Was one of them P0123 or P0118? Check the wires to the TPS. And to the ECT sensor. They both share the same reference voltage from the PCM. WAG, try using the old TPS.

Regards

Jean Marc Chartier

 

I have both these codes. I haven't messed with either the TPS or the ECT. I found these codes erronious at this point as they are on a seperate system. Or are they, is that a bad assumption on my part. I have appropriate fuel pressure. I am just looking for spark... Original TPS along with rest of fuel system.


I was under the assumption that I wouldn't get any voltage across this circuit until the PCM sent a signal to fire. Should there I have voltage to that all the time?

Verrified Cam and Crank sensors by hand cranking engine and watching voltage bounce from 0-5V. I do have a multimeter, and am somewhat familiar with testing circuits.

If you are saying I should have volts to the coil from the wire harness plug then I don't have anything and I will try and trace it back from there. I am to guess we are talking a bad wire to the PCM.

No inductive timing light.

Thanks for the help any more would be great.

-TAG

 

Check of the obvious, Is the inertia fuel shut off reset? Key on engine off disconnect TPS and measure the voltage between the Ref signal and the return Signal. Should be between 4-6 volts. It is the two end contacts on the connector Gray with Red stripe and Brown with White stripe.

JMC

 

You should have Vbatt on the high side of the coil whenever the switch is in RUN or START. The low side of each coil is "grounded" by the PCM in order to fire the coil (simplified). Power to this circuit is provided by fuse F30 in the under-hood fusebox. However, if this fuse opens, the PCM power relay will not energize and it appears that your PCM has enough power to pull OBDII codes. If there is no power at the coil, look for an open circuit between the coil and this fuse.

Bad TPS or ECT will not normally cause a "No Spark" condition unless shorted to ground. If in doubt, just unplug them (along with the MAF) to remove them from the circuit. The PCM has the capability to substitute built in programming for certain sensors that are unhooked. You will, of course, get a CEL under these conditions if you ever get the engine to run.

He already indicated he has fuel pressure. That rules out the inertia switch.

With both of those codes (any others?), I would suspect either a short to ground of the Vref voltage (+5V) or a fault in the PCM itself.


Steve

 

Two things to check for:

1) Is the theft light flashing?
If so, that can cause your no spark concern.

2) Did you/engine builder- install the crankshaft trigger wheel?
Without it, the crank sensor (CKP) would not operate.


You can take a DVOM , set it to "hertz" and connect it to the 2 terminals on the CKP. There should be a jump on the readings when the motor is spun over.