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Tim Mohr
Tim Mohr, ASE MASTER TECH
Category: Ford
Satisfied Customers: 13070
Experience:  30 YRS EXPERIENCE, FOREIGN AND DOMESTIC, GAS AND DEISEL
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1994 ford f150 5.0 cranks but wont start - intermittent problem

Resolved Question:

1994 ford f150 5.0 cranks but wont start - intermittent problem
Submitted: 3 years ago.
Category: Ford
Expert:  Tim Mohr replied 3 years ago.
Hi, my name is XXXXX XXXXX most common problems with no starts is the ignition module or the fuel pump, when it dies, do you know if it has spark to the plugs?
Customer: replied 3 years ago.
Cranks but wont start. Problem is intermittent. Hot or cold. totally random.

Has injector pulse (noid light blinks) but no spark out coil tower during no start. Coil has 12v with key on and while cranking.

I was able to put on a new ign module during the no start condition but it did not fix it. Problem remained the same with no spark at coil tower.

Havent been able to do the test light test across the primary side (should blink while cranking) as the vehicle is running fine at the moment.

If it fails the above test... what can cause me to have injector pulse but no signal to ign coil? I think that would be where my problem is at.


Expert:  Tim Mohr replied 3 years ago.
if it has power to the coil, but does not blink, then its either the ignition module or the pick up coil in the distributor, if it does flash when cranking, and there is no spark, then its a bad coil, or abad rotor in the distributor that is intermittently shorting to ground,Tim
Customer: replied 3 years ago.
It is not the ignition module because i put a brand new one on it and the engine still had no spark at coil tower (with coil wire disconnected from coil tower and spark tester in its place connected to ground).

Can the pcm prevent the ign module from controlling the coil on this vehicle. I was a GM driveablility tech for 10 years so i am familiar with the circuits involved however i have little experience on Fords.
Expert:  Tim Mohr replied 3 years ago.
Yes it can,but is rare, check for the test lite flashing when cranking, on the neg side of the coil, if it flashes, then replace the coil if it doesnt, then replace the pick up coil in the distributor,Tim
Customer: replied 3 years ago.
I dont think it could possibly have injector pulse unless the pick up coil is working... where else would it get a reference signal to control the injectors? I disagree that the pickup is the reason for the no spark.
I already stated that i cant do the test light test as the vehicle is operating correctly and obviously it would be a faulty coil if it flashes with no spark out the tower.
Again, when the problem returns if it fails the test light test (no blinking while cranking) and it has injector pulse (which should mean the pickup coil is fine), and a new ign module didnt help, then what is the next thing to test?
Expert:  Tim Mohr replied 3 years ago.
from the ecm, but we also need to know if the test lite flashes, when its dead, if it does, then the ecm is good, the ignition module is good and the pick up is good, as is the wiring for all of this, but if it doesnt flash and you have injector pulse, then you most likely have a bad ecm,Tim
Customer: replied 3 years ago.
So to be more specific. During the no start if the test light does not blink at the ign coil primary side and i have good injector pulse...

Is there not a test that i can perform with a volt-ohm meter on a particular circuit, wire, terminal, component, etc. to determine that i actually have a failure in the ecm as opposed to a possible intermittent open circuit at a connection point somewhere?

I would rather not just throw parts at it...
Expert:  Tim Mohr replied 3 years ago.
yes, there are test for the entire ignition system, but on the fords, it requires a break out box, do you have one?
Customer: replied 3 years ago.
of course not... gm tech remember? LOL
Expert:  Tim Mohr replied 3 years ago.
it is neccessary for the diagnostics on the early ford fuelm injection, they dont provide any other way of checking these,unfortunately,but lets assume it does not flash, and you have injector pulse, then it is most likely a bad ecm, we would assume the pick up to be good, if it has injector pulse, so this would leave the ecm, or a bad wire, and they would need to be ohmed out, when its dead,Tim
Customer: replied 3 years ago.
I know it is most often the ignition module that is the culprit with these symptoms. In this case it is obviously not since the new one didnt fix it. My question is...

Is this year vehicle with this engine known to have a bad ecm that can cause the problems i have described to you? Also is it something that I might be able to try the "tap test" on the ecm to get it to act up?

Expert:  Tim Mohr replied 3 years ago.
No, it is rare that an ecm will cause this, but it has happened, tap test on the ecm usually does work on this, ford makes a break out box, it is installed inline with the ecm, and has all the pinouts to test when its dead, without, you would be shooting blind, but knowing if the coil is recieving a ground when cranking is important here, without know this, its impossible to even guess,Tim
Customer: replied 3 years ago.
I know, but i cant test it till it becomes a no-start again.
Basically i dont believe an ignition coil can cause these intermittent symptoms. In my experience an ignition coil either works or it doesn't or they show that they are breaking down under high temperatures/heavy load conditions causing performance issues. If this engine starts it always runs perfectly under any and all conditions. It never dies while driving or exhibits any performance issues whatsoever.
A bad ecm makes sense considering the intermittent nature of the problem. I just hate to spend that kind of money if it doesn't fix it.
I dont think i can return it once it is installed.
Expert:  Tim Mohr replied 3 years ago.
No, there is no way to test it unless its dead, if it only does it when starting, then it can be a coil, the spark can betested for color, brite blue is good, yellow or orange is weak, and will usually cause a hard start, but run good once runniong, but there is no test to do, until its dead again, everything will show good when its running,Tim
Customer: replied 3 years ago.
I have let it run for about 5 minutes (at idle) with an in-line spark tester adjusted to 20k volts and it had a good strong spark the entire time. At what setting and for how long do you think i could run it (or drive it) to try to stress that coil into failing if it is the culprit?
Or on the other hand how long will be long enough to say that the coil is working fine?
Expert:  Tim Mohr replied 3 years ago.

i would run it to temparature, then shut it off and let it sit, and see if you can duplicate the no start, how long does it usually go inbetween no starts?

Customer: replied 3 years ago.
totally random. I am not driving it regularly because it is so unreliable. I would say it was doing it once every other day or two when it was being used as a daily driver.

Expert:  Tim Mohr replied 3 years ago.
then its going to be difficault to diagnose, how long does it stay dead?
Customer: replied 3 years ago.
i typically only get about 3 tries before it starts back up and runs perfect again. It has taken quite a while to get the information i have about it now. I will try to get it to do it again by stressing the ign coil with the spark tester. If it will cooperate and mess up again i can determine if its the coil since that is a simple pass or fail test with the test light. I will also remove the ecm so that if the coil is not getting a signal I can try the tap test or disconnect it completely to see if the signal returns to the coil hopefully confirming a bad ecm.

Expert:  Tim Mohr replied 3 years ago.
another test when it dead, disconnect the spout connector and see if it fires, this is the connector you remove to set the timing, if it starts, then its in the ecm, the spout connector takes the computer out of the system,Tim
Customer: replied 3 years ago.
cool. I know exactly where it is. that will be easier and faster than unscrewing the connector bolt at ecm.

I also located a service bulletin online that explained a repair procedure to a spliced connection to the module from the ecm located in the harness that could cause the exact symptoms i am having. after i removed the wiring loom and unwrapped the foil tape i discovered that the wires had already been wrapped with waterproof electrical tape so i assumed that the splice had already been inspected and repaired at a dealer. It was a very high quality tape that i dont think a shadetree mechanic would have on hand.



Expert:  Tim Mohr replied 3 years ago.
yes, that is an old bulletin, it most likely has already been done
Customer: replied 3 years ago.
So do you think 20,000v is sufficient to run inline with the coil tower to load it up? I think it takes around 30k to fire the plugs under compression so that would have it somewhere around 50k volts right?
Expert:  Tim Mohr replied 3 years ago.
yes, 20k should load it up, this coil only puts out 50k
Customer: replied 3 years ago.
ok. One last thing i just remembered... the truck does have what feels like a torque converter shudder under load from time to time (it goes away if u tap the brake pedal with you left foot to disengage it). I just looked and there is a white connector on the passenger side of the transmission just above the drain pan lip. Can i disconnect that to prevent the torque converter from engaging or is that an electronically controlled valve body in which case it might not shift at all if disconnect it? If so, do you know which wire (color) is the TCC solenoid circuit?

I am pretty sure that what i am feeling is not the ign coil causing a driveablity problem and is instead the TCC as it feels exactly the same as any number of chevys i have driven with the same feeling.

Expert:  Tim Mohr replied 3 years ago.
if it goes away when tapping the brakes, then its a bad TCC solenoid, you cant disconnect this with out effecting the shifting of the trans, replace the TCC solenoid to fix this problem,Tim
Tim Mohr, ASE MASTER TECH
Category: Ford
Satisfied Customers: 13070
Experience: 30 YRS EXPERIENCE, FOREIGN AND DOMESTIC, GAS AND DEISEL
Tim Mohr and 2 other Ford Specialists are ready to help you
Customer: replied 3 years ago.
ok. thanks for your help

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Tim Mohr
Tim Mohr
Ford Mechanic
13070 Satisfied Customers
30 YRS EXPERIENCE, FOREIGN AND DOMESTIC, GAS AND DEISEL