Another crank, no start issue...

[acfortier]

Alright, this is baffling me and I'm getting to the point where I want to dump this thing off and have it not be my problem anymore 

 

I've had intermittent no start situations before, which would "go away" after a few minutes - few hours. This time, it hasn't come back, and it's been nearly a week.

 

I've got no power to my ignition coil. I believe I should have voltage with the ignition ON on one of the wires, and while cranking the other wire should be pulsing. I get nothing on either with ignition ON or cranking the engine. Can someone confirm that voltage on one of the ignition coil inputs should be present when the key is in the ON position?

 

I've ohm tested the CPS between B & C terminals, I get infinite resistance, which seems to be correct for an HO engine. Unclear which terminals I need to probe to see the AC voltage (or if that is even a valid test for an HO engine, seems it might be for Renix only?).

 

After toying around with it some more, it finally started after sitting for over a week. The only things I truly did to it to make any difference were:

1. Swapped Starter relay for ASD relay
2. Unplugged ignition coil (input side, to test) and plugged it back in
3. Unplugged CPS (to test) and plugged it back in

 

And it fired up (albeit with an awful ticking noise, yay noisy lifters).

 

Can anyone point me in the direction of what I should be testing next?

[ghetdjc320]

I dug deep in my schematics but I used this diagram to create my standalone efi setup. This is a factory diagram for my 91 HO MJ

[ghetdjc320]

Here is a clearer shot of the ignition coil wiring 

 

[ghetdjc320]

The side that feeds switched 12v to the coil also feeds 12v to the injectors and pin 30 on pretty much all the relays

[acfortier]

Makes sense. And this voltage should be present with the key in ON position? Are you suggesting I check voltage at another source from that same circuit? If there's no voltage anywhere else, is it a bad computer?

[cruiser54]

See if the coil is made by Magnetti Marelli. If so, it is automatically highly suspect. 

[acfortier]

Coil is less than a year old, Accel brand. Definitely not the coil--there's no voltage going into the coil, so the problem exists before it.

[ghetdjc320]

My Accel clip failed as well on my MJ. Check for power at the fuel injector connectors. The power should be coming from pin 87 of the ASD relay. Check for power while cranking also

[acfortier]

Wow, I think I figured it out. I had pulled on wires before, but I really got it to stall:

 

https://drive.google.com/file/d/1oBMERZFDGaNahknZ-v2yk5wv8ZUyANM8/view?usp=sharing

 

Not sure if it's my solder seal connection or if the wire is just loose in the connector after all of these years. Ended up buying the Accel adapter pigtail, and will re-splice and see if that fixes my issue.

[cruiser54]

Nice find. I love wiggle tests. 

[acfortier]

Well, was driving it the other day and completed died, and wouldn't start. Just crank and crank and crank...

 

Getting 0.3V at the ignition coil when cranking, other wire is 12V. I'm pretty sure it should be 5V when cranking. Another oddity is testing the voltage on the ECU side at the CPS plug. One wire gives my ~8.4V, the other ~3.5V when the key is ON. Is this normal? Not sure how else to test the CPS on a HO other than the resistance (which is still infinite)

[Spinnakerblue89]

By CPS do you refer to the Cam Position Sensor or Crank Sensor? I just want to be sure because sometimes I see confusion between the two. 

 

I'm not sure if this is truly relatable, but I'll bring my 1993 ZJ 4.0 into the equation. I had a crank-no start scenario three years ago that was somewhat out of the blue. I forget what it was, but something electric failed and when I replaced it, my cam position sensor died out too- and I heard that it can sometimes fail in near unison with a certain electrical component. 

 

I thought I'd throw that out there, for what it's worth. 

[acfortier]

Crank sensor, one on the bellhousing.

[acfortier]

OK, finally had some time to play around with this over the weekend.

 

I had previously replaced the CrPS with a Crown unit when I was tracking down this issue previously (that turned out to be a worn terminal on the ignition coil). Since I pulled the stock one, and still had it, I figured I would put it back in (since it theoretically wasn't bad). After replacing, still the same--crank, no start.

 

I swapped around a few relays for the ASD to rule that out, no change.

 

There was a crusty looking ground with two wires on the driver's side fender near the ECU, that I cleaned and replaced, no change.

 

Cap and rotor didn't seem terrible looking, and was going to replace, but bought the wrong parts--forgot I had put in a '95+ dist. in, so uses a different style rotor.

 

Going to test the sync signal generator:

 

 

If that turns out good, I guess the only other thing it could possibly be is the ECU going bad?

[pizzaman09]

Can't hurt to try another ecu, given the HO ones are under the hood they are suspectable to a much less friendly environment.

[acfortier]

9 hours ago, pizzaman09 said:

Can't hurt to try another ecu, given the HO ones are under the hood they are suspectable to a much less friendly environment.

Not sure what that actually entails. Can I just swap a junkyard one in directly (from the same setup, 4.0 MT) and it should work?

[pizzaman09]

7 hours ago, acfortier said:

Not sure what that actually entails. Can I just swap a junkyard one in directly (from the same setup, 4.0 MT) and it should work?

From my minor research, you should be able to swap in any ECU from 91 to 96.  I'd personally unplug the battery and wait a few minutes before swapping the ecus.

[Torq_Shep]

On 7/13/2024 at 6:19 AM, pizzaman09 said:

From my minor research, you should be able to swap in any ECU from 91 to 96.  I'd personally unplug the battery and wait a few minutes before swapping the ecus.

Should work just fine. Any 1991-1995 4.0L ECU I believe should be compatible. IIRC the difference between a MT and an AT is only in the wiring harness and it's very minor. 

 

I am sure you did this already but did you pull the codes? I believe you can just turn the key 3 times and it should give you blink codes on the check engine light. I just tried it on my truck though and realized that lightbulb is burnt out on the cluster I just put in. I don't recall the codes ever being particularly useful but might be worth a try. 

 

 

[acfortier]

5 hours ago, Torq_Shep said:

Should work just fine. Any 1991-1995 4.0L ECU I believe should be compatible. IIRC the difference between a MT and an AT is only in the wiring harness and it's very minor. 

 

I am sure you did this already but did you pull the codes? I believe you can just turn the key 3 times and it should give you blink codes on the check engine light. I just tried it on my truck though and realized that lightbulb is burnt out on the cluster I just put in. I don't recall the codes ever being particularly useful but might be worth a try. 

 

 

I haven't in awhile, and when I did, there was no codes--but let me give that a shot again.

[ghetdjc320]

I doubt the ecu is the culprit. Those things are fully potted and rarely have any issues. 

[acfortier]

2 hours ago, ghetdjc320 said:

I doubt the ecu is the culprit. Those things are fully potted and rarely have any issues. 

I'm all ears for other suggestions 

 

It's at my parent's house, next on the list is to check the ASD relay terminals in the PDC and if any CEL codes come up.

[ghetdjc320]

If you haven’t done so yet, check the battery, ground and power cables along with the main harness connections. It’s been my experience that each time I go down the rabbit hole tracing resistance, sensors and circuits 90% of the time it’s something much simpler like a bad battery, loose ground, power or corroded connection. Any one of those factors will throw off all subsequent readings and will make it seem like your problem lies elsewhere. 
Ensure the pcm is getting a solid 12+v even while cranking. If you trace out that diagram I put in the second post, you may be able to track the culprit. 
 

Is the fuel pump running? If you pull a plug, can you check for spark? The engine only needs a few minor items to actually fire. Tracking down which one of those basic items you’re missing will probably get things moving the right direction. 
 

And for some odd reason, I’m still suspecting your coil . Probably because I had an accel one die on my HO also. 
 

 

[acfortier]

2 hours ago, ghetdjc320 said:

If you haven’t done so yet, check the battery, ground and power cables along with the main harness connections. It’s been my experience that each time I go down the rabbit hole tracing resistance, sensors and circuits 90% of the time it’s something much simpler like a bad battery, loose ground, power or corroded connection. Any one of those factors will throw off all subsequent readings and will make it seem like your problem lies elsewhere. 
Ensure the pcm is getting a solid 12+v even while cranking. If you trace out that diagram I put in the second post, you may be able to track the culprit. 
 

Is the fuel pump running? If you pull a plug, can you check for spark? The engine only needs a few minor items to actually fire. Tracking down which one of those basic items you’re missing will probably get things moving the right direction. 
 

And for some odd reason, I’m still suspecting your coil . Probably because I had an accel one die on my HO also. 
 

 

Battery seems fine--it's been sitting for at least a month or two now and will still crank over no problem when testing. Terminals on the battery are clean, ground cables are clean and tight. I'll double check it all, but I don't think that's the culprit in this case (although it would be nice if it was just a bad ground, lol).

 

I'll have to find a higher res image of what you shared--can't make out any of the writing.

 

Fuel pump is running. Don't need to pull a plug, not getting spark, because the coil isn't getting 5V to fire. So, definitely not the coil (at least, not yet) since it's not getting the signal from the ECU to fire.

[ghetdjc320]

7 minutes ago, acfortier said:

Battery seems fine--it's been sitting for at least a month or two now and will still crank over no problem when testing. Terminals on the battery are clean, ground cables are clean and tight. I'll double check it all, but I don't think that's the culprit in this case (although it would be nice if it was just a bad ground, lol).

 

I'll have to find a higher res image of what you shared--can't make out any of the writing.

 

Fuel pump is running. Don't need to pull a plug, not getting spark, because the coil isn't getting 5V to fire. So, definitely not the coil (at least, not yet) since it's not getting the signal from the ECU to fire.

Just remember, all of this can be due to battery not giving a full 12v during cranking 

Grab a replacement coil from the auto parts store and throw it on along with a good battery if you have an extra or a jump. It’s really hard to catch that 5v pulse on a regular meter. I’ve tried, and was also convinced of a bad ecu. Have had this happen on 3 HOs so far. Each time it’s been either coil or battery. 

[acfortier]

4 minutes ago, ghetdjc320 said:

Just remember, all of this can be due to battery not giving a full 12v during cranking 

Grab a replacement coil from the auto parts store and throw it on along with a good battery if you have an extra or a jump. It’s really hard to catch that 5v pulse on a regular meter. I’ve tried, and was also convinced of a bad ecu. Have had this happen on 3 HOs so far. Each time it’s been either coil or battery. 

Interesting... it's worth a shot! Thanks for the tips. Coils are cheap enough, and I can bring the battery to get tested at the same time. Honestly, not sure how old the battery is--it's been in there since I purchased it.