720 electrical gremlin hunting

[dayid]

Vehicle: 1986 720 2wd Z24 manual, regular-cab long-bed, ST

 

 

I've got the worst type of problem - intermittent. I've had it occur with and w/o my AC on, same for headlights, regardless of speed, regardless of gear, sometimes after running for nearly an hour, othertimes just minutes after starting - where it seems like my ignition cuts out.

 

I've got a 1-year old alternator, my charging system light isn't on, my voltometer reads properly, and I've recently removed - polished - and replaced all the grounds on my truck to ensure good connections. This past week I replaced my auto-choke-relay (since I was hearing it click when this occurs - and I wasn't sure if it was the cause or just another symptom). I replaced my ignition switch just to verify that it wasn't the culprit.

 

What happens lasts just a portion of a second - but I've had times where it's last probably 2-3 seconds - is that my truck acts like someone came along and turned off power to the ignition. It acts just like if you were running down the road and turned the key from IGN to ACC and then back again.

 

The wiring for this system looks so simple in the FSM and Haynes, but I can't wrap my head around where else my problem could be. I've got a multimeter and I'm "OK" with it, but it's hard to test a circuit/system when it's so intermittent - any suggestions?

[Sealik]

Idle cut solenoid?????

[poor-mans-720]

sounds just like a loose connection some where... but where?? million dollar question... if you unplug the diesel solenoid it could do it, fuel, ign... so on....

[datzenmike]

Idle cut would only affect it when idling and not if driving.

 

Possibly a bad (Green?) fusible link?This supplies the ignition switch and to both coils. With the engine running move the fusible link wire and connectors and see if the motor quits.

[dayid]

So tonight driving home from my midnight shift it died... and completely died - not jerk as it died and then lurched back to life again. Since I'd not been sure if it was the ignition switch or not, I looked down at my new lit-ignition switch - no light. So I check fuses, etc, and find everything good. Unfortunately (and fortunately) my main fusible link had burnt up before I got the truck, so I have all individual inline fuses there instead of the normal fusible link. It's made debugging some things easier, and others a little more PITA. After checking the fuses, got back in the cab and it was happy again.

 

I'm *really* hoping you guys are right about the loose connection at where the fusible link had been - that would coincide perfectly with it working again after I pulled on the wires to get a better look at the inline fuses. I'd been partially writing off a loose wire since it sometimes does this at 70MPH on the highway, sometimes at 55MPH on a smooth road, and I've even had it do it while sitting idle at a stop light not moving at all - but then going down dirt road (or over speed bumps, etc - where it jostles the truck more) it kept on going.

 

Tomorrow I plan to re-evaluate the links where the old wiring harsness is tied in to the new replacement wires. I'll be the happiest guy driving a 720 if it ends up just being having to rejoin a connection! Intermittent is the worst!

[datzenmike]

Unfortunately (and fortunately) my main fusible link had burnt up before I got the truck, so I have all individual inline fuses there instead of the normal fusible link. It's made debugging some things easier, and others a little more PITA. After checking the fuses, got back in the cab and it was happy again.

 

 

Tomorrow I plan to re-evaluate the links where the old wiring harsness is tied in to the new replacement wires. I'll be the happiest guy driving a 720 if it ends up just being having to rejoin a connection! Intermittent is the worst!

 

This would have been good to know in the original post. Any changes from stock is pertinent information.

[poor-mans-720]

check your battery connections. There is a little clip that clips onto my + side of my battery, goes down tot the starter, and some other stuff. But i know that if you unplug it, or dont have it clipped on all the way it could give you these problems. Also check your battery terminal connections themselves. Sometimes when i go to start my truck it makes an attempt to start, then i just hear a click, if i get out and wiggle my + connector on my battery, all is good again. might check that also...

 

There shouldnt be that many connections that will cause it to quit, right? leave her idling, and start wiggling or unplugging and seeing if it dies. if not, plug it back together. obviously not that connection. Do this one at a time, dont want to get mixed up, or miss putting one back together. sure way of finding things that will cause your motor to shutoff, and checking to make sure there is a good connection.

[dayid]

Any changes from stock is(sic) pertinent information.

If I listed everything that isn't stock there'd be no more room left to talk about the problem :rofl:

But seriously, I didn't even think about it since that was done a year and some 12,000 miles ago.

check your battery connections. There is a little clip that clips onto my + side of my battery, goes down tot the starter, and some other stuff. But i know that if you unplug it, or dont have it clipped on all the way it could give you these problems.

Yeah, that's the fusible link that we've been talking about.

 

Also check your battery terminal connections themselves.

These I replaced when I got the truck also since the stock thin-bands were cracked.

 

In re-checking the splicing this morning the white/black-striped wire on the ignition-switch PAS connector (that takes a 15amp fuse on the fusible link to ignition) looks like of the X# strands of wire in it, only maybe 75% were clamped well by the connector at the ignition plug end. Removed this post of the connector and re-spliced it and re-inserted. AFAIK that end of the connector is 100% stock, and until trying to debug this the only time any portion of the dash was out was when I replaced the speedometer cable back 5 months and 5,000 miles ago. Even then, I only had the gauge cluster out, not the steering column/area where the ignition switch is, so who knows. I had since wired in the two switches and push button start - but that was many miles after this had originally occurred, so while I may have worsened it then, I couldn't've originally caused it.

 

I've only driven maybe 50 or so miles since redoing that end of the connector, but sometimes this problem had slept for 50-100 miles before re-popping up, so I'll just have to drive more to see.

 

I think this is where I really need to write myself better notes in the FSM, since "WL" wire being "White with black line" isn't very clear to me, and of all the tables/definitions/symbols they give in the electrical section, I haven't noticed any that describe the wiring colours/striping details.

[poopypants720]

If I listed everything that isn't stock there'd be no more room left to talk about the problem :rofl:

But seriously, I didn't even think about it since that was done a year and some 12,000 miles ago.

Yeah, that's the fusible link that we've been talking about.

 

These I replaced when I got the truck also since the stock thin-bands were cracked.

 

In re-checking the splicing this morning the white/black-striped wire on the ignition-switch PAS connector (that takes a 15amp fuse on the fusible link to ignition) looks like of the X# strands of wire in it, only maybe 75% were clamped well by the connector at the ignition plug end. Removed this post of the connector and re-spliced it and re-inserted. AFAIK that end of the connector is 100% stock, and until trying to debug this the only time any portion of the dash was out was when I replaced the speedometer cable back 5 months and 5,000 miles ago. Even then, I only had the gauge cluster out, not the steering column/area where the ignition switch is, so who knows. I had since wired in the two switches and push button start - but that was many miles after this had originally occurred, so while I may have worsened it then, I couldn't've originally caused it.

 

I've only driven maybe 50 or so miles since redoing that end of the connector, but sometimes this problem had slept for 50-100 miles before re-popping up, so I'll just have to drive more to see.

 

I think this is where I really need to write myself better notes in the FSM, since "WL" wire being "White with black line" isn't very clear to me, and of all the tables/definitions/symbols they give in the electrical section, I haven't noticed any that describe the wiring colours/striping details.

sounds likely to be the culprit

 

the key on the wiring diagram available in the haynes manual actually provides color abbreviation codes and Ive seen a colored version of the diagram somewhere before... I can try to find it if you like.

[dayid]

the key on the wiring diagram available in the haynes manual actually provides color abbreviation codes and Ive seen a colored version of the diagram somewhere before... I can try to find it if you like.

Ah, I have a Haynes but it normally stays on the shelf since I find the FSM in general has better information. I'll have to double-check my Haynes and maybe break out the coloured pencils on the FSM. :thumbup:

[dayid]

sounds likely to be the culprit

 

the key on the wiring diagram available in the haynes manual actually provides color abbreviation codes and Ive seen a colored version of the diagram somewhere before... I can try to find it if you like.

Broke out the ol' Haynes last night - found the table you were talking about. No idea how I never had noticed it before, but it clears up a LOT when looking at these darn schematics!

[poor-mans-720]

i read in another thread on here, sounded like he was having the same issue. His was a dying alt. So you might want to try and test that. could be that you have a drain on the battery somewhere. Also all the connections on the back of the Alt, and starter itself. The little clip on the starter could be a bad connection...

[datzenmike]

 

I think this is where I really need to write myself better notes in the FSM, since "WL" wire being "White with black line" isn't very clear to me, and of all the tables/definitions/symbols they give in the electrical section, I haven't noticed any that describe the wiring colours/striping details.

 

WL would actually be White with Blue stripe wire. :)