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Unread 03-23-2004, 10:22 AM   #1
m-keith
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87 YJ 4.2 stock - no spark - can I test ECU?

I guess for starters, I should give an intro & a hello since this is my first post here; so.....hello. I'm a long time car guy (former SCCA club racer) but not a Jeep owner. I like Jeeps, but the last thing I need is another high maintenance mechanical device in my life. I already have 2 old cars & an old boat. I help my brother-in-law maintain his Jeeps which brings me here. If anyone here is into SJ's, then you may have seen an occasional post from me on the FSJ list before.

I need to know if there's a way to test the ECU; the one behind the glove box, not the ICM (it's OK). Also, how can I test the pick-up inside the distributor? The Haynes manual I'm using doesn't list a test for it, but I'm sure there is one. I'll try my best to give some background about the problem without making this post too long.

Here are all the gory details:

Installed new cam, lifters, & timing kit; also a new starter. Now, I know what ya'll are thinking, & no, I'm not 180 out. I checked more times than I care to admit.

Wouldn't start; had no spark. It was running when he brought it to me, albeit with an occasional stalling problem. A bad backfire through the carb led us to the lifter problem. Quadruple checked everything I did & replaced battery cables which were in poor condition; still nothing.

Yellow wire to the coil had broken off from its connection. Repaired it...& the green one....still no spark.

Coil's secondary resistance out of speck; replaced it....nothing.

Was only getting 4 volts to the coil (battery's good). Found a kill switch had been spliced into the hot wire off the ignition switch. Wires leading to the kill switch were very hot (I'd left the ign switch in the ON position while I was tracing wires. I found a connection for the kill switch in the kick panel (I guess it was relocated at one point) that was 2 wires twisted together & left bare. Took that out of the loop....nothing, but now I was getting 10 volts to the coil. I also replaced the ignition fuse. It wasn't blown, but its casing had become very brittle.

Found that the yellow wire to the coil was getting hot when I left the ign switch on. Traced it to a red wire that ran into the main loom, which was then spliced to about 9 yellow wires, one of them being from the ignition switch. This red wire was very hot, & I could trace the voltage drop by probing through it at various points. I replaced it with a 10 gauge wire & replaced that big splice with a junction box. Now, I'm getting 11 volts to the coil but....nothin'.

Disconnected everything from the junction box except the wire from the ignition swich & the wire to the coil & got full battery voltage (12.5) to the coil. Connecting all of the other wires one at a time, coil voltage stayed above 12 until.....I connected the wire that powers the ICM. Then, I dropped to 11 volts at the coil. In fact, If I only connected the ignition switch, coil, & ICM wires to the junction box & left all the others disconnected, I only got 11 volts to the coil.

Had Auto Zone test the ICM, & it's good.

I removed the distributor, leaving everything else attached, & spun it by hand with the ignition switch on. I got no spark until I accidentally touched the body of the distributor to ground, then I got excellent spark. I thought I had the problem licked. After reinstalling the distributor, I checked the ground wire by probing the black wire at the harness side of the ICM connector with my multi meter grounded to the block, & I got perfect continuity; so that's not it.

Here's where I'm at now:

11 volts to the coil. The solenoid wire that runs to the coil has voltage when I turn the ignition switch on, & the solenoid wire that runs to the ICM pulses my test light when I crank the motor. The tach needle does not bounce when I crank the motor, but it does seem to bounce once after I release the key. This is consistent with the occasional single spark I've gotten after releasing the key throughout all this testing. I've traced every wire that I can without pulling the dash & the fuse box, & they're all good. I've pulled the ECU. The harness connector & all the pins look good. I don't have a small enough torx bit to remove the screws that hold the case together; so I can't see inside it.

Without a good trouble shooting guide for this ignition, I feel like I'm groping around in the dark. Right now, I'm theorizing that the bare connection from the kill switch touched ground for long enough to burn up something inside the ECU. How sound is this theory? Is there anything else I've overlooked?

Thanks in advance for any input, & apologies for the lengthy post.

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Unread 03-23-2004, 03:04 PM   #2
m-keith
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Alternately, if someone in the Houston area has a known working ECU that I could borrow for a few minutes, I'd sure appreciate it. I believe any wiring problems that would kill it have been resolved, & if I do kill it, I promise to replace it.
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Unread 03-23-2004, 03:31 PM   #3
alextremetj
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Darn, I wished you would would have done this about a month earlier, I just threw away my old ECU that I no longer need in my Jeep. Sorry about that.


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Unread 03-23-2004, 06:51 PM   #4
sentinal02
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well, seems like you ruled out just about everything else. I looked in my chilton's and this is test number one. if the engine doesn't start:
1) with ignition off (all test performed this way) disconnect harness at MCU and test between MCU connector pin 59 and dist (+) wire at the dist (orange wire) for continuity

continuity checks out ok?

if no: repair the wire as you've got a break (if you find no continuity during any of the tests this is the repair for it)

if yes: test pin 18 and dist (-) wire (violet wire) at dist for cont.

if yes: test pin 47 and orange wire of ignition module for cont.

if yes: test pin 20 and engine ground for cont. test harness ground for tight connection at right side of cylinder block.

if yes: turn ignition switch on w/o starting engine. test pin 1 and ground for battery voltage. turn switch off.

if no, then you're not getting power to the MCU, repair the wire.

if yes: test pin 58 and the "S" termnial of the starter solenoid for continuity.

if yes, replace MCU and retest.

*Note: before replacing MCU if engine fails to start, check for failure of ICM, coil dist etc.

but it looks like you already did that. anyway, I hope that helps. with any luck you'll find a broken wire or something. if you need a pic of the harness and pin numbering let me know and I'll scan it out of the book here. good luck!
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92 YJ 4.0L Ax-15 231
5" springs, 1" shackle 31's or 35's depending on my mood
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Unread 03-24-2004, 12:30 AM   #5
m-keith
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Thanks; that'll help out a lot. If you could get a hold of that, I'd appreciate it. The only thing I haven't tested, other than the MCU, is the pick-up inside the coil. Found a test for it today; so I'll test it tomorrow along with the MCU. Actually, I hope it's one of these two components rather than a broken connection 'cause if it is a broken connection, it's broken somewhere behind the dash.
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Unread 03-25-2004, 08:54 AM   #6
m-keith
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How can I adjust the air gap for the pick-up inside the distributor? I believe that either the pick-up is bad, or the air gap is out of spec. It's doesn't seem to be generating a signal, & the air gap is about 30 thousanths. The Haynes manual doesn't cover it at all, & looking over it, I can't find a way to adjust the air gap. Heck, the Haynes manual doesn't even provide a spec for the air gap; so I don't really know what it's supposed to be. 30 thousandths just seems wrong.
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Unread 03-25-2004, 12:59 PM   #7
sentinal02
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Quote:
Originally Posted by m-keith
How can I adjust the air gap for the pick-up inside the distributor? I believe that either the pick-up is bad, or the air gap is out of spec. It's doesn't seem to be generating a signal, & the air gap is about 30 thousanths. The Haynes manual doesn't cover it at all, & looking over it, I can't find a way to adjust the air gap. Heck, the Haynes manual doesn't even provide a spec for the air gap; so I don't really know what it's supposed to be. 30 thousandths just seems wrong.
I checked chilton's and they list nothing for the air gap either. my guess is it's not adjustable, but I'm not at all familiar with the 4.2 dist so can't say for sure. anyway, here's the connector diagram for the harness. sorry it took so long.

http://www.geocities.com/silentsenti...u_harness.html

it's a good sized pic so give it a minute to load. i didn't want to shrink it too much and make it too hard to read. HTH
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RIP: '88 YJ 2.5L Ax-5 NP231
Posi-Loked. Herculined. Optima yellow top. 1" Shackle, 2" BDS. Cragar 397's Aussie front.

92 YJ 4.0L Ax-15 231
5" springs, 1" shackle 31's or 35's depending on my mood

Last edited by sentinal02; 03-25-2004 at 01:01 PM..
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Unread 03-26-2004, 06:02 AM   #8
m-keith
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Update- have spark....still doesn't run

Turned out that the pickup sensor was bad. I replaced it, & now I have spark. However, it still won't run.

By the way, a couple of helpful tips I learned that I figured I'd pass along. First, if you can't spring for a factory shop manual (or in my case, my bro-in-law hasn't bought one yet), & your Haynes or Chilton manual doesn't provide the info you need, go to the library. My local library has all sorts of professional type Mitchell & Chiltons manuals; the kind that an independent shop would buy. I found an excellent guide for trouble-shooting the ignition system. The second thing I learned (from the same trouble-shooting guide) is that the red wire I talked about in my original post is supposed to cause a voltage drop to the coil. It's referred to as a resistance wire. Don't do what I did & replace it with something else. The system is designed to deliver about 6 volts to the coil under normal running condition. During cranking, the wire spliced in from the I terminal of the solanoid delivers full battery voltage to the coil.

As for my problem now, I'm not 100% sure, but it could be battery related. I have spark, I can smell fuel (I cranked with the throttle open to make sure it wasn't flooded), & my timing's still spot on. However, under cranking, battery voltage is dropping to 10. It's supposed to deliver a full 12 volts when cranking. I've been keeping the battery on a charger, but it may be worn out anyway. I'll have it tested today. If that doesn't take care of the problem, I'll be pretty close to the end of my rope.

Thanks for all the input & suggestions.
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Unread 03-26-2004, 03:33 PM   #9
sentinal02
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glad you got spark. try throwing a set of jumpers at it and seeing if it will start then. that should tell you if it's the battery or not.
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RIP: '88 YJ 2.5L Ax-5 NP231
Posi-Loked. Herculined. Optima yellow top. 1" Shackle, 2" BDS. Cragar 397's Aussie front.

92 YJ 4.0L Ax-15 231
5" springs, 1" shackle 31's or 35's depending on my mood
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Unread 03-30-2004, 12:47 PM   #10
m-keith
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It's alive!!!

Finally got this thing running last night. There were two more problems causing it not to run. 1st, after R&R'ing the distributor so many times, I forgot to verify that I was at TDC #1 the very last time I pulled the distributor. So, of course, I was 180 out.

After that, It still wouldn't run. A quick squrit of starting fluid down the carb, & it ran for 2 seconds then promptly died. Decided that some dirt inside the carb must've worked itself loose. Removed the fuel line going into the carb to spray some cleaner inside, & I find.....nothing.....dry as a bone! Removed the line at the pump....nada. So, after all that's happened with this truck, the fuel pump packed it in on me. Funny thing is that I tested that fuel pump when I first put the motor back together.

1 new fuel pump later, & she fired up just fine. Lesson learned- Just because ya smell fuel doesn't necessarily mean it's gettin' fuel. A good bit of cranking will cause the carb to draw enough fumes through the fuel line to make you think you're getting fuel.

Thanks for everyone's input & for tollerating my exceedingly long original post. Although I don't own a Jeep, I'll probably be around on a regular basis. The curse of the previous owner is very strong throughout this Jeep. Every time I look at it, I see something else that it needs.
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