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Old 05-28-2012, 11:00 AM   #1
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Default The dreaded IOD fuse blowing - Jeep won't start

DISCLAIMER: I posted this on another forum but have not gotten any help, so came here hoping for a better response. I know - long weekend all the really knowledgeable people are probably out wheeling instead of hanging around answering questions! But if this makes me guilty of "cross posting", go ahead and whack me appropriately.

Anyway, here goes:

I pulled into get gas about a week ago, and went I went to start up, no go. Turned over fine, but I didn't seem to have a spark. Found the IOD fuse was blown. Bought one at the gas station, fired right up, and away I went.

Went into town Saturday to get a part for another vehicle, and couldn't start. Checked the IOD. Sure enough, it was blown. Replaced it, and it blew again before I even got the key to the Start position (son was watching it for me). Replaced it, and it blew immediately.

Towed the Jeep home.

I've done a LOT of searching on various forums, and it's clear the IOD (Ignition Off Draw) fuse is involved in a lot of circuits. TOO many - this is lousy design!

Since I can't get my hands on a decent diagram (yes, I'm going to buy the FSM!), and I'm therefore shooting in the dark, I started listing what I know about the Jeep that might be related to an electrical problem. The only thing that is NOT stock is the radio, and the only electrical problem I've had is the door locks & window switches.

When I got the Jeep, the window lock switch on the driver's door was broken (floppy loose) and the switches on each door didn't work, as if the broken switch were stuck in the "I'm not letting anybody else open these windows but ME!" position. Also, the electric door lock switches on the driver's and front passenger doors were inop. (No electric door lock switches on the rear doors.) I had gotten a replacement switch assembly from a junkyard, and it all looked identical, but it didn't work at all, so I put the old one back in and have just been running that way.

I remembered that, last Tuesday, I had tried fiddling with the driver's door lock switch just on a whim, and it worked. This was a surprise. The next time I tried to start the Jeep (about an hour later, at the gas station) was the first time it failed to start, and I found the IOD fuse was blown. Replaced it, drove away, and was fine until yesterday.

Hmmmm. Messed with the door lock switch, and then couldn't start. Click the image to open in full size. What would that have to do with the ignition system? Well, okay, it's something to check, anyway.

So, I pulled all the door switches, unplugged them, and replaced the IOD fuse. It didn't blow. Started the Jeep. No problem.

Yay!

Oh.

Not so fast.

Since the door switches seemed to be the source of the problem, and I had them all out, I decided to clean them up and see if I could make sense of the pinout and find out why that junkyard switch didn't work. I disassembled both driver's door switch assemblies (mine & the junkyard) and compared the innards. I can't see any difference. I measured the individual switches on both and they seemed functionally identical (measuring from pin to pin with the switch in each possible position). I replaced one of my individual switches with one from the junkyard assembly (because it fell apart when I took the whole assembly apart), and I also replaced the window lock switch, since mine is broken. (That didn't help - the j/y window lock switch is ALSO inop. Both show zero continuity (open circuit) in either position. I've seen a lot of posts about this, so this was obviously not one of Jeep's finer design features. )

Cleaned up the parts, put it all back together, and things went downhill faster than an avalanche.

Now I can't roll up any of my windows, and the Jeep won't start.

Ah, but that good old IOD fuse is just fine!

In fact, every fuse in the beast is fine. I have checked every single one, in the PDC under the hood and the panel under the driver's side dash. All good.

I have also metered every single pin of every single switch on the driver's door switch assembly. Except for the window lock switch, all are working properly. All are 6 pin, double throw switches. All 5 of them (1 for each door window, one for the door locks).

Can anyone help out with a diagram of the power window circuits? Better yet, give me a clue about how the power windows might affect the ignition circuit! That makes NO sense to me!

I'm looking online for a service manual - the real deal, not a Chilton or Haynes, but that will take a few days to get here. Meanwhile, I could sure use some help.

I know my way around electricity (I'm an engineer - it's what I do), but without a circuit description or a diagram I'm shooting in the dark.

Ah, what a great way to spend a long weekend, and I have to get to work tomorrow!
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Old 05-28-2012, 12:10 PM   #2
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Thank God I have manual windows and door locks.
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Old 05-28-2012, 12:35 PM   #3
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I can't specifically answer any of your questions off the top of my head,
but I do have the Chrysler FSM for the 1996 XJ.
(and family & friends in by-God West Virginia)
I've also got a background in field service so If you can ask specific questions I should be find it in the manual for you.
Hopefully the wiring didn't change too much in 2 model years.

Drop me a PM and I'll send you a way to get in touch.
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Old 05-28-2012, 12:51 PM   #4
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I sent you a PM.
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Old 05-28-2012, 01:04 PM   #5
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Didn't get the PM (yet) do they normally take a while to post?
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Old 05-28-2012, 01:22 PM   #6
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Quote:
Originally Posted by Marcucco View Post
Didn't get the PM (yet) do they normally take a while to post?
I've been away from the computer, trying to figure this out and pulling my hair!

PM on its way! And thank you!
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Old 05-28-2012, 05:20 PM   #7
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Quote:
Originally Posted by BlueRidgeMark View Post
I've been away from the computer, trying to figure this out and pulling my hair!

PM on its way! And thank you!
You're welcome. Hope it helps you solve the problem.
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Old 05-28-2012, 06:21 PM   #8
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Okay, here's where I am.


I have disconnected the window switch assemblies from all of the doors. I have also pulled the 30 amp power window circuit breaker (in the passenger compartment). This should mean there is NO power going to any of the windows or their switches.

The IOD fuse still blows. Sometimes as soon as I put in a new one. Sometimes after a minute or a few seconds, sometimes when I start the Jeep.

As of right now, it has started every time I've tried it, since disconnecting the switches.

I now have the full FSM and I'm going to spend my evenings this week working through the circuits so I understand them, and next Saturday I'll dive in again.

I hope!

The interesting thing is that the FSM says that the IOD circuit should draw a maximum of 20 milliamps:

Quote:
Battery drain should not exceed approximately 20MA (20 milliamps=0.020amps).
Yet the IOD fuse is a 20 amp fuse! That's what's there, and that's what the PDC cover says it should be.

So that circuit is fused at 1000 times the expected current draw.

That's weird.

I jumpered each of the window motors to put my windows about an inch down, and I'll just live with that this week, while I try to get a handle on how this thing is supposed to work.

The annoying thing is that I can't find anything to tell me what is powered by the IOD fuse.
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Old 05-29-2012, 08:35 AM   #9
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I don't know very much about the electrical system, but I believe I read in a thread that a bad ground for the PCM will cause it to draw too may amps.
Since the power to the PCM flows through the IOD fuse, would/could that cause it to blow?
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Old 05-29-2012, 01:41 PM   #10
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Quote:
Originally Posted by BlueRidgeMark View Post
Okay, here's where I am.


I have disconnected the window switch assemblies from all of the doors. I have also pulled the 30 amp power window circuit breaker (in the passenger compartment). This should mean there is NO power going to any of the windows or their switches.

The IOD fuse still blows. Sometimes as soon as I put in a new one. Sometimes after a minute or a few seconds, sometimes when I start the Jeep.

As of right now, it has started every time I've tried it, since disconnecting the switches.

I now have the full FSM and I'm going to spend my evenings this week working through the circuits so I understand them, and next Saturday I'll dive in again.

I hope!

The interesting thing is that the FSM says that the IOD circuit should draw a maximum of 20 milliamps:

Yet the IOD fuse is a 20 amp fuse! That's what's there, and that's what the PDC cover says it should be.

So that circuit is fused at 1000 times the expected current draw.

That's weird.

I jumpered each of the window motors to put my windows about an inch down, and I'll just live with that this week, while I try to get a handle on how this thing is supposed to work.

The annoying thing is that I can't find anything to tell me what is powered by the IOD fuse.
Let me know if you need more. IOD is F1. Note that F1 is fed by F14 40 amp off the battey bus.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.
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Old 02-08-2013, 04:51 PM   #11
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Well, for various reasons, I haven't been on the forum much. Here's where I am:

The no-start condition is a heat problem. If I ice my PCM, the Jeep will start. Every time.

It's not the IOD, because it will start in cool weather with or without the IOD in place.

That's all I know at this point! I'm limping along, but need to get the no-start problem fixed before hot weather gets here.

More later...
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Old 02-09-2013, 12:55 AM   #12
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Quote:
Originally Posted by BlueRidgeMark View Post
Well, for various reasons, I haven't been on the forum much. Here's where I am:

The no-start condition is a heat problem. If I ice my PCM, the Jeep will start. Every time.
I bet if you open it up you'll find a hairline crack where one of the pins is soldered to the board. The fun part might be getting at it under the potting compound some of those have slathered around.
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Old 02-09-2013, 09:34 AM   #13
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Yep, I'm fully expecting it to be a cracked solder joint. I'm in electronics, so I have no hesitation getting in there to find it. I just have not had time.

Thanks for the tip on where to look first!
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Old 04-03-2014, 04:12 PM   #14
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Quote:
Originally Posted by Radi View Post
I bet if you open it up you'll find a hairline crack where one of the pins is soldered to the board. The fun part might be getting at it under the potting compound some of those have slathered around.
Hey what are you talking about opening up? I'm having same problem. My windows work for about 20 sec then they quit and the 30amp breaker is very hot when I pull it out to cool down. Then when I put the breaker back in cool they work again until it heats up.
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Old 04-03-2014, 04:30 PM   #15
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Quote:
Originally Posted by BlueRidgeMark View Post
If I ice my PCM, the Jeep will start. Every time.
Quote:
Originally Posted by Radi View Post
I bet if you open it up you'll find a hairline crack where one of the pins is soldered to the board.
Pretty sure he's talking about the PCM.
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Old 04-03-2014, 04:30 PM
 
 
 
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