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post #1 of (permalink) Old 03-28-2008, 12:40 PM Thread Starter
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Ground strap routing and rear pinion seal

Hi folks,

got a couple issues going on right now w/ my 1998 TJ (4.0L) that I am going to address this weekend, and I have some related questions.

1. The under-hood ground straps are completely rotted off, have been for some time, and I suspect this may be the cause for some electrical problems I've been having. I do have a set of replacement straps, but I'm unsure of the original/proper routing. I think one was from hood to firewall, and there's at least 1 other strap. Can't recall if there were 2 or 3 total.

Where should these straps be routed? IIRC one was connected to a spot on the engine block, where exactly on the block does it connect?

2. The entire underside of my rear diff has a thin coat of gear oil on it. The plug is not leaking, so that means it's either the cover gasket or the pinion seal.

There is also some slow dripping from the cover/housing mating area, but this might just be because it's the lowest point of the housing so the oil naturally migrates there before falling to the ground. I am concerned it may in fact be the pinion seal because if it were just the cover, I wouldn't expect it to be covering the entire housing, since the wind from driving should spray it back and not forward.

This started right after I had a brake line replaced, and I suspect some brake fluid got sprayed from the junction block on the rear axle while the guy was bleeding the lines. I figure it probably ate and ruined either the gasket seal, the pinion seal, or both. I am going to replace the gasket this weekend, but obviously the pinion seal is a relatively major undertaking.

Does the above hypothesis sound valid? Would brake fluid be able to destroy/compromise the pinion seal material (silicone I'm assuming?), or should the seal be intact and maybe I just need to wash the undercarriage to get rid of the brake fluid residue and the seal should still be good? (As well as replace the cover gasket and gear oil.)

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post #2 of (permalink) Old 03-29-2008, 12:46 AM
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The forum is all messed up for how messages are listed, so it may take a few days for the experts to find your questions and post answers.

On the grounding straps, the most important is making sure that the negative post on the battery has a good connection to both the frame and the engine. From the factory the battery negative connects to one of them and then a connection from that to the other (sorry can't recall which order). Some here say it is best to go directly from the battery to both frame and engine. Then there is a whole other issue of making sure all of the portions of the body are grounded to the frame, hence the ground strap from the hood to the firewall.

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post #3 of (permalink) Old 03-29-2008, 01:35 PM Thread Starter
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It's the pinion seal

Hey Rick,

thanks for the info on the grounding. I am also going to take a look at the FSM for my 87 YJ to see if there are any similar grounding diagrams. I have a FSM for my TJ also, but I lost it several years ago and haven't been able to find it.

On the diff leak, I just pressure-washed my undercarriage to get rid of the brake fluid and gear oil that was all over everything. Drove it around for a few miles and then got under it and saw a line of gear oil straight from the pinion input to the back of the diff. So, it's the seal that's leaking unfortunately.

Seems awfully coincidental with when I had the brakes done so I think there may be something to the "brake fluid" theory. Oh well, not much I can do I guess, other than fix it.

On the other hand, I researched the seal replacement a bit and it doesn't look like as big a job as I'd originally thought. So this weekend or next I'll have to gather up the parts and then just get it over with. If anyone has any tips or links to helpful info regarding this work please let me know, I will take any help I can get.

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post #4 of (permalink) Old 03-30-2008, 02:06 AM
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Big ground strap from battery (-) to engine - it has to carry the starter current.
Then it can have a smaller one between the engine to the BODY. It carries the lights and accessories.

Hood can have a strap too - especially if you have electronic ignition or FI.

It's also a good idea to ground the frame to the engine too - sometimes it carries things like gas sending unit etc that isn't grounded to the body.

It's a good idea to have multiple body and frame straps too. It's not possible to have too many, but it is possible to have not enough.
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post #5 of (permalink) Old 03-30-2008, 08:30 AM
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New grounding braid and straps goes a long way in fixing grounding problems in a Jeep. However, that said it doesn't fix the problems in the body or the frame that can lead to grounding problems. As example if the frame rusts, it will do so at the joints, and often at the welds. What appears to you as a complete frame is electrically many resistive joints and pieces.

To truly refresh the ground system, why no just run multiple cables from a main grounding block near the battery, to the engine, inside the tub, and to the rear of the tub, each terminated to a separate grounding block. All devices near these blocks would then be grounded to these blocks. That doesn't mean you can't use the tub or the frame.... just don't rely on it as a good ground. In this way you've got a good grounding system that should give several years of service before needing to be refurbished.
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post #6 of (permalink) Old 03-30-2008, 04:26 PM Thread Starter
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FWIW... I got the new grounding straps installed and have verifiably good ground now. Unfortunately I also found that the reason my taillight doesn't work is not because of bad ground, but because one of the connector wires is broken/cut.

I'll have to replace the lamp assy. but I don't think anyone will stock it with the harness connector already attached, so I may be in for getting screwed by the dealer, unless I can find a boneyard around here with one.
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