Rear axle question (solved) // RWAL confusion... (still confused)

[norgeek]

Kinda confused about a couple of things and decided to make a thread about both.

It's not a great picture, but hopefully someone can help me out a bit..

What does this little rubber hose do? I can't find it in any online sources or in my manuals. It seems to end above the fuel tank with a screw capping the end. At first I kinda got my hopes up thinking one of the previous owners installed an air locker and never got to the compressor installation... but when I checked the spare truck it had the same thing there. Looking at ARB installations it seems like the air pressure line should be on the other side anyway. So.. any ideas? Some sort of ventilation?

While you're already looking at old axles, could you perhaps also help me identify it..? I was under the assumption that it was a 10.5 14-bolt until I had a proper look at it... I just can't make the bolt pattern fit with any 10,5's I've seen pictures of. It rather looks like (some) 9.5 bolt patterns, but my axle is a full floating 8-lug unit and as far as I've figured out the 9.5 is neither. '91 2500 4x4 Suburban.

My second issue is sort of regarding the RWAL... seems to be a common enough problem-source in itself, and I'll probably end up removing the whole thing to get rid of the dim brake warning light.. Anyway, what confuses me;

In my 1991 GMC Light Duty Truck R/V and P models Electrical Diagrams and Diagnosis Manual(!), on page A-116 (and -17), there is a diagram for the RWAL controller. According to this, Connector C103, Pin F/420, should be connected to the brake switch and should receive a +12V feed when the brakes are applied. Essentially, the top-most cable at the back of the big black box next to the brakey stuff.

RWAL question one; In the diagram (also in my factory Service manual) the cable specified is supposed to be purple... yet on both my Suburbans it's a clear, bright yellow?! Is the cable color differentiation a GMC/Chevy difference - both mine are Chevy - or am I missing something obvious?

RWAL question two; What really confuses me however, is the lack of a brake signal. With a volt meter between said pin and ground, I get a reading of ~0.0x volts and ~0.xx depending on whether the glow plugs are engaged or not. My brake lights are working as they should, but I'd like a 'clean' brake signal for my aux. brake lights and trailer coupling. I can't figure out a way to reach the actual brake switch without removing quite a lot of stuff first so I was kinda hoping to take a shortcut until I get around to replace the whole steering column next winter. As far as I understand, this cable will also have a signal when 4x4 is engaged, but that's not a problem for my use.

 

[dyeager535]

Just so I look smart with first response, the first pic is the breather, second is the 14SF, 9.5"

Should NOT be a screw in it, if there is, needs to be replaced with a vent and/or filter of some sort. In certain conditions a plugged vent will force fluid out past the differential cover seal.

I know nothing of the RWAL, but I can tell you that once in a great while the diagrams will be wrong. Are you certain you are looking at the right bodystyle? There will be no difference between Chev and GMC.

Depending on what you need for brake light stuff, I'd strongly recommend (if it works in your application) to go buy one of the trailer wiring adapters that are purpose made for these trucks. I don't know you or how you do things, but I will say that screwed up trailer wiring has got to be about 50% of the source of lighting issues on trucks in general. The ready-made adapters, again, if they work for what you need them for, eliminates splices and mistakes that normally cause all sorts of grief.

 

[norgeek]

Thanks for the quick reply

That would clear up the rear axle issues.. funny how wrong I could get the things I thought I was sure of :P I'll make sure to look into the breather blockage next time I'm in my garage. Could be it's properly filtered already, was hard enough just getting half a hand in there..

As for the wiring I'm quite confident as it's sort of what I do by trade (...as opposed to the mechanical stuff ). The problem is that I can (barely) get away with red rear turning indicators on the car, but absolutely not on a towed trailer. Norwegian/European rules and restrictions. I'll probably end up doing a 'heavy duty' solution in the back with a good power feed, relays and a fuse box in the back just to be sure.

As for wiring color, I first thought it could be one of the typos in the manuals as I've spotted quite a few already. After googling "pin f" and "rwal" everyone seems to be referring to the top wire as purple though, and that sort of reconfused me.

 

[dyeager535]

The stock piece wasn't a filter, it was a plastic little cap thing...the front diff should have the same setup. I just mentioned filter because one of the aftermarket fuel filters would work if you can't find the right breather.

Glad you know wiring, that makes it easier.

Only thing I can think of on the purple wire is that the '88+ C/K may be using that color? I've seen GM use wrong diagrams/pictures before, obviously a cut/paste from a prior manual, or different body series, but if everyone else says the same thing, then maybe there is something you are looking at wrong. But since you are mentioning the specific pin, it indicates to me you know what you are looking at.

Someone on here with RWAL on an R/V should be able to look for you to see how theres is setup. Might want to post a picture of where on your truck the pin/connector is though, to make sure you are looking at the exact same thing.

 

[norgeek]

Yeah, better make sure

Yellow, top wire is the 'offending' one. The rest are - underneath the grime - as described in the manuals (A through E, bottom to top): Black, Brown, Orange, White, and Tan/White.

The hits I've gotten on google are mostly regarding the 90/91 square body suburban and blazers, rather than the next-gen pickups.

Quick edit; After taking another look, I realized just how easily accessible both the brake switches are (D'oh!!). I'll obviously be able to do something here.. which I guess means both my actual issues are sorted out already :P Still find the lack of input signal and wire color mismatch interesting enough to leave the thread overnight....

 

[dremu]

It's been a while since I had something as (relatively) new as your truck, but I seem to recall there being multiple types of ABS/RWAL controller. There was a Kelsey-Hayes with a big "K-H" on it, but there might have been another kind as well.

And yeah, I'd hit the wires at the brake pedal anyway, be easier to get to them there.

-- A

 

[Big Ray]

fwiw, my brake light did the same thing twice. I simply unplugged the rwal computer up there by m/c. Just a few minutes the first time. 2nd time I left it off all day. replugged it and haven't had a problem since...