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Hub (portal Axle) Running Really Hot... Next Steps?

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#1
Hi Guys,

After a 40km journey in my 1984 416, the one axle (right rear) is running very very hot. The point of cooking my brake fluid, I believe (which is why I lose brakes after driving for 30mins+).

It does not leak oil, and is properly filled. Oil looks nice and clean at first glance.

My understanding is that it can be either of the following:
  1. The brakes dragging (though it doesn't feel like it - but I have to jack it up and test to be sure). If so, might be caused by bad caliper.
  2. Bearings / gears in the portal axle going bad. I'm told to drain the oil into a clear glass bottle, and carefully examine in bright light for bits of shiny metal.
Any other preliminary advice for me? How scared should i be?

thanks!
 

paulventer

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#2
Hi Dawid

I agree with your potential culprits, but I would lean towards the brakes binding.

On the rear calipers you need to manually adjust the play between the calipers and the disc - IIRC there are two adjusters on each caliper, one for each brake pad (can't remember what the manual says, but free play should be set to a mm or two between the brake pads and the disc). It could be that the brake pads were replaced, but this adjustment was never done, which now causes the pads to touch the discs, creating the heat. Or brake fluid could have leaked onto the brake pads, causing them to swell slightly and touch the disc.

There is a good write-up with lots of pics here http://www.billcaid.com/2008/UnimogBrakeRepair20081110/UnimogBrakeRepair20081110.htm (although Bill Caid's mog is a 1300L, the brake calipers are the same as those on a 416).

To eliminate the portal bearings as a culprit, I would do the following:
1. Drain and inspect the oil as you indicated above
2. jack up the wheel, grab it with one hand on top and one on the bottom and try and shake/move the wheel - if there is a significant amount of wear in the bearings, you should be able to feel free play in the wheel assembly.
3. while the wheel is jacked up, rotate the wheel and feel if there is any significant resistance - this will indicate a binding brake
4. same as 3. above, but listen/feel for grinding noises which would indicate loose bearings/pieces of bearings in the hub (you don't want to hear this as this is where things become expensive!)

Keep us posted with your findings.
 
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#3
Thanks for your valuable advice, Paul! I'll keep you guys posted with my findings.

I hope the fact that I get spongy brakes due to overheating brake fluid (even after a fresh DOT4 fluid replacement) also points to the binding brakes option - I would image it would take *significant* heating of the portal to transfer that much heat to the brake fluid otherwise. And with it not leaking at all, and being filled with oil, that just doesn't make sense (I hope!).
 
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Dawie Louw

G-Wagen Club SA
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#5
It could also be a binding caliper, that is the piston is sticking.

If you wish to try and pinpoint the source of heat, you could try using an infrared therometer. I have one you could borrow.
 
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#8
Quick update - after a 10km drive, the problem is almost certainly the brakes. The portal axle housing itself is almost cold, but the brake disc was super hot - and more so on the right rear than on the other wheels. I live just around the corner from a (reasonably priced) brake specialist shop, I'm going to let him take a proper look at all the brake calipers just to be safe - I don't really want to mess around with brakes on a 3.5ton truck. I don't really have good workshop space, if I were to do it myself it'd be an exercise in frustration.

They're squealing when braking in anyway, so I might just replace all pads while I'm at it - I don't know how many years these pads were in.

My reasoning is sound, right? If the problem were bearing(s) in the portal, the portal housing would be much hotter than the brake disc after a 10km high speed drive on the highway? Going to drain an examine the oil today for extra evidence.
 
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Dawie Louw

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#9
My 2 cents: Yes, I think you are on the right track. To have the slave cylinders opened and checked is a good idea. One observation though: Age will have no effect on brake pads. Provided they are not contaminated with oil, grease or brake fluid, only the thickness of the remaining pad is all that counts. Also check that the discs are still within (thickness/wear) specs.
 
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#10
Useful discovery today - the overheating wheel is the only one from which the internal mud / guard plate (basically encloses the whole brake disc / caliper, preventing mud, rocks - and air! - from reaching it). The other wheels don't have it on - Tim removed them, apparently left this one on as an experiment.

Not sure if all Mogs have this guard fitted (curved metal plate - I'll post pics). I'm not talking about the normal circular rear plate - but instead the half-drum shaped enclosure running inside the wheel as it were.

Could be as simple as normal overheating because of that guard. It'd be too much of a coincidence otherwise. I think that "normal" cars and 4x4s spoil us, but these are giant, non-ventilated discs. The must retain heat like nothing else, especially if there is no airflow.

I'm going to remove it first, and test - before touching anything else. It's simple enough, if only I can find a jack / stands tall enough to safely jack this beast up (my tallest 10ton jack doesn't reach the axle). Finding one tomorrow.
 
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paulventer

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#11
Dawid - I think those guards were an optional extra. I have seen some mogs locally with those guards, but most I've seen are without them.

I doubt the guard would cause overheating of the brakes, seeing that it was available as an option from MB. It could however be that the guard has collected mud/gunk which is rubbing against a moving part, although I suspect the culprit to be a binding brake caliper.
 
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#12
Yes, I removed the guards, and still, the problem remained - albeit less so, because of the much better airflow.
Anyway, back from my trip this week-end, it was the usual story, with the brake heating up a bit too much after 50km of high speed driving, causing spongy brakes.
I slowed way down, but it wasn't a safe area to stop.

Anyway, after some vigourous pumping to avoid crushing a car, at some point it coughed up a bunch of brake fluid out the fill bottle up front. I don't know if there was a big air bubble (I *did* bleed the brakes though), or what happened, but it didn't happen again, and after that the brakes were somehow much better?! The final 50km journey home after that, which involved a fair amount of stopping at traffic lights after long, fast, downhill stretches on a main road - the worst kind for truck brakes - saw perfect brake behaviour.

I haven't had time to investigate further. But, when I took the rear wheel off to remove the guard plates, the caliper did look quite sad, evidence of old leaks etc - I think the calipers need some love, and then I'll be fine.

I'm happy about the fact that during all this, the hub reduction gear housings never got hot, so all is well for now. Had a great weekend exploring what my Mog can do, I'll post pics later (in a new thread).
 

paulventer

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#13
They say a picture is worth a thousand words, so here's a thousand words on how to adjust the rear brake pad free play - for when you get to adjusting them ;)


416 rear brake adjustment.jpg
 

Dawie Louw

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#14
Very useful information. Not many disc brakes have any adjustment.

In my experience, low usage/older vehicles tend to have sticking brake pistons and it may be worthwhile to disassemble the piston and then clean and ligthly hone the cylinder.
 

paulventer

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#15
I suspect the rear calipers aee adjustable because the hand brake, which is not hydraulic, uses them.

The front calipers are not adjustable - the hydraulic system compensates/adjusts as the pads wear.
 
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#16
I never posted a conclusion to this thread :)
Ferdi (from MogDoc) re-did all my brakes - refurbished calipers, skimmed discs, new brake pads, and a general health check of the system.
Hot wheel hub problem disappeared completely, and my mog can stop now! (no more brake fade)

It's really nice driving a Mog that can stop faster then she can go...
 
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#18
Hi David
You can buy a IR thermometer from the guys at Comunica in Midrand.I bought one there year ago.
Hi! I actually did buy one from them about a year ago.
In May last year, it turned out that my hub bearing was very worn, which was the real cause of the heat problem (and not dragging brakes).
My Mog has been running great since then.
 
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#19
Hi! I actually did buy one from them about a year ago.
In May last year, it turned out that my hub bearing was very worn, which was the real cause of the heat problem (and not dragging brakes).
My Mog has been running great since then.
Sorry I dont visit this site often enough.
Too busy with other projects .Next one already started buying materials to build new alu clad camper on my mog.