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Old 04-04-2022, 10:48 PM   #1
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Tiffin Bus 37AP fan drive gearbox

Well I thought I would take the time and service the belt driven fan drive gearbox, 23000 on it and has never been done. Tiffin recommends every 10000 so it was overdue. Removed the bottom air shroud to access the box and inspect the driveline. Tried several times to get the top fill plug out First, because if you cant get that one out no need to open the bottom drain plug. Stripped out the 1/2" plug - 3/8" drive allen because it was actually glued in. So now I have to take the box out. Glad I did because I needed to address the input shaft yoke keyway was worn so bad had too much play in it, thought the play was gear lash in the box, but no it was the yoke. All 3 plugs on the box were glued in not to mention the top plug was Not Vented, but a solid 1/2" plug. I had to heat the box around the top plug using my Map gas to 160 degrees via my temp gun so I could remove the plug. Thats the reason both input and output shaft seals were sweating oil covering the box. Well I wanted to replace the seals called Tiffin and they said the seals were not available but I could buy a new box and Curtis Machine said they could not talk to me about the parts for a 412M type F gearbox because it was Tiffin specific. Since the driveline was out I just reinstalled with u-joints with zerks. Now to find the seals. Found a Curtis drawing online for the 412 box showing the seals SK 12458 but they are 2" and will not fit the case opening, too big. After 2 days of searching found them via an internet search 1.25 X 1.968. There is a Difference between the gearbox 412 and 412M, I guess M is for motorhome. Anyway pulled out old seals, which were also glued in and compared them to the replacement seals part number 12192TB-BX and were an Exact Match used in the trailer axle industry on 2000 lb axles. I did have to apply diff sealant to the housing perimeter before inserting the seal so as to secure it and seal the opening as the mfg did in the beginning. Just thought I would give some info of my experience in case someone else is having the same troubles. Refilled the gearbox with 24 oz. 75W-90 synthetic gear oil as recommended by Curtis Machine via their website covering the 412M gearbox. Sorry for the long rant
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Old 04-06-2022, 11:26 AM   #2
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fan drive

Thank you for the great information and detail it should help many owners. Is it a recommended maint. ?
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Old 04-06-2022, 01:44 PM   #3
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Can you send me a PM as to where you got the seals. Im going through that now. I see they are on Amazon. Are they both the same size as I did not take my gear box out yet
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Old 04-06-2022, 10:38 PM   #4
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Thank you for the great information and detail it should help many owners. Is it a recommended maint. ?


Yes I would not let it go over the recommended 10000 mile service interval. My coach has 24000 on it and the oil was Black. Only 24 oz. capacity and the new oil was clear letting me know I needed to service this gearbox. Its a Pain in the a(* but had to be done
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Old 04-07-2022, 05:25 AM   #5
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Does your have a breather on it as mine is a solid plug on top?? Did you add a breather??
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Old 04-07-2022, 08:30 AM   #6
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Does your have a breather on it as mine is a solid plug on top?? Did you add a breather??


Yes it now has the vent plug in the top plug location. Some here have installed extensions but I chose not to.
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Old 04-07-2022, 08:58 AM   #7
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Some views of the 2019 Bus Powerglide fan gearbox. It is now suspended from a horizontal plate. Previously it was mounted on top of the plate. Yes it's a bear to access. I've used a small battery powered pump to suction the oil out when I couldn't remove the bottom plug. The side plug should be opened to overflow when full to capacity. I've gotten to it both from above and below, but now I leave the servicing to Bay Diesel!!

Yes the vent plug is important, it minimizes pressure buildup when warm, otherwise the oil can blow past the seals and create 1) a mess, or 2) failure of the seals. It's not a cheap replacement, it was around $2000 on my 2010 Bus.

Someone on the TRVN forum recommended using 75w-140 oil instead, when seal minor leakage is present, as it remained thicker than 75w-90 when hot. Also shown is my previous 2010 gearbox(5 yr, 30,000 mi, likely never sealed) with leaky seals.
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Old 04-09-2022, 04:41 AM   #8
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Pulled mine out over the last 2 days and got the seals on Amazon. Set of 4 for $14.00. Alll rebuilt and back in. Will button it all up today. Got to re-install fan and a few minor bolts to tighten up and put bottom cover on. What a pain in the Arse that was.
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Old 04-09-2022, 08:15 AM   #9
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Pulled mine out over the last 2 days and got the seals on Amazon. Set of 4 for $14.00. Alll rebuilt and back in. Will button it all up today. Got to re-install fan and a few minor bolts to tighten up and put bottom cover on. What a pain in the Arse that was.


Well Steve, if any thing like me your felling great after the job and happy it's done.
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Old 04-09-2022, 11:39 AM   #10
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Has anyone used synthetic oil?
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Old 04-09-2022, 11:54 AM   #11
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Yes it now has the vent plug in the top plug location. Some here have installed extensions but I chose not to.
A friend with a Bus sent this post to me as he knows I've had issues with this gearbox. On the Newmars with a Freightliner chassis, they have been using this box for years. Both my 2014 DS and my 2019 DS have it. For what ever reason, Freightliner puts a 3" riser in the top of the box with a cap screw that has a vent hole drilled into both sides of that screw.

On both my coaches, that vent has puked oil out since new. I'm guessing about 10% -20% of the Newmars had the oil leaking issues. It was odd as they all used the same riser and vent cap. The leak REALLY makes a mess of the engine as the fan blows the oil everywhere.

As far as I know, I was the first to install an extension on my 2014 DS and passed along what I did to others who copied what I made. When my 2019 DS arrived, I figured what are the odds it would leak, well I guess pretty good as it was puking oil. This time it was a good thing, as my brand new coach had a cracked fan blade which the oil actually highlighted. Off I went to Freightliner. They replaced the fan blade and the gearbox saying the seal was bad, I knew better, but didn't argue. Sure enough, a few months later I had oil puking out. I did the same repair I did on my other coach. I used the parts that were there and added an 18 hose so that reached the top of the engine bay where I reinstalled the vent on the end. I secured it to the roof of the bay.

Since this had been an issue twice, I looked at my friends bus and saw that the Tiffins didn't have a vent just a plug. I wondered how they did that since mine was obviously building enough pressure to puke oil.

With all that said, my guess why only some of the Newmars were puking oil was the orientation of that little breather cap with the holes in opposite sides. I think in certain positions it was allowing air from the from to actually suck oil out. I actually spoke to the Freightliner CEO Jeff about two months ago and told him they've probably replacing several hundred thousands of dollars worth of fan drives under warranty thinking the seal was leaking, when it was the vent. He was going to look into it.

I post this long story just to warn you to keep an eye on the vent and make sure it doesn't leak oil after the next few trips.

Lastly, the Newmars come with zerk fittings in that short driveshaft. The ones on the u-joint are impossible to reach from either the top or the bottom unless you have 3' monkey arms. I finally made a tool my last oil change. It's a small grease gun with a short flexible hose and a two-foot solid tube that I can reach up there and access that zerk.

https://www.irv2.com/forums/f103/fan...de-343698.html

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Old 04-09-2022, 09:41 PM   #12
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Don, I read your post about the box extension before I did my job and in my mind is a good idea, but I chose not to do it at this time. Going out this month and will see how the top vent will work and am hoping it will not leak. The u-joint off the belt drive I can reach along with the shaft zerk its the one on the input shaft what will be the problem, but I am up for it. Thanks again for the reminder and if it does puke out the oil I will be adding the extension
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Old 04-19-2022, 05:40 PM   #13
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Well Steve, if any thing like me your felling great after the job and happy it's done.
Yes clean gear box. Whole area cleaned up. Also found a drip on my lower rad hose over to intercooler and fixed that. Had to drain off all antifreeze and replace lower rubber hoses and clamps and while at it I flushed system and put in all new coolant.

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Old 04-19-2022, 06:31 PM   #14
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After thinking about what Don said about the extending the vent I did just that. Thought about possibly having the new vent leaking and I did not wish to do the job again because of not extending it and having oil slung all over the engine and radiator.
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