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Transmission Leak(s)
TRANSMISSION LEAK(S)
Before our last trip to New England I discovered our transmission was several quarts low on oil. I topped it off with Mobil Delvac synthetic ATF that I get at NAPA. Upon return home from the trip, down a couple quarts again. Time to find the leak(s). Put the bus up on the ramps to change engine oil. (place markers for photos all at end, I can not figure out how to post photos in line with the text)
bus on ramps
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See trans fluid all over the trans pan and the mist of fluid/dust on the undercarriage aft of the trans out put end. Also with the bus running I see transmission fluid dripping – a drop every second or so – from vent at the bottom of the retarder accumulator solenoid – red circled fitting. You can see the small puddle under the rear by the crack in the pavement. The big hose fitting on the left of the accumulator marked with red arrow will be referenced later.
accumulator
Get out the engine degreaser and the pressure washer and clean most of the oil off the trans pan and surrounding undercarriage. Change the engine oil and let the rest of the stuff dry off. Next morning take of the plastic shields between the rear axle and the output end of the trans. Miserable, dirty greasy, oily, gritty job, in hindsight the dirtiest part of the whole job. Scrape ¼” of goo from the shields and wash well to make the reassembly a little more pleasant. See that the rear face of the retarder, which is a module on the back end of the transmission, is oily and clean of dirt and grime between about 7 and 3 o’clock pretty much confirming a rear seal leak.
Dig around on PC and find posts by several intrepid souls who have replaced their own rear trans seal and learn much from them. Order seal Allison number 29552705 $80 from DieselDash, turns out to be a sister company of Stewart and Stevenson. Unhook rear end of driveshaft (heavy) from trans output yoke and pull yoke off the output shaft – no puller needed, just light tap with hammer.
SLIGHT PROBLEM
Flange wear – Often the surface of the shaft or other rotating part gets a groove worn in it by the foreign matter that embeds in the rubber of the seal over time. Turns out my output flange had a groove worn in it. Not real bad but enough you could catch a fingernail in it.
flange wear
Five primary options
- Leave as is and hope for the best, area was not too rough just a smooth groove, new seal will work – maybe – or maybe for a while.
- Replace flange – prices out at about $1300 not a favored option
- Repair existing flange – driveshaft shop weld up grove and remachine – did not price out
- Repair sleeve – actually a common repair method in industrial and automotive power transmission. $80 ENGINE CRANKSHAFT SLEEVE – SKF 99473
- An old farmer’s trick – reassemble new seal slightly shallower or deeper than “normal” in its bore and have seal lip ride on “virgin” area of flange sealing surface. My choice.
As the seal bore in the Allison is a good bit deeper than the thickness of the seal assembly, I drove the seal .06” below the outer face of the bore by using at small (.06 thick!) washer held against the seal and tapping lightly with a small hammer until the washer was flush with the end of the bore, doing this around the circumference after first having seated the seal flush with the bore (the factory original depth) with a piece of flat hardwood.
seal bore and countersunk seal
Bolt the output flange back on, then the ujoint/driveshaft using new bolts and straps Spicer 6.5-70-18X kit about $10-$20 depending on where you buy it:
ON TO THE ACCUMULATOR LEAK
This assembly called the retarder accumulator is basically a hydraulic cylinder with trans fluid on one side and air on the other of a floating piston. When your system calls for retarder action the electrical solenoid valve in the plumbing on the aft end of the accumulator allows air pressure into that end of the accumulator and pushes the piston forward and thus trans fluid out the other end into the retarder, starting retarding action. When you end retarder activation the air solenoid closes and dumps air pressure on the aft end of the accumulator and fluid pushes the piston back and dumps the air out the vent. In my case the seals on the floating piston had begun to fail and it was allowing trans fluid which I presume remains under some pressure to ooze past and get into the air side and back flow out the vent.
A call to Prevost reveals accumulator assembly no longer available, replaced by 9 part numbers, the first of which is an overhaul seal kit Allison number 29535973 obsolete replaced by 29563811 $545. Thank them for their time and pursue other options. With some research come up with some part numbers for the accumulator assembly – 29522225 and the now obsolete overhaul seal kit 29535973. Find the assembly NOS on ebay for $300 and the original obsoleted seal kit also NOS available for $20. I buy both.
Remove the accumulator – a story in itself – to disassemble and overhaul with the seal kit, figure I’ll do that first and if it does not work out use the new old stock assembly. Short story on the removal of the accumulator is the hydraulic line between it and the retarder has a huge flare nut (red arrow in earlier photo) hydraulic fitting that take a 2” wrench. I must be getting weaker in my old age or the nut was super tight. I had a 2 foot long 2” combination wrench open end on the nut, both hands on the ring end and both feet on the tire and pulled with all I had including some loud grunts and could not break the nut loose. Got mad and got a chain and come-a-long and with two cranks the thing popped loose. Photo taken after it popped loose hence the slack.
rigging
Turns out the accumulator version I have has steel cylinder with aluminum end plates. It’s tied together with steel low grade bolts. Steel bolts in aluminum for 20 years is a recipe for problems.
Through much penetrant and vigorous banging on the bolts with hammers, drift pins, swearing and light twisting get 3 of the 4 out without breakage. One broke, had to drill it completely out and retap the hole. Just happened to have an 8” long 3/8 x 16 bolt lying around and even more miraculously could find it. Clean everything up and reassemble with new seals and copious trans fluid on all seals. $500+ for 5 little pieces of rubber is crazy.
Put it all back together, topped off the trans fluid and took it for 90 mile road test. All good!
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11 Replies
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Good information. Great that you tackled the job at hand; and won!!
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Dave,
Great write-up. I grew-up in McAlester. Made the drive to Talihina many times!
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The transmission output seal is surprisingly straightforward to change. Just 6 bolts. Even a caveman (me) could do it.
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I wish I found this thread before I changed my output seal last time, about 5 months ago. After I changed it out, leak seemed to be gone, but after a recent trip, I have trans fluid everywhere again. I suspect it is the yoke surface chewing up that seal. I guess I’ll have to figure out if I do the “farmers trick” or get that sleeve -I had no idea those existed!
And BTW, holy hell, can this create a mess! I have never been so dirty in my life, and I am about to do it again. Hopefully this will be the last time. I did not really clean much up under the bus last time, as I was on a time crunch, but this time I feel compelled to try to clear all the crud out. It’s just going to take a lot of de-greaser.
Not sure if I have a leak with the retarder, but I doubt I can tell until the leak at the seal is taken care of.
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Engine degreaser and a pressure washer is a good start. To get the final greasy film off this stuff (from Walmart) is amazing. I thought brake cleaner was good but this product literally makes greasy gunk fly off surfaces.
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Thank you for the excellent write-up!
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Thanks for the info on the brake cleaner. My local Walmart had the non-chlorinated version, which might not be quite as good, but I got it anyway. 10 cans later, I think I have it clean enough for my work.
I have decided while under there, to replace both u-joints. Just learned the hard way I am going to need a puller for that.
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If you ever decide to change u-joints, I highly recommend a tool like the following. There are multiple brands, Tiger Tool is the original, but I could not wait for them to get them in stock, so I went with oemtools. Works very well.
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Do you have a link to purchase?
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Here are some pictures of the yoke, before and after the installation of the speedi-sleeve. I guess I did not pay very much attention to the condition of the shaft when replacing the seal about 6 months ago. It’s clearly quite bad, and I have no doubts that the re-leaking of the transmission was due to wearing down the new seal.
With the yoke installed, I install the transmission-side of the driveshaft first, as it’s easier to bolt that up because it uses the u-joint straps. The differential-side is a bit more difficult because you have to install the caps to the u-joint after you line up the u-joint in the yoke. You definitely want to support the driveshaft with a strap when you do this. Also, having those holes in the woke pointing up/down make the job much easier, as you can much more easily align the u-joint in those holes before inserting the caps. I cleaned those holes with a light sanding before installing any of the u-joints, and that also made it a lot easier.
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