1986 Automatic Transmission Leak

[birdyhues]

Hi! I’m new to the forum and to transmission maintenance, but a long time GM owner with good analytical skills. I need some help identifying the source of a transmission leak in my 86 C10.

A few months back, the power steering went out. Around that time I also noticed a good sized pool of transmission fluid under the truck. I don’t drive it much, but I keep it on a trickle charger and have taken it on short distance drives since then.

Recently, it’s started having trouble shifting into reverse, and only reverse. I did some googling and it seems likely that this is due to low fluid, which isn’t surprising given the puddle from earlier.

I took it to a store a few miles from home recently and it struggled to shift from a low gear to a higher gear in drive once in the way home.

So, now I’m looking to fix it and want to try some things on my own to build my skills first. I’m planning to change the fluid and filter since that seal is probably needing to be replaced, but I got under it today and got some pics of the pan and noticed fluid in some other places that may need attention. I’d love to know what I’m looking at and if I should plan to do anything more than the fluid and filter while I’m at it. Looks like a new pan might be a good idea, too, for example.

 

[NickTransmissions]

Replace your pan gasket (DO NOT USE a cork gasket), speedometer cable/housing seals snd rear transmission mount. Check the governor cover seal, replace that also if leaking. Double check the selector shaft seal - replace if leaking.

 

[Blue Ox]

I'd also suggest giving it a cleaning. You can use a spray-on engine cleaner and a garden hose. Leaking fluid migrates all over and makes it impossible to tell where it's coming from.

It's also preferable to clean it before you do the service work Nick recommend so you don't get grit and crud in it while you have things opened up.

 

[KDizzle]

Hoping I’m not breaking etiquette here, by hijacking this thread. I have a small leak around the shifter of a TH700 (86 K5, 5.7L). When you say selector shaft seal, is that the location you are referring to? I cleaned it off some before taking pic.

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Also, I found this in my transmission pan. Any idea what it may be?

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Prior to changing fluid/filter, I’ve noticed shifting to PRND is more difficult.

I’d appreciate any assistance!

 

[squaredeal91]

Lube locker makes really good pan gaskets. Maybe replace trans mount too.

 

[Matt69olds]

The seal is called the manual shaft seal. Some people call it the selector shaft seal. Go to your nearest transmission shop, chances are they will have one. There is a cheap tool you can buy to replace the seal without removing the transmission pan, otherwise you need to drain the fluid and disassemble a few things. There are lots of YouTube videos that will explain it far better than I could here. Just make sure you file down any rough edges where the linkage fits over the shaft, if not the shape edges can cut the seal and cause a leak worse than what your trying to fix.

No clue what that might be in the pan.

 

[KDizzle]

Already have pan/filter/fluid removed. I’ll check out this tool, YouTube videos and disassembly options. As for the seal, looks like I can pick one up at Summit. Thankfully, it’s close by.

Hoping item in pan is something insignificant…

Thank you for the reply!

 

[NickTransmissions]

Already have pan/filter/fluid removed. I’ll check out this tool, YouTube videos and disassembly options. As for the seal, looks like I can pick one up at Summit. Thankfully, it’s close by.

Hoping item in pan is something insignificant…

Thank you for the reply!

You dont need the tool if the pan is already off. Just take loose the 15mm selector shaft to rooster comb nut with a 15mm wrench, tap the rooster comb towards the valve body, pry the band style spacer off the selector shaft with a flat blade screw driver then withdraw the shaft. Then remove the seal with a small pry bar. Install the new seal with a hammer and 15mm deep socket (9/16 will also work). Make sure it's as square in the bore as possible before you pound it in or you'll bend/break it.

 

[KDizzle]

Awesome! I’ll try and “knock that out today or tomorrow”…Sorry, couldn’t resist. I’ll be sure to post pics for follow up.

Thank you!

Any idea on pic with item found in pan??

 

[83Stepper]

Hi! I’m new to the forum and to transmission maintenance, but a long time GM owner with good analytical skills. I need some help identifying the source of a transmission leak in my 86 C10.

A few months back, the power steering went out. Around that time I also noticed a good sized pool of transmission fluid under the truck. I don’t drive it much, but I keep it on a trickle charger and have taken it on short distance drives since then.

Recently, it’s started having trouble shifting into reverse, and only reverse. I did some googling and it seems likely that this is due to low fluid, which isn’t surprising given the puddle from earlier.

I took it to a store a few miles from home recently and it struggled to shift from a low gear to a higher gear in drive once in the way home.

So, now I’m looking to fix it and want to try some things on my own to build my skills first. I’m planning to change the fluid and filter since that seal is probably needing to be replaced, but I got under it today and got some pics of the pan and noticed fluid in some other places that may need attention. I’d love to know what I’m looking at and if I should plan to do anything more than the fluid and filter while I’m at it. Looks like a new pan might be a good idea, too, for example.

Transmission fluid, like a lot of the other fluids in these old girls, have a tendency to migrate and make you think it's coming from somewhere, only to be not even remotely close at times. Like previously mentioned, hose her down with some cleaner or wipe it down thoroughly and then check for the origin of the leak. Could be multiple places, or just one. Instead of just throwing parts at it and process of elimination, wipe it clean and monitor it to trace it back to the root of the leak. Not going to overwhelm you with all the places it could be leaking from, we'll save that for another time lol.

 

[NickTransmissions]

Already have pan/filter/fluid removed. I’ll check out this tool, YouTube videos and disassembly options. As for the seal, looks like I can pick one up at Summit. Thankfully, it’s close by.

Hoping item in pan is something insignificant…

Thank you for the reply!

You need to either quote my post or mention me if you want a timely response as I don't get notifications for subsequent posts in any thread i post in (that's intentional).

Check the pawl facing end of your parking rod - that thing in the pan could be the riveted portion of the rod...If that's what it is, replace the parking rod assembly...It would explain your issue w/getting the thing to go into park, if so....