GM Machanic Help

[Skywalker]

I have the 1970 LT1 with the Original 1970 matching Turbo 400 and have had her now for about 3 months. Today, the 3rd time as my driveway is slanted and I always pull in so fluid from tranny runs to the rear. Today is the 3rd time the tranny pukes about a 1 ft wide and four foot long streamer down the driveway. I pull her up on the ramps and look to see it's coming from the yoke on the front of the driveshaft and out onto the universal joint! This is not a seal leak for sure. I now since I have two turbo 400's and seen many yokes when I was younger there is a small hole in that yoke and I have always assumed it was so trapped air could get out as you slide the driveshaft into the tranny. But that is where the fluid is coming from! So, my question is, who, what, when, where and why? I'm sure I could plug that little hole but I'm sure it was there from the factory so what gives? lease:

 

[O Holy P]

Never seen a hole in a yoke, usually the air just gets pushed out the front through the play in the splines. I'd plug the hole with some silicon or some JB Weld and see if that fixes it. If not, then it has to be a drive shaft/yoke seal.

Another option would be to level your driveway.

 

[expat1970]

its called a welsh plug,you need to take driveshaft off and have a new yoke put on,best fix.hope this helps

 

[Skywalker]

I did pull in backward this time until I get this fixed.

Never seen a hole in a yoke, usually the air just gets pushed out the front through the play in the splines. I'd plug the hole with some silicon or some JB Weld and see if that fixes it. If not, then it has to be a drive shaft/yoke seal.

Another option would be to level your driveway.

 

[Mr Bogus]

its called a welsh plug,you need to take driveshaft off and have a new yoke put on,best fix.hope this helps

old timer... welsh plug.. (glad I am not alone)

you can always just check the price on a fresh yoke... we never changed out the plugs at the dealer... just throw new stuff at them.. I would suggest you check the tail shaft housing bushing and seal while you are there..

 

[Skywalker]

OK, never heard of a welsh plug and I'm googling just to see what it is with no luck. My Z28 also has a turbo 400 and yoke has the small hole as well and it does not leak, what is the difference since I don't know of any plug within either of these two Turbo 400's? ALso, I have noticed that the modulator valve that of course controls shift points on the one leaking has stoped shifting out of low on its on when you run the RPM's up high. Sounds like I need a new modulator valve and I have already checked the hose and cut the end and it's tight with no leaks.... if that modulator is bad and it must be since she stopped shifting out lf low on her own, could that be building up pressure and pushing out fluid as well? ???

its called a welsh plug,you need to take driveshaft off and have a new yoke put on,best fix.hope this helps

 

[lavinrac]

Welsh(welch) plugs are the same as expansion plugs here in the states. just a different name. FYI

old timer... welsh plug.. (glad I am not alone)

and just think im not that old.

 

[Skywalker]

So, are we saying that there is to be an expansion plug that goes in that small hole in the front driveshaft yoke? The parts stores here has not clue unless this is only in a new yoke?

Welsh(welch) plugs are the same as expansion plugs here in the states. just a different name. FYI

 

[O Holy P]

just tack weld it shut then grind smooth

 

[blusmbl]

There isn't a welsh/freeze plug to fill a tiny, drilled hole like you're describing. My old Mopars have never had a hole like you describe. Even with that present, you have to have a pretty severe driveway angle to get fluid to leak out of the yoke. That doesn't seem normal to me, at all.

Try parking so that the hole in the yoke is always upright. May require an inspection mirror or hole in the floorpan.

I'd second the vote for welding it shut.

 

[Zoinks!]

old timer... welsh plug.. (glad I am not alone)

you can always just check the price on a fresh yoke... we never changed out the plugs at the dealer... just throw new stuff at them.. I would suggest you check the tail shaft housing bushing and seal while you are there..

+1. I've had some yokes leak before, but we always just replaced them.

OK, never heard of a welsh plug and I'm googling just to see what it is with no luck. My Z28 also has a turbo 400 and yoke has the small hole as well and it does not leak, what is the difference since I don't know of any plug within either of these two Turbo 400's? ALso, I have noticed that the modulator valve that of course controls shift points on the one leaking has stoped shifting out of low on its on when you run the RPM's up high. Sounds like I need a new modulator valve and I have already checked the hose and cut the end and it's tight with no leaks.... if that modulator is bad and it must be since she stopped shifting out lf low on her own, could that be building up pressure and pushing out fluid as well? ???

This may sound like a dumb question, but have you added fluid back to trans since the described leaks happened? If not, then that's most likely why it stopped shifting. My advice at this point with the information given would be replace the yoke (and if it were me I'd go ahead and replace the tail shaft seal too), fill the fluid back up to normal, and see how she does.

There isn't a welsh/freeze plug to fill a tiny, drilled hole like you're describing. My old Mopars have never had a hole like you describe. Even with that present, you have to have a pretty severe driveway angle to get fluid to leak out of the yoke. That doesn't seem normal to me, at all.

Try parking so that the hole in the yoke is always upright. May require an inspection mirror or hole in the floorpan.

I'd second the vote for welding it shut.

A lot of GM yokes do have a tiny little hole in them. The hole is normal, the leak is not.

 

[Skywalker]

Well, driveway is pretty steep of an angle as they all are here in Colorado and I don't know why they do that. The fluid leak, I always immediately pull down to ground zero if you may where it level and put about a pint back in it to full level and never over fill! So, if the modulator is bad could that cause pressure inside that builds and then pukes out the tranny fluid. Remember, this is not a slow leak as once she goes it's like it puked and is about a 1 ft wide and runs about 4 ft long and it's so far had done this when I have not drove her for days every time she leaked. Is there any relations between a bad modulator and my issue at all?

+1. I've had some yokes leak before, but we always just replaced them.



This may sound like a dumb question, but have you added fluid back to trans since the described leaks happened? If not, then that's most likely why it stopped shifting. My advice at this point with the information given would be replace the yoke (and if it were me I'd go ahead and replace the tail shaft seal too), fill the fluid back up to normal, and see how she does.



A lot of GM yokes do have a tiny little hole in them. The hole is normal, the leak is not.

 

[Mr Bogus]

yokes with the hole have that hole for a reason..

as the rear end of the car goes up and down, the drive line changes length.. some yokes vent vacuum/pressure down the splines, others are going to vent out that pin hole.. I would discourage plugging that hole unless you know things are going to work out ok..

Without some venting, you load and unload the output shaft and that is not real good on thrust washers.. if you look inside some yokes, there is a seal inside the thing..

Skywalker, the vacuum modulator lets the transmission upshift at "high" vacuum and downshift at "low" vacuum.. (works as a load sensor).. at WOT or low vacuum conditions, the governor takes over duties based on output shaft speed.. for racing, both come out in favor of a manual valve body

 

[Skywalker]

Thanks Bogus, not sure I still follow what this happens and only occasionally though. So, if I go buy a brand new yoke there is something there that has been lost or work off of my old yoke and this will fix the problem. And would you agree that since the tranny no longer shift out of low on her own it must be broken and could that cause this thing to puke in my driveway by building up inside or internal pressure?

yokes with the hole have that hole for a reason..

as the rear end of the car goes up and down, the drive line changes length.. some yokes vent vacuum/pressure down the splines, others are going to vent out that pin hole.. I would discourage plugging that hole unless you know things are going to work out ok..

Without some venting, you load and unload the output shaft and that is not real good on thrust washers.. if you look inside some yokes, there is a seal inside the thing..

Skywalker, the vacuum modulator lets the transmission upshift at "high" vacuum and downshift at "low" vacuum.. (works as a load sensor).. at WOT or low vacuum conditions, the governor takes over duties based on output shaft speed.. for racing, both come out in favor of a manual valve body

 

[Mr Bogus]

hmmm you say it wont shift out of low? if you pull the shifter down into low and then upshift, does it go then?

back under the car..

check that the vacuum line is connected at the vacuum modulator (rubber hose to metal pipe).. then check it at the motor.. (usually a 2" chunk of rubber hose again going into a specific fitting at back of intake)

make sure car is on stands, start engine and pull the hose at back of transmission.. should have engine vacuum..

ALSO.. if there is ANY oil in that hose, replace the modulator (do not loose little pin either, it needs to go in new modulator)

Could be the modulator is the leak and the upshift problem.. but you gotta crawl under there and look..

 

[Skywalker]

She will shift out manually no prob! I have been under her which was about two months ago and cut the hose and put it back on but did not check for vacuume since everthing was working except the shift out on it's on thing! At taht time did not see fluid but obviously need to check again. Done the same thing at the rear of the Holley on the Intake and all was good. But, need to check again now that you are pointing it out. Pretty sure it will be good but will determine in just a few. I have a facuume gage and will check starting at the rear of intake. Thanks.

hmmm you say it wont shift out of low? if you pull the shifter down into low and then upshift, does it go then?

back under the car..

check that the vacuum line is connected at the vacuum modulator (rubber hose to metal pipe).. then check it at the motor.. (usually a 2" chunk of rubber hose again going into a specific fitting at back of intake)

make sure car is on stands, start engine and pull the hose at back of transmission.. should have engine vacuum..

ALSO.. if there is ANY oil in that hose, replace the modulator (do not loose little pin either, it needs to go in new modulator)

Could be the modulator is the leak and the upshift problem.. but you gotta crawl under there and look..

 

[VANNWRAY]

IT WAS A COMMON PROBLEM 20 YEARS AGO AND THE QUICK FIX WAS A PIECE OF CARDBOARD SILICONED IN AS A PLUG OR YOU CAN WELD IT UP . EITHER WAY WILL WORK AND THE TRANNY WILL VENT THROUGH THE CASE , NO WORRIES . I'VE SEEN IT DONE A MILLION TIMES .

 

[Zoinks!]

IT WAS A COMMON PROBLEM 20 YEARS AGO AND THE QUICK FIX WAS A PIECE OF CARDBOARD SILICONED IN AS A PLUG OR YOU CAN WELD IT UP . EITHER WAY WILL WORK AND THE TRANNY WILL VENT THROUGH THE CASE , NO WORRIES . I'VE SEEN IT DONE A MILLION TIMES .

Just because "it works" doesn't mean it's the proper fix. I've seen PLENTY of things that worked that I would never even consider doing on one of my own vehicles.

 

[VANNWRAY]

YOU CAN ALWAYS TAKE IT TO A TRANNY SHOP AND MAYBE THEY HAVE THE CORRECT GM CURE . JUST A THOUGHT .

Just because "it works" doesn't mean it's the proper fix. I've seen PLENTY of things that worked that I would never even consider doing on one of my own vehicles.

 

[blusmbl]

If oil can leak past the 6" of tailshaft/yoke splines to the weep hole at the end of the yoke and leave a quart of fluid on the driveway every time it's parked on an incline and the converter drains out, I'd be willing to wager air could do the same. I see no problem whatsoever eliminating the weep hole.

However, looks like there is plenty of fuss over this on multiple other forums. There's an additional seal on the tailshaft that is most likely bad:

Leaking Driveshaft Yoke

4L60E Tailshaft Sleeve

transmission fluid leak