I6 3spd Transmission Mount
 

Hello,

I have been investigating a vibration under cab when exceeding 50mph. Have replaced wheels/tires and it made no difference. Took a look under the truck and found that the transmission did not have a crossmember/mount. Is that normal for an I6/3spd?

Thanks in advance.

Re: I6 3spd Transmission Mount
 

yes. mine is the same way. seems odd that the tranny is cantilevered off the back of the motor. i was thinking of fabing one myself. Fatigue wise, that cant be good for the motor mounts or bellhousing/tranny and bellhousing/engine interface.

Re: I6 3spd Transmission Mount
 

I have a 6cylinder 3 speed combo and I have a crossmember although it's almost rusted in half!:lol:

Re: I6 3spd Transmission Mount
 

Thanks for the replies.

Does anyone have a picture of a stock or fabricated crossmember/mount they have made?

If anyone has recently purchased one, please let me know what you got.

Re: I6 3spd Transmission Mount
 

the 6cyls manual trans trucks that I have had and have all had the two front motor mounts and the rear mounts were on the bellhousing. The trans is just left hanging there. The vibration you might what to check could be the driveshaft (u-joints or hanger bearing).

Re: I6 3spd Transmission Mount
 

do you have a 2 peice drive shaft? If so you might look at your carrier bearing.

I have a 3 speed w/overdrive (longer), with only the crossmember on the bell housing, with no issues.

You might also want to check to make sure all your bell housing bolts are tight and rivets on the cossmember are not broke.

Re: I6 3spd Transmission Mount
 

No need in second guessing GMs engineering. No matter that it looks crazy, the trans hanging off there is sturdy enough that you don't need to supplement it with other mounts. Those engineers were pretty smart.

You think it looks weird having a 3 speed hanging in mid air, you should see a granny 4 speed truck. That's a lot of iron.

Re: I6 3spd Transmission Mount
 

If you use your truck like a work truck and/or off-road, a crossmember on the back of the 6 cylinder 3 speed may cause your bellhousing or transmission to crack. This is because of the length of the engine to the rear of the transmission and the twisting or flexing of the drive train or frame when going over uneven terrain. The rigidity of the three mounting areas prevent enough flexing.

Re: I6 3spd Transmission Mount
 

My truck has a 1 piece driveshaft. U-joints seem tight ( No clunking when shifting into reverse ). I will check that the bell housing bolts are tight and rivets on the crossmember are not broke.

Thanks for the info/advise.

Re: I6 3spd Transmission Mount
 

Another thing to check is your harmonic balancer. Mine slipped the outer ring and it had a vibration at certain rpm. Of course, it was impossible to get the timing right too.

Re: I6 3spd Transmission Mount
 

I agree.

I forgot to mention that I have a slight vibration on mine at about the same speed. I know it's an out of balance driveshaft since I've had tires rebalanced, replaced u-joints, yoke asm, driveshaft bearings and checked mounts, etc.

The drive-line shop said about $50 to balance it a few years ago. I just haven't gotten around to doing it.

Re: I6 3spd Transmission Mount
 

Try spinning the the rear u-joint 180* in the rear yoke. It worked for me after i had mine out and put it back in it vibrated so I figured i didn't put the u-joint back in the same way it came out.

Re: I6 3spd Transmission Mount
 

I'm cheap. I always put a big hose clamp around the driveshaft and rotate it a bit at a time until the vibration goes away. It takes a few test drives and crawling under the truck to get it clocked right, but free is free.

Re: I6 3spd Transmission Mount
 

^^^^ yep, you could do that too. Fine tuneing LOL

Re: I6 3spd Transmission Mount
 

Low Dough, Tx Firefighter... I haven't tried the 180 turn on my 68 but I did use the large hose clamp on my 69. Worked pretty good and might be worth a try.

Re: I6 3spd Transmission Mount
 

I'm not sure I'd notice the vibrations in my truck with the mirrors whistling, 4-speed howling, wind coming in the cowl, back bed clunking, metal dash and doors etc.:lol:

I don't think these trucks were meant to be quiet.:gmc2:

Re: I6 3spd Transmission Mount
 

Thanks for all of the great suggestions.

I am definately going to try spinning the the rear u-joint 180* in the rear yoke and the large hose clamp if necessary.

Will report back with results.

Re: I6 3spd Transmission Mount
 

as far as the clocking u joints go. you want to make sure that the grease fittings are clocked 180* out. if you have both grease fittings on the same side this will cause a vibration.

Re: I6 3spd Transmission Mount
 

Well, I found a very loose bolt on the drivers side bellhousing/trans mount. I tightened it up and the problem is slightly better.

Rotated the rear u-joint 180 which did not seem to make a difference.

While driving it to work this morning, the problem seems better in the cab but the same towards the rear.

I plan to inspect the adjustable panhard bar and maybe try the hose clamp to balance out the driveshaft.

TX FIREFIGHTER - do you have any other instructions/details on the hose clamp fix?

Re: I6 3spd Transmission Mount
 

Re: hose clamp... I used two of them on my old 69 for added weight. It's an install and try it technique. I kept turning mine in 1/4 turn increments until it got better.

You have to make sure that you have isolated it to an out of balance driveshaft. And even then, it may be bad enough that the clamps may not help...

Re: I6 3spd Transmission Mount
 

I tightened up the panhard bar at the frame and axle. They were pretty loose.

It seems to have made the vibration worse but I believe the vibration has been isolated to the driveshaft. The transmission and rearend don't seem to be bouncing/rumbling around.

Something strange happened this morning on my way to work, the headlights in my rear view mirror looked like small white ovals/circles instead of a solid light, at speeds above 50mph.

I think it is time to try the hose clamps to balance the driveshaft.

Re: I6 3spd Transmission Mount
 

Wanted to follow up thinking it may help someone in the future.

I ended taking my single piece driveshaft to a joint and clutch shop ( one recommended by Longhornman - thanks Andy ). They installed new lifetime u-joints, straightened and balanced the driveshaft.

It turned out being the best $98.50 I ever spent on my truck. It is driving great. My original problem started at 50mph and the vibration was terrible.

I would say doing driveshaft maintenance, tightening up some loose bellhousing/trans mounts and tightening up the panhard bar has eliminated 95% of the problem. There is still a tolerable vibration/shimmie from 50-55 mph but outside of those speeds it rides like a new truck.

Thanks to all for the help.