Misterious Shimmy 45-55mph OD

[79Scott]

Hey guys,
My 86 Silverado has a shimmy between 45-55mph while in overdrive. Problem has been there for a while, front wheel bearings, tie rods and idler arm have been replaced. Seems odd to me that it does it in overdrive. Going to get the tires balanced in the next week or so. Thoughts on what it could be?

 

[Rickf]

If the shimmy goes away after 55mph, look at the driveshaft u-joints.

 

[HotRodPC]

Sure it's not a tire balance issue? And it does it in OD only? So if it's doing it, and you downshift to 3rd it goes away immediately? Do you notice or feel the converter lock up when it happens? I wonder if it's not the converter lock up clutch in the coverter going bad? You can unplug the converter lock up on the driver side of the transmission. If that solves the problem, then you know your lock up clutch in the converter went bad. Just leave it unplugged until you can replace the converter but be careful and mindful of the transmission temp since the converter locking up does keep the trans cooler.

 

[79Scott]

I've checked the u-joints and there isn't any play in them.
It does it in OD and as soon as I shift it to D it stops for the most part. It almost feels like motor is lacking power or has a stumble but its not the motor if that makes sense. I know the truck was set up for towing at one point (had a tranny cooler, receiver hitch and electric brake setup) so it could be the converter. Which by the way is a 700R4.

Thanks Guys

 

[79Scott]

Sure it's not a tire balance issue? And it does it in OD only? So if it's doing it, and you downshift to 3rd it goes away immediately? Do you notice or feel the converter lock up when it happens? I wonder if it's not the converter lock up clutch in the coverter going bad? You can unplug the converter lock up on the driver side of the transmission. If that solves the problem, then you know your lock up clutch in the converter went bad. Just leave it unplugged until you can replace the converter but be careful and mindful of the transmission temp since the converter locking up does keep the trans cooler.

Nailed it HotRod, uplugged it and took it for a ride last night in OD and it was fine. Thanks for the help guys!

 

[1987 GMC Jimmy]

I was thinking the same with the lockup, although the shimmy is front to back, not side to side, and there should be a distinct chattering sound. I’ve had the same problem with one for years. I put some of that Instant Shudder Fixx in the transmission, and it helped a lot. Mine was real bad so I used several tubes of it. I’d rather do that and wait for the transmission to give out than run it without lockup. The transmission is prone to getting too hot without that 1:1 ratio in OD. I say that, but the transmission is still going strong at 286K without being rebuilt yet, and I’ve had a new converter sitting in a box for around three years.

Also, checking for play in u joints like that has not served me well in the past. I’d hear clunking noises under the vehicle, but the driveshaft and joints were as tight as could be. Pulled them off, and the needle bearings were dry and machining themselves into the trunnion. Replaced and no more problems!

 

[HotRodPC]

Thanks for the comeback and update on the problem. It's helps everyone who searches for the same problem, even on google and brings in more members.

 

[JoeR Jr]

Nailed it HotRod, uplugged it and took it for a ride last night in OD and it was fine. Thanks for the help guys!

You need to check the secondary(high voltage part) ignition. It may just be a misfire. The hardest time for a spark to jump a gap is when you are in 4th gear with the converter locked. If a shake goes away when you unplug the converter it could be a miss.
It's time for you to check your plugs, wires, cap and rotor.
Joe

 

[79Scott]

You need to check the secondary(high voltage part) ignition. It may just be a misfire. The hardest time for a spark to jump a gap is when you are in 4th gear with the converter locked. If a shake goes away when you unplug the converter it could be a miss.
It's time for you to check your plugs, wires, cap and rotor.
Joe

Thanks.