Suburban vibration issue

[PrairieDrifter]

Ok so I have a 1984 suburban with a 400 that I believe was swapped before I got it, to the best of my knowledge suburbans didnt have the 400 as an engine option before 85. But it has a 700r4 behind it. The problem is that it has a vibration only at really low rpms when it kicks into overdrive, I can't tell if it's the engine or if it's a u joint (or some other problem)The hubs Werent locked in and was in two wheel drive. It doesn't seem to make sense since its an extremely noticeable vibration that ONLY happens in overdrive at low rpm and that's the only time it happens any ideas?

 

[89Suburban]

It could be the converter lockup clutch slipping, I have that problem with mine so I disconnected it. When it shifts into OD at low rpms the clutch would chatter and slip and grab.

 

[bucket]

The problem could also be exaggerated by tall gearing. Do you know what RPM the shudder occurs?

And '79 or '80 was the last year for the 400 BTW.

 

[PrairieDrifter]

I don't have a tach In it right now but I'm guessing 1000 to 1200 about maybe a little Lower maybe a little higher Im not quite sure but. The parts sheet under the hood says it has 3.73 gears in it. There's no noise just a low vibration it's very noticeable though. I believe it has to be something in the trans with the overdrive. That's the only gear it does it.

Everytime I've went on alldata or some of the parts books my uncle has 84 didn't have a 6.6 400, but an 85 came with that engine option? Not sure.

 

[bucket]

Nope, no 400 option after '79 or '80. I know for a fact it was still available in '79, pretty sure about '80 too. But for '81 and every year after that, I'm positive there was no 400 option.

 

[bucket]

And your 400 with have a dipstick on the driver's side, correct?

 

[PrairieDrifter]

Yes the dipstick is on the drivers side. And the harmonic balancer tells me that it's a 400 just by looking at it

 

[bucket]

Yes the dipstick is on the drivers side. And the harmonic balancer tells me that it's a 400 just by looking at it

Driver's side dipstick was '79 and earlier.

Have you disconnected the lockup TC to see if the vibration goes away?

 

[PrairieDrifter]

I know that the 400 was swapped with the original motor. I'm completely sure that all data has a 400 as an engine choice after 84. Why? I have no idea. And yes the 400 was discontinued in 81.

I haven't been able to do anything with the burb this past week.. 30 below with the wind in ND...too cold to work outside haha. Once it starts getting above zero I'll unplug the lockup connector and see what happens. This is the first chevy I've had with a 700r4. Not the most knowledgeable on them