Deceleration dive...

[Rescue 1]

Okay got a question for you driveline guys. We have a 1972 Chevy 3/4 ton 4X4, with 14,000 original miles.

Here is the problem, while driving the truck under acceleration it goes straight down the road, however as soon as you lift off the fuel pedel it dives to the right. I don't mean a small pull, I mean it heads for the ditch.

We have checked the following, both front tires, both front wheel bearings, rear brakes, front brakes, steering gear box( tight), pitman arm(tight), tie rod and ends, ball joints (upper and lower), motor mounts, rear axles(truck does have posi), front axles, locking hubs, etc. Everything that I can think of to check that has anything to do with the steering.

Any help or guidance is greatly appreciated. BTW this is not a new problem this truck has done this for awhile(several years). Considering it is a firetruck it doesn't have many miles so I don't see how many things can be worn out. I realize that it is hard to diagnose a problem by my descriptions, however if you email me or post a question for me here I will do my best to answer it, Thanks.

[Bill72BB]

I had an old truck that did the same thing it was a bent tie rod.

Bill

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72 3/4ton Custom Camper BB,400 Dynamax Cermax headers , air,ps,pb,leafsprings
tilt,3.54 . 82 chevy silverado sb ,ps,pb pw,pd,air 350/350 Flowmaster 40.
Torrance Calif.

" If I knew I was going to live this long I would have taken better care of myself "

[ddsmith]

Just a guess. Have you checked the brake hose going to that brake. I've had one in the past look perfectly good on the outside but it would not allow the fluid under pressure to return to the master cylinder. Basically would apply small amount of brake all the time. This is probably not your problem but thought I would throw it out as a possibility.

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66 Chevy C30 Stakebody Dump, PS,PB, 327cu in
71 Corvette Coupe 454 4 speed
79 Chevy 4X4 (Tom's old truck)

[JIMs70GMC]

I think ddsmith is on to something, I had the same thing with an 85 blazer, caliper was seized up(not completely) jacked up the passenger side and could fell the resistance when spinning the tire.

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1970 GMC K25 Custom
Chesapeake, VA
My 70 GMCor Webshots http://community.webshots.com/user/70gmc

[tom hand]

You seemed to have checked the front end parts but have you checked the rear end parts? A lose rear end,bad trailing arm bushings or leaf springs,or bad panhard rod bushings can "steer" the truck places you don't want to go. The way you describe the problem it is not the brakes so it is the front end or the rear. If you can not find the problem take it to get The front end alinened,and if it is a desent shop they will not line it up,they will take you out in the shop and show you why they can't aline it>[your worn out part] Then after you fix the problem that your truck back there to get it lined-up to pay them back for showing you where the problem is.

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72 GMC Sierra SWB almost finished---- 84 Softail
Olive Branch MS

[Longhorn Man]

My old Longhorn had one leaf busted on the right rear pack. The thing would dart to the right every time I hit a big bump, and if it became airbore, even just a little, it would be visable from behind.

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'69 G.M.C. 350/350. Trying to clean up the left over damage from the Dope-Smokin-Old-Man
I've been dubbed the Longhorn Freak/Fanatic/Expert, I just hope I can live up to it.
FINALLY got the HORNIAC...a '70 one ton Longhorn with a Pontiac 350/350 and lots of 'personality'. Check out The Longhorn Webite.
If you need a pic posted, E-mail me at longhornmail@yahoo.com
Andy, in Columbus Ohio

[truckgoddess]

I would say to check all the obvious including a front end allignment and have the toe/in checked. The truck would go stright under pressure of acceleration but will pull when you let it off!

[Scrub]

I had a Detroit Locker in mine and it had some cold cap tires that where pretty far off as far as being the same diameter. The day I drove it home from having the locker put in, I had to find a dirt road home. That sucker did just as you describe. It literally dove off to one side when I hit the gas and slammed to the other when I let off.
You sure yo don't have a locker? A firetruck might have had a locker.

Good luck

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Scrub
70 GMC 1,000,000 + miles
72 K-20 project, 456 Dana60 front, Corp14 rear w/locker, 265R19.5 tires 20-ply. Warn 12k winches both ends, Cross-over steering with raised tie-rod, Powerbox steering, 4500 watt 120-AC power, Air, Hydraulic aux power, 4 inch lift, 5000 lb air-bags both ends.

[Richard]

What comes to mind right away is either a sticking caliper or locking hub, but you said these were checked. I have seen this condition under braking caused by old rubber brakes lines. One will expand more than the other reducing pressure at one wheel. Also other things to check would be spring bushings, broken leafs, shocks, toe, tire pressure. Assuming stock leafs check for degree shims at front axle that would change caster dramatically.

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Richard
1972 K10 Custom Deluxe SWB Fleetside

[Rescue 1]

Rechecked everything today. No problems with brakes, tires or broken leaves. Front end is all tight. No problems with the steering gear box. After driving it almost thirty miles today, it almost feels like the back of the truck is pushing one way or the other. Kinda hard to describe. Rechecked all the u-bolts. Truck does not pull when going down the road under power or coasting. Truck does not pull during braking either. It is weird, when you lift off the gas it immediately dives right, when you step back on it, it feels like it pulls again.

I am considering having them replace the springs(with the extra leaves)with aftermarket 2" lift springs. It's not that we really want the lift but we have 32" tall tires that we don't want to have to change. I thought it might eliminate the problem. Besides it would give a much better ride plus new poly bushings in the springs. Any thoughts?

[Scrub]

Try swapping the rear tires from one side to the other and see if that changes the dive to the other side. You said it had a posi rear. What happens if the posi doesn't have posi fluid?

I still think the rear end is locking up like a detriot and your rear tires are not the same diameter. THat will cause you to dive one way under power and the other when you let off the gas. Switching the tires should make it go the other way. It could be as simple as tire pressure being different too.

good luck

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Scrub
70 GMC 1,000,000 + miles
72 K-20 project, 456 Dana60 front, Corp14 rear w/locker, 265R19.5 tires 20-ply. Warn 12k winches both ends, Cross-over steering with raised tie-rod, Powerbox steering, 4500 watt 120-AC power, Air, Hydraulic aux power, 4 inch lift, 5000 lb air-bags both ends.

[Scrub]

I have another idea. The rear might be moving front to back some. I thinking that you could jack up the front end so you could roll the front sideways on a floor jack. Have someone sit in the seat and hold the brakes on hard. Have another one of your fire buddies move the front of the truck back and forth. THat should be really hard to do as the back axle should be holding the rear end tight. If there is any play in the rearend mounts, the play sould show up.

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Scrub
70 GMC 1,000,000 + miles
72 K-20 project, 456 Dana60 front, Corp14 rear w/locker, 265R19.5 tires 20-ply. Warn 12k winches both ends, Cross-over steering with raised tie-rod, Powerbox steering, 4500 watt 120-AC power, Air, Hydraulic aux power, 4 inch lift, 5000 lb air-bags both ends.

[Rescue 1]

Okay I'll try the jack thing. I'll also try the tire swap. We have checked the tire pressures so I know they are okay. I may try to swap the tires front to rear just to see if anything changes. Thanks.

[Rescue 1]

Okay here is the latest...

Pulled the rear diff cover half to change the fluid and half curiousity to see if I had any spare parts. No spare parts.

Here is what I found, yes it has a locker (we knew that), yes it seems to work okay. However if I rotate the right rear tire it unlocks and turns free from the driveshaft. If I try to turn the left rear it turns the driveshaft. Better explanation..it's like the right side unlocks but the left side doesn't. If I move the left tire back and forth once in awhile I can get it to unlock, however it locks back up when I turn it the other way. The right side unlocks right away but the left side doesn't. Make sense?

I pulled to cover to change the fluid to be sure it had the "additive" for a locking diff. The fluid that I drained out didn't look bad, but how bad can 90W look?

As the plot thickens...

[Scrub]

That sounds like a detroit locker.
if so, you can play with it and you will figure out that you can make either side unlock by unloading the opposite side. THey are really kind of wierd.

That or it's toast.

Still sounds like it is locking and pushing you to the side.

TW Scannell aka scrub