ts4tomh

1989 F-150 5.0 4wd 5-spd. This has happened twice now, once at about 20mph, and the second time at 55mph. In 2wd both times, constant speed. For the blink of an eye, it felt like the truck had been grabbed by an anchor chain, then let go. The second time, I saw the speedometer waggle. No unusual noise before or after.

I thought it felt like something broke in the rear axle, like a ring gear bolt had backed out, or a roller bearing had worn and thrown a roller out. Inside the differential: clean. Wheel bearings and seals fine, but replaced bearings and seals anyway.

I did find a broken spring in the brake assembly on one rear wheel: the smaller spring with the long hook that runs from the center of one of the pads up to the alignment pin. The other, heavier springs were fine, and all the springs on the other side were fine.

Transmission was low on fluid, but no big chunks, just fine filings on the magnet. Going to check the transfer case in the morning.

Any ideas? This has me stumped so far.

And it isn't even my truck, but it's my problem.

Chris_1

iTrader: (1)

Brake spring is a suspect especially if it was able to jam up between the shoes and drum. Or could it possibly be the motor hicupping - from water in the gas or a connection ? Considering what you've already checked, that's all I've got.

ts4tomh

Thanks, Chris, for the quick reply.

The spring was broken near the alignment pin, into only two pieces, with a small "u" still in place around the pin, and the longer part of the hook still connected to the spring, which was still in place at the other end. The loose tail had been rubbing against the brake drum, with a noticeable rotary scrape near the outside edge of the drum... but not a mark of any kind on either the pads or the contact surface of the drum.

It's hard to describe the events any better than "instantaneous". The motor never seemed to hiccup at all, and the feel of it was that the back end had been grabbed, then let go, before I could even blink. When the second event occurred, I had just glanced down at the speedometer. It "bounced" down about 10 miles an hour, but rebounded back to 55 just as quickly -- not a matter of accelerating.

I've already gone after the motor, replacing the coil and control module after the truck stalled and would not restart a few weeks ago. There, the recommended test of putting a test light across battery + and coil - didn't work: I saw the test light blink indicating control module function, but no spark, so I replaced the coil. Still no go, so replaced the module and it fired right up. And it's hard to imagine how the sheer intensity of the event could be caused by bad fuel, and yet recover completely in the blink of an eye, with no change in the sound of the motor at all.

I remain perplexed. I will post what I find today.

Chris_1

iTrader: (1)

Just for the heck of it, because it's dirt cheap try a couple ounces of methyl hydrate (or gas line anti freeze) in the tank. It won't hurt anything and on the off-chance there's moisture in the gas the m/h will absorb it and burn it off.
Other than that, for something to grab hard enough to cause a noticeable jerk on a 4,000 lb truck moving at 55 mph, you'd think there would be pieces scattered down the highway. Or it should at least have made a mark somewhere ? I'm reaching here but I haven't heard of that happening before and I can understand your concern that whatever it is might grab and not let go.
Most of the things that could possibly cause that would stay broke you would think.
That's why I'm thinking electrical or fuel because they can work, not work and work again in the blink of an eye.
Or maybe whatever it is will start to howl or grind and then you'll be able to pinpoint it

ts4tomh

Chris, I'll try it. Having experienced such things in the past -- water in fuel, intermittent electrical issues, etc. -- 45+ years of driving and maintaining my own vehicles, I can only say: "This doesn't match any of that."

But I'll try it, of course. As you said: cheap and easy to try.

However, since these two events were about 250 miles apart, it sure would be nice to discover an affirmative broken thing, rather than just have it stop with no definitive solution.

I'll post the results as soon as I can get to the end of it.

Thanks, again.

Manuellabour247

You mentioned rear axle, did you check the front since it's 4WD? Just because it's not engaged doesn't mean that it can't be broke. Also check the Transfer case level. Those would be my ideas if you haven't tried them already.

ts4tomh

Transfer case and front axle are one today's list. I will post what I find.

ts4tomh

OK, this seems to be the end of it:

Fluids low in transfer case, transmission, and rear axle, but no metal in any of the them. Bearings on rear axle pinion shaft were loose enough to allow a wobble in universal joint coupling, but examination found no unusual wear patterns, and re-torquing of bearings with a new seal and new compression sleeve & pinion nut was successful.

Motor fuel fine, and with thanks for the suggestions regarding fuel and electrical problem causing a momentary hiccup, those could not present the absolute chest-into-the-dashboard braking event we experienced on two occasions. This wasn't loss of power with resultant compression braking, this was serious lock-up, albeit brief.

In the end, I'm left with only one thing that could have caused this strange pair of events: that broken secondary brake show retracting spring (the one that runs between the cable guide and the top anchor pin. There was no metal missing from the spring (the two pieces "added up" to the right length). There were no marks on either the drum braking surface or the brake pads: all smooth as they should be. But... the spring was broken. Somehow, that must have permitted the brake pads to chatter into position and bind, letting go a moment later.

At least it gave me an excuse to go through my friend's truck, and fix lots of annoying things, from the spare tire mount to the loose pinion thrust bearings, to the pinched and severed wire in the wiring around the transmission. Worth every nickel, in gratitude for the loan of his vehicle.

So, thanks for reading and offering suggestions, all. I really appreciate it.

ts4tomh

No re-occurrence of the violent, split-second lockups, now several weeks running. Broken secondary brake shoe spring was definitely the problem.

Thanks again for the advice and commiseration.