Clunk when slowing and about to stop

[mnwolftrack]

Hello--I've got a '94 that seems to have a bit of a "clunk" when braking. It happens just before I am about to stop, say about 5mph. Just one clunk, like another vehicle bumped the rear end. It doesn't matter how fast I was going beforehand. And despite stepping on the brake pedal gently and maintaining equal pressure, it still seems like there's a clunk that happens just before the vehicle stops. This triggers the ABS light consistently, after X number of occurrences. I don't seem to sense the opposite "clunk" when I take off from a dead stop again (driving forward) but can't rule it out.

Up until then, I couldn't quite feel the clunk (it almost feels like getting rear-ended gently) because there had been slight warpage to the original rotors (so I replaced them all, with new pads too). I've jacked the vehicle up and looked for the obvious but didn't see anything. Wheel bearings are nice and tight all the way around. Pads and rotors were just replaced all the way around. The ABS code points to the sensor under the center console (decel. sensor). I can clear the code, and after about a day of driving to work and back, the light comes on again consistently on my drive home. No other codes. I can almost predict when it will happen. While somewhat rusty underneath, it really isn't that bad. Nothing's rusted or broken off. My only thoughts are a sticky rear brake proportioning valve that "finally" kicks in just before I stop, or, a drive shaft slip yoke that is stiff, maybe a hard "downshift" into 1st by the trans, or loose suspension bushings. Either way, the issue isn't obvious. I've had the front drive shaft off, but not the rear.

Any ideas? It's a strange one!

 

[Bimrunner]

Have you greased your driveshafts? Sometimes they can bind and it creates a bumping as it binds and then releases giving that sound and feeling especially going into or out of first gear. Just something to check.

 

[mnwolftrack]

That's one of the things I was wondering. I was originally going to post this as "yet another ABS issue" but figured I'd focus on the clunk first. I had the front driveshaft off already (replaced bad u-joint) but not the rear. I will inspect the rear driveshaft this weekend by taking it off and manually checking the splines and regreasing while it's apart. I just did a search on "clunk" and found a couple threads regarding the very issue of sticking driveshift slip yokes. I'm hoping this is it! But nobody seemed to mention it triggering the ABS light though. I did find one other thread that maybe had similar symptoms, and that was in regards to someone saying the truck rolled a bit too far after putting in park. E.g. a vehicle will move "slightly" even after putting in park, before it actually stops and stays parked. But mine seems to go a little bit more than I would expect.

 

[Brentbba]

Check all 6 nuts/bolts on your front control arms too. One loose will cause a clunk.

 

[Twinair]

Check the brake caliper bolts. They could be loose causing binding when brakes are applied, enough to cause the ABS to cycle. For some reason these bolts backing out appears to be a common issue.

 

[mnwolftrack]

Thanks--I do know that the caliper bolts are ok. I had just replaced all pads/rotors and they were all torqued again properly. I was actually afraid I was going to snap the caliper bolts during removal. They were on really well (rust) and figured I'd either snap one or round them off. I went so far as to go to the store to get a new 6-point socket to avoid rounding them. I cleaned up the bolts and used anti-sieze to prevent that from happening again. The clunk was happening before and after the brakes were done (I replaced the brakes, in part, to see if this would help fix the issue).

 

[Twinair]

Could be driveline slop and the clunk is when the transmission downshifts to 1st while coming to a stop. Check axle and driveshaft spines and U joints. Grease driveshafts to see if it resolves the problem. I believe someone mentioned before that the slip joint may be binding

 

[Alleycats]

I second the downshift into first (clunck) when coming to a stop theory. Which is normal although maybe loosening kick-down cable would lessen it's severity (the clunk that is).

 

[FZJ80 in KC]

I had this too. Just replaced all of my u-joints and gone. Thought it was in my front driveshaft, but my rear was actually the biggest culprit.

 

[mnwolftrack]

Some excellent suggestions here. I will start by checking the rear driveshaft slip yoke and throttle kickdown cable. I just read a FAQ on this site about the kickdown/throttle cable adjustment and some shifting symptoms also appear to be (mildly) present. Front axle shaft splines (outer) looked ok when I had the hubs off to replace the rotors. I did not pull the caps to check the rear axle shafts. I may have a faint clicking birfield too, but it doesn't do it all the time (sharp slow speed turns).

 

[mnwolftrack]

I had to go for another drive this afternoon, and it dawned on me that I should try shifting into neutral before braking. Sure enough, stopping was smoother and no longer had the clunk. So, ,I think that eliminates suspension issues. Whether I was going 10 mph or 55, the clunk was gone. Shifting back into gear, even while completely stopped with my foot on the brake, the shift into D seems to cause it's own clunk and the vehicle rocks just a hair. I don''t seem to actually lurch forward at all, but the vehicle shifts its stance (if that makes sense).

 

[CruiseOrlando]

You can also pull your rear DS - assuming you have the 7-pin mod - you can lock the center and (with the front shaft in) run off the front shaft. No more clunk = u-joints in rear shaft.

 

[Alleycats]

A certain amount of clunking is normal FYI.

 

[blunderbuss]

Lubricate your drive shafts.

 

[OffGridOverland]

Have you checked your motor mounts?

 

[den]

Sway bar rubbers often wear.. I replaced mine and it fixed my clunk

 

[scrowley]

After greasing shafts including slip joints. Have trans serviced and kick down cable properly adjusted. Imo.

 

[crawlin40]

I vote check DS too. I had a bad u-joint with that sound.

 

[wearsabrowncoat]

Depends on the sound of the clunk. If not driveshaft I say check wheel bearings.

 

[mnwolftrack]

I'm going to start a new thread, as the focus of this has shifted enough. But here is where it has gone to:

Clunk fixed. ABS light NOT fixed.

Further down, I explain/ask if the "IG" wire on the decel. sensor is supposed to have ~12V when the ignition is turned on. I get 0V, nada ,not even OHM's to a ground/short.

Here's some background on how I got to the above questioning of the wire "IG."

Wheel bearings are good and were checked during a recent brake job. U-joints are also all good, just checked today (I did replace a U-joint about 12 months ago). Driveshaft slipshafts were also nice and loose. I took them apart anyway, cleaned them, added new grease, and greased all the U-joints. While driving, letting on/off the gas pedal (quickly) does NOT result in any clunking (which would have indicated U-joints).

I think what made the difference, fixing the clunk, was adjusting the throttle cable to the trans. It was at about 1mm of spacing, and I changed it to almost 2mm, and now it's much better, and that clunking I was talking about, just before I was about to stop, is basically gone. I think the clunk WAS INDEED timed with the trans downshifting from 2nd to 1st, and it was a bit harder than it should have been. But keep in mind, it was never that bad anyway. There appears to have been 2 problems happening at about the same time.

So, I've fixed the clunk, however, my ABS light still comes on (takes several stops before it comes on). It appears to be roughly timed to the # of stops I make, say ball park about 20 times, after I've reset/cleared the ABS light. So after about 20 stops, the ABS light comes back on, DESPITE THE CLUNK BEING FIXED. I have cleared the light several times today and ran several tests to see if I could prevent the light from coming on, and nothing helped.

I even went so far as to shift into neutral, before touching the brakes to slow down, then waiting to lift my foot off the brake again before putting back into D and accelerating, and that didn't help either. Meaning, the trans was never in gear while I was using the brakes, therefore eliminating the driveline from causing "clunks" that the ABS might have been picking up. Before adjusting the throttle cable, shifting into neutral was how I made the "clunk" go away when stopping.

So next, I tried to do some wiring troubleshooting. I seemingly have the FSM, but I don't know if there's a page that explains the decel. sensor (under shifter console) wiring or not. There's a black/white wire on the decel. sensor plug (vehicle side) with silver spots that the FSM claims is "IG" which I take to be ignition. At the sensor side of this plug junction, the wire color changes to red. If I run a voltmeter on this wire, I do not get voltage at all (meaning not even when the ignition is on). So, I'm now wondering if THAT is the culprit. My ABS code is 43, every time. There are a total of 5 wires on the decel. sensor (it looks like there should be 6, but one position is unused at the #5 position). The white wire with black line is ground and it does measure to ground properly. There's also a GS1, GS2, and GST. GS1 and GS2 seem to have around 7-8V with ignition on. I don't know what GST is supposed to have--the FSM doesn't seem to clarify what it is.

So, the question becomes, should that "IG" wire on the del. sensor harness have ~12V when the ignition is on?