FAD Failure

[AEsco48]

Here is a video of how it sounded in my jeep, also a diesel

https://www.jeepgladiatorforum.com/...und-any-ideas-video.67298/page-2#post-1107148

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[npgeorgeuw]

I get it. You have a vibration. You think it is the FAD. My perception is that the FAD is either on or off or broke. The truck should operate without vibration if it is on or off. If it is broke, you need to know that too.

What is causing the vibration? That would be my concern. An easy first thing to do would be to disconnect the front drive shaft and see if the vibration changes. Rotating the tires would be another thing to try.

I’m pretty certain that driveline vibrations aren’t uncommon when a FAD is deleted requiring balancing of the half shafts. It’s seems likely then that with a locked FAD a perfectly normal half shaft would also have vibrations at high speed. Whether or not it’s normal or actually outta spec I’m not so sure.

 

[Andy29847]

I’m pretty certain that driveline vibrations aren’t uncommon when a FAD is deleted requiring balancing of the half shafts. It’s seems likely then that with a locked FAD a perfectly normal half shaft would also have vibrations at high speed. Whether or not it’s normal or actually outta spec I’m not so sure.

I've never seen a "balanced" half shaft. Doesn't mean it doesn't happen, but I haven't seen that.

The FAD, in or out, should not create driveline vibrations. This is assuming a suspension and running gear properly designed, properly installed, and in good condition. Very few wheelers have FAD and almost all of them are driven on the highway.

The original poster here seems technically astute. His Gladiator is lifted more than normal (4"), is equipped with bigger than normal u-joints for the driveshaft (1350), and has intentionally been tuned good highway manners (6.5° of caster). Throw in the REV axles on the shelf in his shop and I thing we have identified someone who goes places most of us will not go. I expect that caster could be dialed back until the vibrations went away without ill effect.

 

[JTR178]

Thanks for the answer. I'm still running at stock height but I'm looking. I've read that the stock driveshaft should be replaced if you lift over 2" - 3" inches. Then I read where the actual lift many owners get from their kits exceeds the advertised lift. It sure confuses a guy.

Driveshaft replacement is based more off front suspension travel than lift height even though the two are closely related. There's a magic number, I think more than 28.5" of travel, requires an upgraded driveshaft. That's why Metal Cloak's 30" of travel on their 2.5" lift recommends a new drive shaft while most others 2.5" lifts don't.

 

[Andy29847]

Driveshaft replacement is based more off front suspension travel than lift height even though the two are closely related. There's a magic number, I think more than 28.5" of travel, requires an upgraded driveshaft. That's why Metal Cloak's 30" of travel on their 2.5" lift recommends a new drive shaft while most others 2.5" lifts don't.

I came from the TJ world. There were 2 reasons to change a drive shaft when lifting your Jeep. The first was length. The second was u-joint angles. Suspension travel affects both.

 

[rharr]

Has anyone checked to see if the drive shaft is out of phase? maybe the installer separated the front and back and then slide it back together with out thinking? maybe the shaft mfr had a bad day and his dog died and wasn't paying attention?

 

[npgeorgeuw]

I've never seen a "balanced" half shaft. Doesn't mean it doesn't happen, but I haven't seen that.

The FAD, in or out, should not create driveline vibrations. This is assuming a suspension and running gear properly designed, properly installed, and in good condition. Very few wheelers have FAD and almost all of them are driven on the highway.

The original poster here seems technically astute. His Gladiator is lifted more than normal (4"), is equipped with bigger than normal u-joints for the driveshaft (1350), and has intentionally been tuned good highway manners (6.5° of caster). Throw in the REV axles on the shelf in his shop and I thing we have identified someone who goes places most of us will not go. I expect that caster could be dialed back until the vibrations went away without ill effect.

I don’t disagree with you, I’m just saying that it’s not an uncommon thing with RCV half shaft with FAD delete kits. You can find the posts on here, that’s where I read them (or the JL forum, can’t remember) regardless, either their (rcv) QC sucks or it’s an issue in general with them at high speed and we just don’t see it that often since the FAD isn’t engaged the vast majority of the time? I’m certain the OP should get things looked at and that there are other variables at work, too. If his half shaft is unbalanced it would explain it based on what’s happened with other people with locked FADs at least occasionally.

 

[Andy29847]

I don’t disagree with you, I’m just saying that it’s not an uncommon thing with RCV half shaft with FAD delete kits. You can find the posts on here, that’s where I read them (or the JL forum, can’t remember) regardless, either their (rcv) QC sucks or it’s an issue in general with them at high speed and we just don’t see it that often since the FAD isn’t engaged the vast majority of the time? I’m certain the OP should get things looked at and that there are other variables at work, too. If his half shaft is unbalanced it would explain it based on what’s happened with other people with locked FADs at least occasionally.

If I read his posts correctly, he is not using the RCV half shafts at this time.

 

[Rusty PW]

I don’t disagree with you, I’m just saying that it’s not an uncommon thing with RCV half shaft with FAD delete kits. You can find the posts on here, that’s where I read them (or the JL forum, can’t remember) regardless, either their (rcv) QC sucks or it’s an issue in general with them at high speed and we just don’t see it that often since the FAD isn’t engaged the vast majority of the time? I’m certain the OP should get things looked at and that there are other variables at work, too. If his half shaft is unbalanced it would explain it based on what’s happened with other people with locked FADs at least occasionally.

I had RCV axles in my Power Wagon. Used to run it at over 100 mph with no vibration.

 

[npgeorgeuw]

I had RCV axles in my Power Wagon. Used to run it at over 100 mph with no vibration.

yeah so I’ve seen. Was there FAD delete involved?

 

[Rusty PW]

yeah so I’ve seen. Was there FAD delete involved?

No.

 

[Andy29847]

I don’t disagree with you, I’m just saying that it’s not an uncommon thing with RCV half shaft with FAD delete kits. You can find the posts on here, that’s where I read them (or the JL forum, can’t remember) regardless, either their (rcv) QC sucks or it’s an issue in general with them at high speed and we just don’t see it that often since the FAD isn’t engaged the vast majority of the time? I’m certain the OP should get things looked at and that there are other variables at work, too. If his half shaft is unbalanced it would explain it based on what’s happened with other people with locked FADs at least occasionally.

The OP seems to know where his vibration comes from. It is the pinion angle in relation to the driveshaft. When on the highway, The OP did not have any vibration before because the FAD was working (front axle disconnected) and the front driveshaft was not turning. Off-road vibration was a non-issue because of low speeds.

The OP has 2 acknowledged 2 possible contributors to driveline vibration. The biggest thing is that he has dialed in a lot of castor (6.25). The OP is also running bigger than normal u-joints.

Your reference to RCV axles is confusing because the OP is not running RCV axles at this time. He has a set of RCV axles, but they are on the shelf.

I'd like to point out that RCV axles are made for JL/JT Jeeps so that you can use the FAD or not use the FAD.

FAD delete kits make the axle the same as it has been for 50 years. FAD, if working properly, is not an issue with regards to vibration.

 

[Oak]

- had a long drive, noticed some driveline vibrations highway 60+ MPH 1.5-2k RPM (mostly around 1.5-1.6k -- this is a diesel 8th gear)
- shortly after arriving home got a service 4wd light on dash ( i thought this was due a failed repair of the FAD connector harness that broke on trail last yr)
- double checked FAD connect which seems intact
- when I jack up the vehicle can engage 4wd & lockers
- with vehicle jacked up when you push front tires forward it spins the front drive shaft.

My conclusion is FAD failure, likely actuator -- maybe coupler. I don't hear any grinding noise coming from the front axle so less likely think it's shattered.

I've got a big overland trip planned to leave in 2 days so no chance this will be fixed by then...plan is to just drive real slow on the highway to avoid the drivetrain vibrations. Hopefully it holds up....

Has this happened to anyone? Do you think the diagnosis makes sense?

Do you have a tazer? If so check to see how it set for front axle? Ask me how I know what it's Iike to drive in 3 wheel drive lol.

 

[jnack]

Update:

Very weird, I drove over 500 miles with the front driveshaft disconnected and meanwhile the service 4wd light went off. When I got to my destination for the trip I reconnected the front drive shaft and the FAD worked fine. Did over 400 miles off-road & no issues, drove home with driveshaft connected.

My conclusion is that it's electrical due to messed up connector @ the FAD. Replacing FAD actuator & connector from wiring harness to make this more reliable

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