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FAD Failure

jnack

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- 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?
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Jteakus

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Can you get it on 4 jack stands? If so put it in 4lo and watch the tires. Next engage lockers rear then front then disengage both. See what your tires do.
 

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I would think that if the FAD actuator failed, that it would stay disengaged. There is a spring that keeps the fork and coupler in the disengaged position without electrification. Then the actuator electrifies, it pushes the fork to the engaged position. It’s pretty easy to pull the FAD actuator off and see where the coupler is.
 

Andy29847

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It sounds like you believe that the FAD is stuck on the locked position. If so, that won't hurt anything but your gas mileage. I'd suggest taking off the FAD cover and looking inside. Here is a video I found that may shed some light:

 
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Advntrbound

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Dumb question but does a MT have a FAD?
 

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jnack

jnack

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Can you get it on 4 jack stands? If so put it in 4lo and watch the tires. Next engage lockers rear then front then disengage both. See what your tires do.
did this, that's how I tested it. seemed like everything worked as expected. with lockers on it seemed that tires were moving at same speed though hard to say 100%.

I would think that if the FAD actuator failed, that it would stay disengaged. There is a spring that keeps the fork and coupler in the disengaged position without electrification. Then the actuator electrifies, it pushes the fork to the engaged position. It’s pretty easy to pull the FAD actuator off and see where the coupler is.
hmm this is a good point about the spring. i had considered pulling the cover to take a look inside BUT at this point the main reason I haven't done that is in case something in there is broken that upon removing the cover it by chance prevents me from getting everything back in working order I'm not really ready to take that risk now since I leave for a big trip in under 36 hours with no chance of getting it fixed. I'd normally just pop it open if I didn't need my 4wd to be working

It sounds like you believe that the FAD is stuck on the locked position. If so, that won't hurt anything but your gas mileage. I'd suggest taking off the FAD cover and looking inside. Here is a video I found that may shed some light:

thanks for sharing that video, I actually watched it as well. in his case he heard a sort of pop which I didn't hear. though fingers crossed mine isn't all crushed up in there. I actually have RCV axle shafts ready to install similar to him but I'm going to try to get this FAD issue fixed under warranty before I put those in
 
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jnack

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It sounds like you believe that the FAD is stuck on the locked position. If so, that won't hurt anything but your gas mileage. I'd suggest taking off the FAD cover and looking inside. Here is a video I found that may shed some light:

i'd say the other thing that it could "hurt" is wear on driveline -- could wear out my driveline u-joints quicker with the harmonic vibration at highway speed so I'm going to try to just keep it real slow lol
 

IanNubbit

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Most likely a electrical failure, consider it lucky it failed ingaged. With the FAD engaged you should not have driveline vibrations, if you do, id be looking at front driveshaft issues
 
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jnack

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Most likely a electrical failure, consider it lucky it failed ingaged. With the FAD engaged you should not have driveline vibrations, if you do, id be looking at front driveshaft issues

should depend on driveline angles. i'm running 4" lift with about 6.5 caster. i was considering going solid axle shafts from RCV but when i spoke to them they said for higher lifts to expect driveline vibrations on the highway so I decided to go with the 2 piece axle shafts (haven't installed yet). maybe wouldn't be an issue closer to ride height or small lift like 2"?
 

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whats the diagnostic code the vehicle is throwing? It is possible it is related to an abs sensor. My truck had a bad abs wire which caused the 4wd service light to pop on. I used my scanner and it told me left rear speed sensor out of bounds. I switched it and everything cleared up.
 

piroman683

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also, are you still driving it around and still having vibrations after you got home and checked everything out?
 
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jnack

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whats the diagnostic code the vehicle is throwing? It is possible it is related to an abs sensor. My truck had a bad abs wire which caused the 4wd service light to pop on. I used my scanner and it told me left rear speed sensor out of bounds. I switched it and everything cleared up.
also, are you still driving it around and still having vibrations after you got home and checked everything out?
haven't checked codes...good idea lol -- i considered ABS sensors, checked all which are visually intact. the last time I tore off an ABS speed sensor I also got warning lights for traction control/ABS which I did not get this time, so I assumed that was not the cause.

yea, continued vibrations but only at highway speed mostly 1.5k RPM up to about 2k RPM and go away after that.
 
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jnack

jnack

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possibly wheel balance too
unlikely bc that is more of a vibration you can feel in the steering wheel & see the vibration in the cab, whereas I'm experiencing harmonic vibration -- which is primarily audible. additionally, wheel balance issues worsen at higher speed but in this case it's RPM dependent ie. if going 65 but go above 2k RPM goes away therefore isn't not wheel balance issue
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