Bad gem or dash switch?
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Mountain Pass
Joined: Feb 2011
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Bad gem or dash switch?
I've been chasing down a intermittent rumble, noise only in a4wd and at highway speeds.
I learned today that's it's switching to 4H by itself but only lasts a second or two. The dash light stays lit for a4wd and I hear no clicking from the relays when it happens though.
I figured it out when I accidentally switched to 4h at 70mph and it made the same noise/vibration.
I checked the dash switch and the ohms are in range but they were pretty jumpy, could be just my meter though.
The transfer case motor is about 6 months old.
Another issue is sometimes a4wd does not kick in our it seems delayed.
Could it be a bad GEM module?
I learned today that's it's switching to 4H by itself but only lasts a second or two. The dash light stays lit for a4wd and I hear no clicking from the relays when it happens though.
I figured it out when I accidentally switched to 4h at 70mph and it made the same noise/vibration.
I checked the dash switch and the ohms are in range but they were pretty jumpy, could be just my meter though.
The transfer case motor is about 6 months old.
Another issue is sometimes a4wd does not kick in our it seems delayed.
Could it be a bad GEM module?
Fleet Owner
Joined: Apr 2008
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From: 0,0,1
The contacts in the transfer case motor are what signals the 4WD control module to change the lights on the dash. Check your connection down at the motor and make sure it is tight and dry.
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Mountain Pass
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I leaning to believe the meter is fine,I checked a couple of other things and its fine.
I was having the same problem before I changed the t case motor, I'll check the connections though.
Could I have a bad transfer case? What else could cause a uncommanded switch to 4h with out triggering the dash lights.
4x4 works fine when I switch it, although sometimes it will switch back to a4wd then back to 4h on its own.
I was having the same problem before I changed the t case motor, I'll check the connections though.
Could I have a bad transfer case? What else could cause a uncommanded switch to 4h with out triggering the dash lights.
4x4 works fine when I switch it, although sometimes it will switch back to a4wd then back to 4h on its own.
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Mountain Pass
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yes i have.
I thought ive read before that if one of the hall effect(speed) sensors on the t case is bad while in a4wd it would switch to 4h on its own. Is this correct?
My issue only happens at speeds of 70mph and up while in a4wd mode.
I thought ive read before that if one of the hall effect(speed) sensors on the t case is bad while in a4wd it would switch to 4h on its own. Is this correct?
My issue only happens at speeds of 70mph and up while in a4wd mode.
Fleet Owner
Joined: Apr 2008
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From: 0,0,1
If the sensor is bad as in it is open or shorted then it should NOT engage the front axle unless the switch is turned to 4hi or 4 low. If the sensor is worn then I'd say you could be getting fluctuations from the sensor at high speeds, like you're reporting.
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Mountain Pass
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I see. Is there any truth to the system engaging 4H if it can't get proper info from the speed sensors? If so, that seems to be my problem.
Then I'm curious though why doesn't the 4x4 light trigger when it defaults to 4h.
Then I'm curious though why doesn't the 4x4 light trigger when it defaults to 4h.
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Mountain Pass
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Also forgot to note thay when I put the dash switch to 4H sometime it will switch back to a4wd and sometimes it will switch out to 2h(i assume because neither the 4x4 or the awd lights are lit).
This only lasts for a second or two then it will go back to 4h,all on its own.
This only lasts for a second or two then it will go back to 4h,all on its own.
Logistics Pro
Joined: Jan 2001
Posts: 3,740
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From: Central NYS
Thank You for adding the year of your Expy. 
That tells me that your year vehicle had a "Front Axle Actuator Motor Asembly".
I can't remember if vacuum engages the front axle shaft or keeps if from engaging, but at higher speeds your engine generates much less vacuum than say at idle. The lower vacuum, or if you have a vacuum leak it may let the actuator engage the front axle shaft to the diff (or not fullly disengage it) and that may be creating the rumble noise you hear.
The dash switch will be in A4WD and the transfer case may be in A4WD, but the front axle shaft maybe engaging the front diff so it may feel like 4H, but the front driveshaft will not be powered by the engine.
alloro, which way does that vacuum work?
That tells me that your year vehicle had a "Front Axle Actuator Motor Asembly".
I can't remember if vacuum engages the front axle shaft or keeps if from engaging, but at higher speeds your engine generates much less vacuum than say at idle. The lower vacuum, or if you have a vacuum leak it may let the actuator engage the front axle shaft to the diff (or not fullly disengage it) and that may be creating the rumble noise you hear.
The dash switch will be in A4WD and the transfer case may be in A4WD, but the front axle shaft maybe engaging the front diff so it may feel like 4H, but the front driveshaft will not be powered by the engine.
alloro, which way does that vacuum work?
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Mountain Pass
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This system uses vacuum to connect and disconnect the axle. That was my first thought that the issue is vacuum related, but after I checked everything out with a vacuum pump, everything seemed fine.
I would take it to the dealer for diagnoses, but I fear they will not find the problem and still would have to pay the fee.
I would take it to the dealer for diagnoses, but I fear they will not find the problem and still would have to pay the fee.
Logistics Pro
Joined: Jan 2001
Posts: 3,740
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From: Central NYS
Or you couldn't find a vacuum leak?
BTW, did you check the 4WD vacuum solenoid with your vacuum pump to be sure it is not leaking vacuum through the solenoid to the front axle actuator even though the solenoid is not energized?
And for the second or two that it lasts, is that how long the itermittent rumble you hear lasts?
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Mountain Pass
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So are you saying that after checking with your vacuum pump, it seems fine, as in, it is no longer making the rumbling sound?
Or you couldn't find a vacuum leak?
BTW, did you check the 4WD vacuum solenoid with your vacuum pump to be sure it is not leaking vacuum through the solenoid to the front axle actuator even though the solenoid is not energized?
Couple questions: How did you learn that it switching to 4H by itself?
And for the second or two that it lasts, is that how long the itermittent rumble you hear lasts?
Or you couldn't find a vacuum leak?
BTW, did you check the 4WD vacuum solenoid with your vacuum pump to be sure it is not leaking vacuum through the solenoid to the front axle actuator even though the solenoid is not energized?
Couple questions: How did you learn that it switching to 4H by itself?
And for the second or two that it lasts, is that how long the itermittent rumble you hear lasts?
While I'm not 100% sure it's switching to 4h on its own, when I accidentally switch to 4h on the freeway, it made the same noise and vibration ad it does when I'm in a4wd mode and the intermittent problem occurs.
Yes the rumble and noise generally lasts a second or 2, the truck also pulls to the left when it happens.
During the 15 min commute to work, somedays it will happen once and somedays it will do it 4 times.
Maybe for some reason the transfer case clutch is kicking in causing those symptoms since it only happens at speeds above 70mph while in a4wd mode.
I'm thinking about wiring a test light to the TOD relay to see if it triggering during the issue
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