Hello,

I have a 2002 F150 4x4 5.4L. I'm trying to figure out what's wrong with the 4x4. The 4x4 light comes on for a second when I start the truck. When I turn the 4x4 switch to 4H, no light, no relay clicks. Same with 4L. I checked all fuses for 4x4 and GEM, in both fuse boxes (dash and under hood). I have good vaccum at both vaccum relays. I have constant power at both vaccum relays, checked with indicator light. I have good switching to ground on 2WD vaccum relay, also checked with indicator light connected to positive battery terminal. I have no switching to ground on 4WD vaccum relay, checked in same way. I also checked transfer case motor plug connection which was good. I even beat on the transfer case motor housing with a hammer for ****s and giggles but that didn't change anything.

I guess the 4x4 vaccum relay not getting a signal through ground to activate is where I'm stuck at. Any suggestions on where to go next? Thanks in advance!

Justin

 

When you turn the truck on the 4x4 system runs a self test.

If it fails that self test, it disables itself. From that point on it ain't going to work. Trying to troubleshoot it is going to not work either because it has disabled itself.

A very common cause is the shift motor fails. They are defectively designed. Sometimes some of them ran over range when a plastic stop broke and they lost their position.... Mine just died an electronic death and no longer worked. Yes it's trial and error with parts but it's always probably the most likely place to start is with a shift motor. There's actually a code thrown in the PCM that can be read as well for four-wheel drive failure.

Vacuum isn't going to affect anything. I mean it may not actually work and shift the front end, but the computer won't know that. The computer only knows if the front end solenoids electrical circuits are complete. It has no idea if there's actually vacuum or if the shift actuator actually moves, or if the solenoid is actually work, or if the shift fork actually engages the axle if it moves. BTW the shift fork has little plastic tabs on it that always fails and requires replacing. It's cheap and about an hour's work, and a good chance to change the front differential fluid while you're at it.

Then there's the possibility of the wet and corroding GEM causing problems.

So you really got three possibilities if it's not fuses/relays:
Bad GEM
Bad wiring someplace in the four-wheel drive circuits
Bad shift motor

It's almost never switches, etc

 

Yeah, it seems like a bad GEM or shift motor like you said. I only mentioned the vaccum because I wanted to list everything I checked. Do you know if the signal for the 4x4 vaccum relay comes from the GEM or the shift motor? They are both pretty pricey parts. I'd like to at least make an educated guess before I replace one! Could I check for power to the shift motor with the switch in 4H? If there is power there then it would have to be the GEM, right?

 

If it fails the self test when you turn the vehicle on, the ESOF system is disabled. There is no reliable testing you can do when the system is disabled , it's all rendered inactive. No dash lights, no relay clickey clickey, etc. Dead. As in not working. All of this works through the GEM.

Unless you have other wet GEM symptoms, there's not much reason to suspect that first. You can look at your GEM to see if it's corroded. But the easiest path forward is to buy a new shift motor, and install it. And then see if the problem clears up. It's probably 100 to $150.

Some particularly thrifty people have studied how the shift motor worked before, and applied test voltages to it on the bench to make it do what it's supposed to do. In order to check it out without spending the money. One particularly industrious person then tested continuity of every circuit until they found the problem with was a pinched or broken wire someplace. If you do some searches for this you may find what other people have done like that. Who's got time for that? If it's not the shift motor I'd take it to the dealer. They got guys who know the wiring and can find a fault in an hour, when it would take me weeks. There are trouble codes the dealer can read, it would identify the faulure cause I don't know if you get to read them in forscan. My dealer read mine for me, told me it was the shift motor, gave me estimate to fix. I fixed it myself.

 

I can't guarantee these are the GEM trouble codes for our years, but it's illustrative of what variables the GEM is looking at in order to have a successful test. There are lots of reasons it can fail the self-test...... Good luck figuring this out without a dealer if you can't read the codes


GEM Diagnostic Trouble Code (DTC) Chart — Electronic Shift on the Fly (ESOF)
GEM Diagnostic Trouble Code (DTC) Chart DTC Circuit Description Test Mode
B1342 N/A GEM Defective On-Demand, Continuous
B1355 1040 (R/LB) Ignition RUN Circuit Open or Short to Ground On-Demand
B1359 297 (BK/LB) Ignition RUN/ACC Circuit Open or Short to Ground On-Demand, Continuous
B1365 32 (R/LB) Ignition START Circuit Short to Battery On-Demand, Continuous
P0500 676 (PK/O)
679 (GY/BK) Vehicle Speed Signal Not Detected Continuous
B1483 511 (LG) Brake Pedal Input Circuit Open or Short to Ground Continuous
B1485 511 (LG) Brake Pedal Input Short to Battery On-Demand, Continuous
P1804 210 (LB) 4WD High Indicator Circuit Open or Short to Ground On-Demand, Continuous
P1806 210 (LB) 4WD High Indicator Short to Battery On-Demand, Continuous
P1808 975 (BR/Y) 4WD Low Indicator Circuit Open or Short to Ground On-Demand, Continuous
P1810 975 (BR/Y) 4WD Low Indicator Short to Battery On-Demand, Continuous
P1812 780 (DB)
682 (DB) 4WD Mode Select Switch Circuit Open On-Demand, Continuous
P1815 783 (G) 4WD Mode Select Switch Circuit Short to Ground On-Demand, Continuous
P1819 483 (R/W) Neutral Safety Switch Input Short to Ground On-Demand
P1820 782 (BR/W) Transfer Case LO to HI Shift Relay Coil Circuit Open or Short to Ground On-Demand, Continuous
P1822 782 (BR/W) Transfer Case LO to HI Shift Relay Coil Short to Battery Continuous
P1824 779 (BR) 4WD Electric Clutch Relay Circuit Open or Short to Ground On-Demand, Continuous
P1826 779 (BR) 4WD Electric Clutch Relay Short to Battery Continuous
P1828 781 (O/LB) Transfer Case HI to LO Shift Relay Coil Circuit Open or Short to Ground On-Demand, Continuous
P1830 781 (O/LB) Transfer Case HI to LO Shift Relay Coil Short to Battery Continuous
P1832 605 (R) Transfer Case 4WD Solenoid Circuit Open or Short to Ground Continuous
P1834 605 (R) Transfer Case 4WD Solenoid Short to Battery On-Demand, Continuous
P1838 GEM activate shift motor relays and no movement of the shift motor was detected from the contact plate inputs. Continuous
P1846 771(P/Y) Transfer Case CONTACT PLATE "A" Circuit Open Continuous
P1850 770 (W) Transfer Case CONTACT PLATE "B" Circuit Open Continuous
P1854 764 (BR/W) Transfer Case CONTACT PLATE "C" Circuit Open Continuous
P1858 763 (O/W) Transfer Case CONTACT PLATE "D" Circuit Open Continuous
P1866 INTERNAL TO TRANSFER
CASE NOTE: If this DTC is stored, the transfer case cannot be shifted until the DTC is cleared and the ignition switch is cycled. Repair any other DTCs retrieved first and re-test.

Transfer Case System Concern, Repair Required.
GEM activate shift motor relays and no movement of the shift motor was detected from the contact plate inputs. Continuous
P1867 INTERNAL TO TRANSFER CASE Transfer Case CONTACT PLATE — General Circuit Failure (One or more plates open or short to ground). Continuous
P1876 145 (GY/BK) Transfer Case 2WD Solenoid Circuit Open or Short to Ground On-Demand, Continuous
P1877 145 (GY/BK) Transfer Case 2WD Solenoid Short to Battery On-Demand, Continuous
P1891 762 (Y/W) Transfer Case CONTACT PLATE Ground Return Open Circuit On-Demand, Continuous