Hi Folks,

I'm new to this forum and posted the original thread in the wrong area, I know noob mistake

The reference thread is here, thanks for pointing me to the right place jbrew:
https://www.f150online.com/forums/19...when-cold.html

Anyhow, I'm really confused now as I've performed further tests that have only clouded the issue further for me. I connected a vacuum gauge on the 2wd and 4wd vacuum solenoids and found that the blue vacuum line solenoid is always sourcing vacuum, 4wd solenoid I believe. The pink solenoid follows the floor shift lever and sources vacuum when the lever is in 2WD but not in 4wd.

I thought the problem was a bad solenoid but then I measured the voltage to the solenoid and found it's always energized ?? I'm really confused now and would appreciate any input from an expert. Is there a separate CPU for the 4wd system?

Also, what causes the 4wd light to come on? When I move the floor shifter the light doesn't always follow the lever so what tells the light to turn off or not? It seems the light follows the status of the front drive shafts but how does it "know" what state they are in because I didn't see a switch to provide feedback down at the actuator?

Thanks in advance and happy new year.

-- Fish

 

When you shift to 4Hi the lever moves the linkage and shifts the transfer case into 4hi. Positive engagement is accomplished by a shift fork. There is a sensor switch on the t-case that is moved by the Mechanical Shift Cam. If this switch is defective or corroded it will not ground the circuit telling the GEM (Generic Electronic Module) to engage the front axle. It also tells the GEM to turn on the 4Hi indicator light. Check for corrosion at that connector. BTW the connector (black) is just below the shift linkage going into the transfercase. Two wires: a gray one and a light blue with a black stripe. Next time you engage 4Hi and the light doesn't come on ground the gray wire.

.

 

Hi Jean Marc,

That sounds like the best suggestion yet. Your detailed info is really appreciated because I've been wondering how the system "knew" what mode to be in. My money is on a bad switch as that explains a lot. I should be able to crawl under the truck tomorrow and I'll let you know what I find.

Thanks for your input.

-- Fish

 

I crawled under today, in the snow , and found the connector that was described with the gray wire and the light blue with blk stripe. The connector was attached to a sensor that threaded into the transfer case. Is this simply a switch? and if so, what would it be called to find a replacement?

I unplugged the wiring but when I tried to ground the gray wire the 4WD light stayed on, I was having a problem finding a good ground though. I did see the light flicker once so I think I'm still on the right track. Should I be able to short the two wires to get 2WD? The GEM should have direct control of the indicator light, correct?

Where is the GEM located? I would love to be able to tap into that wiring closer to the module if it's located in the cab. I hate hacking wiring that's exposed to the weather.

Anyhow, thanks again for the help and any future suggestions.

-- FISH

 

Those two wires get grounded to tell the GEM what 4x4 position the transfer case is in. Grounding the gray wire is for 4Hi grounding both wires is for 4Lo and not grounding the wires is 2 wheel drive. Connecting them together will not do anything. Unfortunately I do not have the part number for that switch.

Regards

Jean Marc Chartier

 

Hi Again,

If I unplug that connector and the 4WD light remains lit, what could that mean? What else can cause the light to remain on if the GEM is told it's in 2WD by the shift position switch?

Thanks,

-- Fish

 

A faulty gem will keep the light light or a switch that is still in the 4Hi or 4Lo position.

Regards

Jean Marc Chartier

 

Well, it turns out the issue with my 4wd was with the transfer case shift selector switch. The symptoms really confused me, I was really expecting to need a new GEM because I was getting really weird and inconsistent operation of the 4wd indicator lights. The 4wd lights didn't always light up with the other lights when turning the key to ON. I even access the instrument cluster dealer test mode that allows testing of ALL instrumentation and the 4wd light wouldn't always light up.

I followed the suggestion from JMC to ground the wires one at a time at the pigtail and sure enough the 4wd light would follow one lead grounded and the 4wd low light would follow the other being grounded.

Thanks again JMC for the help since I now have a reliable 4wd system and learned a lot about how the system works to boot

-- Fish

 

Good info! Where is the transfer case shift selector switch and where did you buy it? My 97' is having 4wd issues and I can't figure it out.

 

Would you mind posting some pictures of the area pointing out the switch?

 

Black connector with blue weather protector.


Regards

Jean Marc Chartier

 

I own a 02 f150 4x4 lost 4x4 only had it in reverse ...then later lost that... new electric switch in dash...worked maybe 2 weeks... then nothing?...new motor on back of t-case...worked to begin with' but then nothing...now they want to put in a new t-case ?need answers...