Having issues with 4WD. I have an electronic switch. When I turn the selector to 4 HI or 4 LO nothing happens. No clicking in the relays, nothing. I have changed the switch out. Checked appropriate fuses. Changed both solenoids. I have constant vacuum to both solenoids and through the vacuum line from the 2WD solenoid to the actuator on the front axle. I have 12 volts to both solenoids but I do not have ground from the GEM to the 4WD solenoid, only to the 2WD solenoid. I'm thinking I need to replace the GEM but want someone's more experienced opinion before I tear into the dash and pull the GEM. Your help is greatly appreciated.

 

First off, what type of vehicle are we working with?

 

Sorry. 2003 F150.

 

Are you getting a ground signal to the GEM? But the GEM is not sending the ground?

 

How do I check ground to the GEM?

 

First thing I'd be looking for is the fuse that powers the 4wd components. Look in fuse panels under the dash or under the hood. You need the owner's manual to look at the table which says which fuse is for what. Because the panel covers only give fuse numbers. Next to check is, is the transfer case shifting? The servo motor may be failed, halting the 4wd engagement sequence thus preventing the relay to go.

The 4wd on my 2003 is not working either. However, I have power and vacuum to everything. Up to an including visually seeing the actuator control arm moving back and forth when dash is switched. On mine, I suspect the issue is worn and sheared front axle shift fork plastic pads. To fix that, I have to split the axle. Has to be soon too, as there is snow on the ground here already.

Update. I traced the problem with my no-4wd problem to the front axle shift fork, internal. Fixed it today. Yours sounds like an electrical problem, mine was mechanical.

 

Fuses are fine.

 

Just following up on a thought. And forgive me if this sounds basic and already known. Going back to your 4 Hi to 4 Lo nothing happens comment. ........ To shift from 4 Hi to 4 Lo the procedure is
1) the vehicle must be stopped,
2) the transmission must be put into neutral,
3) the switch moved from 4 Hi to 4Lo.
4) the brake released so the driveline relaxes and rolls ever so slightly so the transfer case can spool the planetary collar into Lo gearing.

4 Lo will not even attempt to engage unless the truck is stopped and transmission is in neutral
4 Lo will engage with a very discernible "clunk".
To shift out of 4 Lo, follow the same procedure. Stopped, transmission in neutral, foot off the brake to relax the driveline, "clunk"

Hope that helps!