I have a '99 F150 5.4 Auto 4x4 and I have no 4WD. I have read the FAQ and many posts in the search but can't find out this answer.

I have no power to my 4x4 switch. I have checked all fuses per FAQ and searches, ohm'd them also and all are good. It has a new motor on the case. I hear no clicking in the dash. Is there a relay that supplies power to the switch? When am I suppose to see power at the switch and where does it come from?

Been at this for 2 weeks off and on, I must be overlooking something simple. Any suggestions/help would be appreciated!

Regards, Jeff

 

First thing is to read the first line on this page about 4x4 to get the baseline education on how the system works.
In short it uses a module to control all actions, called GEM.
The dash switch signals the GEM to request the action to begin.
It signals the transfer case motor to rotate to it's first position engaging the case gear train.
A solenoid relay on the fire wall passed engine vacuum to the front axle actuator to lock the axles together, then lights the dash lamp.
So you need to see where the fault is and not run with looking at just the switch.
Good luck.

 

Thats true the GEM could be the cause and that dos have a relay. It cant be a sort because youd blow a fuse. BUT other parts to look at would be the safety switches for the 4x4 to engage. Like the BOO switch for example. Because the truck wont go into 4x4 unless its in N and with the brake applied. So check those circuits out as well because if those are faulty it wont give the 4x4 switch power to turn the motor to put it in 4x4

 

Nope. That's only for low-range. The transfer case can shift into 4X4 at any time on the fly.

On the selector switch, there are two dark blue wires. One is ground (ohm it out), the other is part of a voltage divider formed by a resistor in the GEM module plus whatever resistor is selected by the mode switch. The voltage reading on that wire should change with each switch position.

There are several fuses that supply power to the GEM. You'd need to look them up and check them out.

 

Thank you dutter and projectsho! I have made more progress in 1 hour than I have in three days!

Here is where I'm at. I checked the switch per projectsho's ohm test, and voltage does change at switch with each position, so switch is working. Checked for power at the solenoids and I have power there but they won't click unless I ground the ground wire by probing it to ground then they click.

My conclusion is either relay or GEM?

I appreciate all your replies, Thank You!

Regards, Jeff

 

1. Have you checked fuse F23 in the CJB and F104 in the BJB?

2. Do either the 4X4 or LOW lights come on when the selector switch is changed?

3. Do either of the vacuum solenoids on the engine side firewall "click" when switching from 2H and 4H (have an assistant operate the selector switch while resting a finger on the solenoid)?

4. What is the recent history of the truck? Any odd or unusual electrical problems?

 

1. Yes and a few others that were suggested in searches. All good in the cab and all ohm'd good under the hood.

2.No

3.No. But if I go and probe one of the wires on the solenoids to ground they will click.

4. No

Appreciate the help!

Regards, Jeff

 

I'll just preface this with the comment that without a scanner that can access the GEM, no schematic, and no workshop manual, your prospects of diagnosing this will depend on some luck based on educated guesses.

1. Check the fuses that power the GEM module. In the absence of any other symptoms, these are likely good. All are in the under dash box. F15- Always Hot, F6 - Hot in RUN, F8 - Hot in Accy or RUN, F20 - Hot in START.

2. Does the transfer case motor make any attempt to rotate when the dash switch is changed between the various positions?

3. Can you hear the shift relays (module is located somewhere behind and above the radio, near the base of the windshield) or the transfer case clutch relay (in RPO relay block, somewhere above the glovebox near the base of the windshield) when the selector switch is changed?

If all fuses are good, and there is no response from any of the relays, then the GEM is *probably* defective.

 

Again Thank You for your replies! I haven't had time to look at this any further so far. Hope to this weekend but doesn't look good either. I will check back in when I have more info.

Regards, Jeff

 

Yes the T case can because it is synchronized but the front actuator can not because it is not synchronized is straight sleeve to hub, so for 4x4 high you still need the truck at a stop for the teeth of the sleeve to match the teeth on the hub/drive shaft. Your probably thinking of the newer models that have the auto 4x4 that uses speed sensors to engage the 4x4

 

97 up can switch to 4x4 Hi while traveling on the road.
ESOF is the term used.
Electric Switch On the Fly.
55 mph max in normal warm temps.
45 mph in temps at 32 or lower.
See page 181 in an 02 owner manual or whatever page for any other year.
The transfer case has an electric sync-ro to permit it. It's seldom talked about here.
It's true there is a sleeve coupling that takes place at the front axle but it handles it within reason, by design intent same as a starter gear finds the flywheel without hanging up or being blocked in between teeth.. You very often have to breath the drive train to get it all in by lifting you foot off the throttle for a second or two as you switch over..
.
Lo range is not permitted unless stopped in Neutral and foot on brake.
You can imagine the results of going 4 low at speed with the huge gear reduction that takes place and a shift skeds change. These are a safeties put in place so it cannot be done.
The GEM has inputs to detect all the safety features and must all be present at the same time or no 4L is permitted to take place.
Good luck.

 

Wrong. At least go read the owner's manual for the specific vehicle under discussion so you understand how it's intended to be used by the customer.

There's a reason it's named "shift on the fly", not "shift only when stopped".

 

Don't mean to hijack this thread, but quick question about the 4x4 switch. Should it be back lit like the fan and A/C switches? Mine (son's) 1998 4x4 switch works fine, but is NOT back lit like the others mentioned. Thanks.

 

wwooohooo you can read an owners manual. How many 4x4 related systems have you rebuilt ? honestly. Because you should be able to at least understanding my theory and terminology and get were im coming from vs just saying WRONG. Back up your argument with something i may not know about.

 

Sir, there is no argument and no need to challenge.
This a Ford specific board.
If we don't know, by now, how 4x4 works we will never know.
You have to come up to speed on these Ford systems 97 to 04 before you make a challenge like this.
These systems have been in place for 17 years, nearly unchanged.
After making this reply, i'm not going to answer you again to prove anything because all 'you' have to do is get into Ford service discription of the system operation to verify what is being said here.
And yes I own an 02 4x4 and have Ford's service info.
Good luck.