I recently replaced transmission and now my shift on fly 4x4 does not work. I need some help troubleshooting this. No light when I turn 2wheel, 4 high, 4 low switch. Where is the best place to start.

 

Since they just replaced your tranny, the first thing I'd do would be to inspect the wiring going to the transfer case. They might not have gotten something plugged in correctly.

There are two connectors going to the shift motor on the transfer case. One has 6 wires, one has 2 wires. I'd unplug them and inspect them for damage, corrosion, etc. Make sure they have a good dielectric grease in them and make sure they are plugged back in well.

If that doesn't work, disconnect the 2 wire connector. Put a test light between the 2 pins, have a helper switch to 4WD with the key in the RUN position. You should see flashes at the test light. Report back what you find and we can diagnose it from there.

As an aside, did you do the tranny yourself or did you have it done? If you had it done, you may have the option of just taking it back and saying hey, what's this. And make them fix it. Or at least diagnose it.

Also, let us know what year you have. It's not critical but it can help to know.

 

the truck is a '01 F250 diesel, Will troubleshoot the wiring later in the day or tomorrow. What I find interesting is I have no light on the switch. Could you give me some insight as to the electrical path required to engage the shift motors on the case. I have not torn anything apart yet as want to have some idea of operational sequence before tearing into it.
Thanks

 

Everything surrounds the GEM module, which is a computer module that controls a number of functions in your truck, including 4WD. There are 4 wires going to the switch, 2 for illumination and 2 that go back the the GEM. The switch contains 3 resistors, and simply applies a different resistant value to the wires depending on the position. The GEM reads these values and knows what position the switch is in.

The GEM outputs to two relays. They are on the drivers side firewall in the engine compartment. Two identical relays in a relay box with nothing else inside, usually wrapped with a fabric electrical tape from the factory. Both relays apply ground to the shift motor in their normal position. One relay to each wire. When the GEM wants to move the shift motor, it activates one of the relays, causing it to apply power to that wire, making a circuit path to cause the motor to move. The relays are opposites, so that one relay causes the shift motor to move one direction, the other relay causes it to move the other direction. I can give you wiring diagrams if you'd like.

The 6 wire connector is how the GEM knows the position of the shift motor. Frankly, I've never fully understood how that works. I've read they're simple switches, I've also read that they are hall effect sensors. (Hall effect sensors are generally used to detect movement). The 6 wire connector reports back to the GEM the position of the shift motor. If the shift fails, the GEM will know it because of these sensors and reverse it, then try again. If the shift completes, the GEM will turn on the 4x4 light and then proceed to locking the hubs.

On the passenger side firewall is the PVH (Pulse Vacuum Hublock) solenoid. It has one power wire directly from the fuse panel, and two wires going back to the GEM. The GEM grounds one or both wires to cause vacuum pulses to lock and unlock the hubs. The PVH solenoid applies a full vacuum pulse for about 45 seconds to lock the hubs, and about 1/2 vacuum pulse for 45 seconds to unlock them.

That's the ESOF system in a nutshell.

In later model years, the GEM module was eliminated and replaced with separate modules to performs it's functions. The module for 4WD is called the 4WD control module. So everything I wrote is the same for 2002 and beyond, just replacing 4WD control module with GEM.

 

Fuses.

Check #17 (30A) under the hood, power distribution box.

Check #15 (5A), #19 (10A), #28 (15A) under the dash, central junction box.

Check your owners manual for any others I may have missed.

 

I didn't even think about fuses. I must be getting senile in my old age.

In your owners manual, check for things like "Run/Start Feed", etc. Some of those feeds power a lot of different things.

If by chance you don't have your owners manual, you can get it online here.

https://www.flmowner.com/servlet/Con...ionalEventPage

 

David,
Thanks so much for the technical knowledge. I will start by making sure the wiring on the xfer box is secure and making good contact. Next I therorize I should check signal at the two relays, if no signal there work back to either the GEM or the switch. If the relays check good but have no signal where is the GEM located. Just trying to put together a plan of action.

Thanks;
Monty

 

will be sure to check fuses also. The truck sat for almost a year while fixing transmission. Don't know if that has any effect on anything here just putting it out. Before pulling tranny transfer worked fine.

 

The first thing to do is check the fuses and do the test light at the transfer case, and to check the plugs and wires, etc.

Once you get to the relays, you want to look for corrosion in the relay bases and wiring going to the relay box. Then, a common diagnostic technique is to swap the two relay positions. Often the "shift in" relay goes bad and by swapping them, you move the "shift out" relay into the "shift in" position and it'll go into 4WD, but not back out. This just demonstrates that you need new relays, which are cheap from Ford.

But do the other stuff first. If you get flashes at the test light, you've eliminated entirely the possibility of a bad GEM, relays, fuses, etc.

 

I checked both relays under the hood, they are good. swaped the switch in the cab with a known good one, no go, relays are not energizing when switching switch. I jumpered the relay and now have the truck locked in 4x4. So I am assuming it is not getting the signal to the relays. Any ideas?

 

This could be the old windshield leak/GEM issue. Early SDs had a leak sometimes in the windshield seal that would cause water to drip onto the GEM and cause corrosion and then failure. Solution was to get the windshield properly sealed for starters, and then replace the GEM. The corrosion was not apparent from the outside of the GEM. Note - I haven't seen this problem myself, but I have read it here many times over the years. Yours is an '01 so it may fall into the leaker group. Search on "GEM Windshield" and you should get a bunch of hits.

 

It wouldn't hurt to check for the water on the GEM problem. My truck doesn't have a GEM, so I'm a little out of knowledge in that area. But it's my understanding that the GEM is attached to the Central Junction Box (fuse box) under the dash. Some years they didn't get good seals on the windshield above that area and water could drip down on the GEM and the CJB causing problems.

Now, I didn't suspect that initially, because as I understand it, a wet GEM will usually cause multiple failures beyond just 4x4, because the GEM controls a lot of other stuff too. It still wouldn't hurt to look at it though.

The question that immediately came to my mind is what did the OP do to check the relays. When one of my relays went bad, it clicked just like it should have. The only way I could tell it was bad was to check for continuity across the contacts, both in the normal de-energized state and with the relay energized. I needed a battery to energize the relay and a DVOM to check for continuity to do this. I found that my relay was actually making contact in the energized state, but was not making contact through the normally closed circuit of the de-energized state and that was preventing the shift motor from working.

I also wonder if he did the test light procedure that I outlined and what the result was.