I have a 98 F150 4x4 manual shift, I am getting no voltage to the electric clutch in the transfer case, the 4x4 lights come on like they should, the fuse for that circuit in the engine compartment is OK, my haynes manual has no depiction of that wiring, If you could help me with a circuit diagram or the location of the relay. or what controls this circuit. Thanks in advance.

 

Not sure if this is what you are talking about



This shift on the fly relay is controlled by the GEM ( Generic Electronic Module ), the reason for this, is to check the vehicle speed against the shift mode ( 4H or 4L ).

Do you have the same symptoms when you try to shift into 4L, with the truck in Neutral or Park with the brakes applied ?

Is the overall symptom the 4WD does not work ?

If so, did you jack up and support the front of the truck ( wheel chock, jack stands, normal safety precautions ), turn the key from off to run ( do not start ) and shift the truck into 4WD ( try with both 4H & 4L ), and try to turn the front drive shaft.
If the front drive shaft turns, you are going the correct direction ( transfer case is the cause of 4WD not working )
If the front drive shaft does not turn, you need to look at the front axle disconnect are ( electronic controlled vacuum solenoids ).

 

The symptoms are the same in both low and hi, when the 4x4 is engaged I can turn the front drive shaft and it will turn the wheels but is not engaged with the rear. So I checked for the voltage to the case and it's not there. I can fake the voltage and it works fine, S I may have a bad control module or a relay, I think your diagram you sent will help, Can you tell me where the relay is located? Larry

 

The RPO relay block is under the oval on the dash, towards the passenger side, at least on the 2001+ MY it is. 90% sure it is in the same place on the 98 MY.

The oval is held down with spring clips, plastic trim tool makes it easy to pop up.

Take note of the wire color combination to that relay, depending on your trim level, you can find a few other relays in that block.

When you are checking, make sure the GEM is supplying ground to the coil when you are testing items.

 

Thanks for the help, I am thinking the selenoids are on all of the time as well so I may have a bad GEM, alltough the other stuff works fine, windows, mirrors etc, Larry

 

The 2WD and 4WD solenoids ? That should not have anything to do with the front drive shaft being free.

The item to check is when you shift into 4WD is if the GEM is supplying ground to the MSOF relay. If it does, that would answer the electrical question, could be the clutch coil ??

 

Well I found the relay and checked the operation and it closes and the voltage is on the transfer case side. It drops out after a short time and I still have no 4x4. I read somewhere it was supposed to drop out. Is that right? I also checked the selenoids for the front axle and they were operating, so I guess I will see if the acutator is closing as it should. More later. thanks again for the help, Larry

 

Well good news and bad news,, the relay works as advertised, ties up the front drive shaft, I unpluged the clutch in the transfer case and use the 4x4 lever and the drive shaft ties up to the diff aswell, bad news the acutator is operating to engage the front diff but it will not tie the wheels to the drive shaft, more later. Larry

 

Final up date it seems that the vacuum selenoids were operating backwards it was engaging the front axle in two wheel drive and dropping the front end in four wheel drive, as soon as I have some time to figure out why and what was changed I will fix it so I forced the front end and removed the leads off of the selenoids and I now have 4 wheel drive, thanks for all the help, cheers, Larry