I have 97 4x4 f-150 V6 4.2 litre that in 2wd tries to go into 4wd causing a grinding then a thump. When I put the truck into 4wd there is no grinding or thumping.
I believe one of the vacuum solenoids could be bad. Just was wondering which one would cause it to want to go into 4wd while in 2wd.
The left most solenoid looking under the hood is the 4wd and the other with the pink vacuum is the 2wd.
I have no vacuum from the solenoids but from the lines going into them.

 

There should always be vacuum on the colored line leading from one or the other solenoid. The vacuum keeps it "drawn" into whichever mode is selected. Your diagnosis is correct and the 2WD solenoid should be bad. You know of course that these solenoids only affect the front axle being engaged or disengaged and has nothing to do with it actually being in 2 or 4 wheel drive.

 

Thanks for your reply and info!

 

Welcome to FTE , we're always happy to help!

I had a lot of problems with my axle vacuum system when I first got my '98. Took a while but did get it straightened out. My guess is that the 2WD solenoid goes bad because that's the one activated most all of the time. I picked one up at the salvage yard off of an Expedition. If you go that route grab the lines too since they sometimes crack/break.

 

Will do!
Thanx so much.

 

Did this fix your problem. I have the same problem with my f150. Mine does it occasionally. Left it with a mechanic who kept it for month and found no problem. Drove it for several months, not problem, then this week it did it again. Thanks Lynn

 

There are two electric operated vacuum solenoids on the passenger side firewall.
The inner one pulls vacuum on the Actuator locking the driver side axle.
The outer one puts vacuum on the other side of the Actuator removing the axle lock.
If the Actuator is locking the axle, the solenoid is not releasing, leaking or is sticking or even possibly a ground in the lead to the case motor..
Under the round cap on it's top is the third port. It has a small sponge filter that deteriorates and can be the cause of the sticking.
Another way is to test the lines for vacuum with the motor running and the port for leaking with a hand vacuum pump.
Shops have to go further in testing and not just say it works or doesn't and be lazy about it.
If that is all that is going on, replace it and have it over with.
OEM part is about $50.
It has been superseded with a different part number.
Look a the connector to see which one you have.
The old PN will be rectangle, the new one rounded. They are not interchangeable.
The parts guy may not know this after some 16 to 18 year old parts.
If he tells you there is no such part, try another dealer or ask him to bring out the parts he does have and Id the one you need if you see it..
Good luck.

 

I have a F150 King Ranch. My 4 Wheel Drive wont engage. I started by checking vacuum solenoid. I noticed the plug for the 2 wheel drive solenoid ground didn't transfer to ground for the 4x4 solenoid. The power was in both plugs and I was told the ground switched between when you turn the 4 wheel drive switch to 4 HI. I checked Switch and seemed fine.
Now it gets interesting. I pulled the fuse and tested that it had power on fuse box side. When i did this my truck happened to be in 4 low. It had power. I put fuse in and was going to run somewhere when my truck started bouncing. While backing up i didn't realize my 4x4 lo light was on. I got happy and went to turn 4x4 off and realized I was stuck in 4 Low. After thinking for a while I retraced my steps and realizes if I ground out the non hot side of fuse holder my truck will switch to what ever position my 4 wheel drive switch is on when I turn my truck back on. I am vary lost.

THANKS IN ADVANCE FOR ANY ADVISE.

 

Your missing a lot of what happens to power the firewall solenoids.
What you describe is hard to follow and make any firm sense.
.
When you turn the dash switch to 4H, it causes a relay to operate connecting power to the transfer case motor causing it to rotate to first position.
This connects through the GEM module to the firewall solenoid, operates the solenoid to pass engine vacuum to the front actuator and causes the dash light to come on.
Any failure in the chain will fail 4wd.
You tested voltage supply at the fire wall solenoids, then worked with fuses and caused the Gem to function with the dash switch on without knowing what happened.
Start with how the system functions from new then find where the function stops.
The firewall solenoid will not operate unless the transfer case rotates to the first position so ground from the Gem is connected through to operate the firewall solenoid as a start..
Good luck.

 

Sorry Not good at explaining this problem. So what i know.

1. My four wheel drive works fine but wont engage. I can engage if I pull my (104) 4x4 shift motor and clutch fuse while my F150 is turned off. Then I change my switch to 4 hi , 4 low or 2 wheel drive. Next I put fuse back in and shorty after i start and move my F150 the truck will change to what ever spot I have my switch on.

2. I tested the solenoid with F150 in In 2 and 4 Wheel drive. The vacuum was sucking from bottom hose all the time. It sucked from top hose on one solenoid when the 4 wheel drive was on 4 hi. It sucked from the other solenoid top hose when the 2 wheel drive was on.
The ground was also switching when it was supposed to.
3. The switch only has power to the one leg for the dash light above it. I was told that if it only sent a signal for the other leg it would not register on my light I check power with.

I don't know why I have to pull fuse to make F150 switch between 4 hi low and 2 wheel drive.
I thought it could be the gem module. I would like to know how to test that.
I would also like to know if there could be any other sensor That could be tested.
I think something is faulting the series of operations you mentioned above but don't know how or what to test next.

 

It would help if you would indicate what model year you are dealing with.

You probably should be using a digital voltmeter rather than a test light.
The switch leg that you said had power does not light the dash light.
That leg is powered by the GEM module. The GEM reads the voltage on the other (I,e, "signal return") leg to determine where you have the selector switch set.
(As you rotate the selector switch to different position, it changes the resistance of the switch, which changes the voltage returned to the GEM on the signal return wire. Based on the voltage the GEM sees on this line, it controls the transfer case shift relay module. The relays within the transfer case shift relay module power the transfer case shift motor to rotate it to the proper position.
When it is in the proper position a signal is sent to the instrument cluster indicating which light to turn on.

So, how many volts are on the wire going to the selector switch?
And, what is the voltage on the signal return wire when the selector switch is in each position? (Need to know what year vehicle this is.)
Your test lamp may not turn on bright enough for you to see it because some of the positions on the selector switch have thousands of ohms resistance.

 

2002 f150 king ranch. All that is in my first post a couple post up but forgot the year.

There are four pins/wires. The only one that reads hot from my light tester only reads hot when I turn my head lights on and my dash lights come on. I took switch apart and It goes to a light in the switch which lights ups switch. Not as an indicator light just so you can see in the dark. Have to get a new tester today then I will test volts.

Thanks

 

Yep. I got all that from your first post a couple of posts up. Sorry I wasn't clear enough. It was the model year that was important.



Yep. The one that reads hot with your light tester and is used to illuminate the selector switch. That would be the light blue/red stripe wire in pin #4 of the connector. The reason it appears hot to your test light is because that wire is 12 volts with your lights on. The black wire at pin #1 provides the ground for the selector switch lamp


The dark blue wire at pin #3 is the one that the GEM uses to send voltage to the selector switch and the GEM reads the signal return voltage on the dark blue wire coming out of pin #2. That return voltage is critical for the GEM to figure out what position the selector switch is in. As a matter of fact, that voltage is soooo critical that the GEM doesn't depend on sending battery voltage as it can vary too much, so the GEM sends a controlled reference voltage (called VREF) of only five volts on the dark blue wire going into pin #4 of the selector switch.


Will 5 volts light up your test light thingy?


And because of the different resistors with in the selector switch's different positions, the signal returned to the GEM will be less than 5 volts.


Will your test light turn on at less than 5 volts?


My SWAG is that your selector switch is okay, but you may as well prove that it is indeed good.

 

4x4 fixed itself. I left my key on testing stuff Friday night. I had to jump the truck a little bit ago. I went to buy a new tester and on the way I tried my four wheel drive and it works . Guessing being dead completely reset the gem module or some other computer system. I did fail to mention I didn't hear the relay before when I was switching four wheel drive back and fourth . Now I hear the relay. Thanks for any advice that was given.

 

Have patience,,,,,,,,,,,,it will be back.
The Gem does not reset. It has no 'variable' memory that changes for any reset to occur..
Good luck.