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.
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.
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It has been superseded with a different part number.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..