Problem With Vacume Hubs, Its Not A Leak

[Bai]

I have a 2001 f350. I'm not getting power to the little vacume pump that locks the hubs. I have a wiring diagram howerver I'm still having a hard time tracking down the problem. I'm sure its not the switch in the cab. I've checked all the fuses, I'm not sure where the relays are. I think it has to be that or a bad wire somewhere. Any help would be appreciated. Thanks

[mschn99]

its almost allways the seals that actually seal the knuckle up so the hub can operate........they run pulse vacume to the knuckle so if it is leaking the hub cant engauge.........Marc

[Bai]

There is a 3 wire harness that plugs into the vacume pump. The middle wire is always hot while the key position is in run. The other two wires going to the pump never go hot. However the transfer case locks into 4x4. So its not the switch in the cab. Its somewhere after the wires split going to the transfer case motor and vacume pump motor. It can't be a vacume leak if I'm not getting vacume in the first place can it??? Thanks...

[mschn99]

Originally Posted by Bai

There is a 3 wire harness that plugs into the vacume pump. The middle wire is always hot while the key position is in run. The other two wires going to the pump never go hot. However the transfer case locks into 4x4. So its not the switch in the cab. Its somewhere after the wires split going to the transfer case motor and vacume pump motor. It can't be a vacume leak if I'm not getting vacume in the first place can it??? Thanks...

if your electric vacume pump never kicks on.....than thats the culprate......but usually a vacume leak overworks them and thats what takes it out. its this easy.......with the key on, disconnect the vacume connector on the resevour just in front of the pump........if there is no vacume and the pump doesnt kick on....replace the pump and also vacume check the system to be safe......we did see a run of bad pumps on the early 6.0's.....but i have not replaced many on 7.3's......Marc

[Bai]

If I have the truck running and put a test light on the middle wire its always hot, 4x4 or not. When I put the test light on either of the other 2 wires they don't go hot at all. I switch the 4x4 switch back and fourth but nothing. So I don't understand if the pump isn't getting the juice it needs to work how can it be the pump?? To me the pump isn't being told to turn on. Thats the problem.. Thanks

[mschn99]

the pump also runs your heater controlls, so if they are working right...then the pump isnt eve the problem.....usually they work on defrost only when the pump is out......and you said yourself the pump has power.....it is probabaly controlled on the ground side.....aways has power and then they complete the circuit to ground to turn it on.........turning the 4*4 switch does nothing different to the pump, ill pull up the scematic tonight and walk you throught it......Marc

[Bai]

Thanks I appreciate the help. You just reminded me of something. when i have it in four wheel drive the heater dosen't like to change from one position to another. When you switch from your feet to defrost you can here it trying to change but it takes 5-10 seconds before it locks in. I just went out and played with it. Hopefully this helps... Thanks again