:ybrick:I've searched 3 different forums and can't find one with an answer to my problem. 4Lo engages fine but 4Hi wont engage at all. I hear a relay clicking when i shift into 4Lo but not for 4Hi and the 4WD light doesn't come on for either position. I just replaced the 4WD indicator switch since i had a spare one and it didn't change anything. Its a 97 SR5 w/ 5-speed and V6 and doesn't have the push button selector. Thanks
4WD not engaging
- Thread starter 97SR5T4R
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TheDurk
Elite Member
No, just one system. The heart of the ADD are the two VSV's (Vacuum Switching Valves) on the driver fenderwall. They take vacuum from the manifold, and send it into one or the other of the two long hoses that go down to the actuator on the front diff. First you need to confirm that both VSV's have vacuum available--just pull off the upper short hoses in turn and see if they suck. Then, verify that ONE of the VSV's is passing vacuum into its long hose to the diff. Then switch from 2WD to 4WD and see if the other VSV is now passing the vacuum. Do this, and you will know whether the problem is in the diff actuator (vacuum moves), the VSV's or the switches or wiring going to them (vacuum doesn't move), or the supply of vacuum due to leak or plug (no vacuum at all). It's a simple system and with logical troubleshooting you can find the problem.
ok so both VSV's are getting vacuum but only one is sending vacuum down to the diff, i replaced the VSV's and still the same problem any idea's on where to look next. the relay still only clicks for 4Lo and the VSV's send vacuum when i engage 4Lo but not for 4Hi. This sounds like an electrical issue but not sure where to start looking?
TheDurk
Elite Member
I don't understand your post. Only one VSV AT A TIME is supposed to send vacuum. The system works by allowing vacuum down one hose for 4wd and the other for 2wd. So we need to know whether this switch from one hose to the other is happening when you try to switch drives.
If it does not, and you have new VSV's, the problem IS electrical and it's between the VSV's and the TC, probably a sending switch or connection on the TC.
sorry, so when im in 4Hi there is no vacuum going to the diff but the VSV's are receiving vacuum from the intake manifold. When i shift into 4Lo the blue VSV sends vacuum down to the diff engaging the hubs but not for 4Hi. Im guessing that the blue VSV engages the hubs and the Redish brown one disengages the hubs regardless of if its in 4Hi or 4Lo? So if thats true then im thinking its an electrical issue and im not sure where to start looking?
TheDurk
Elite Member
The TC has separate sending switches for 4HI and 4Lo, since different gears are moving into place. Unfortunately (for you), I have not had to mess with those, so I don't know which is which--I just know they are there. I think they are on driver's side of the TC. First thing would be just to get under there and check for loose or damaged connections. If not, then you have to figure out which is the 4Hi sending switch, get it off, and test it for continuity. They are just simple push switches. I know getting them out is a PIA, but I haven't done it myself.
Hopefully someone who has will chime in here. My problem was at the other end, so I'm sorry but I can't help you more here.
Hopefully someone who has will chime in here. My problem was at the other end, so I'm sorry but I can't help you more here.
Just tested the 4WD indicator switch on the TC and its working properly. I looked for the ADD relay and didn't find it, i dont think i have one. when i shift into 4Lo i here clicking coming from the 4WD computer not from a relay. The connections all in good condition to. I dont know where to look for the problem now. And no relay click sound for 4Hi still. :redbrick:
TheDurk
Elite Member
Here's the problem, I'm sure the issue could be traced if we had a good wiring diagram--but the on-line one is from 2002 after the 5-speed was dead. So I don't really know how that part works between the switches on the TC and the VSV's. Or how to troubleshoot it if it is not the switches or connections on the TC. The 4wd computer may be the problem, but it would be nice to be able to test it's inputs and outputs--if we had the information as to pin outs, etc.
