LJ78 JDM 4WD Problem

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2001Slayer

10 Year 15 Year
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Nov 14, 2006
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I have a 1990 LJ78 Prado SX5, when I first start it in the morning I can push the hub button and the indicator light will come on, I then push the 4WD button and the indicator light comes on. I then switch the 4WD drive and hub button off and both indicator lights go out. So the vehicle is cold and everything works 100% of the time. I then drive the vehicle until it is up to temperature and do the same test. The hub light comes on 100% of the time but the 4WD light will not come on. This is not intermitten it is repeatable each time the vehicle is warmed up. I am pretty confident that everything is truly engaging when cold and I know for sure when it is warm and the 4WD light will not come on the 4WD system is not engaged.

If anyone has any insight into this problem I would greatly appreciate it.
 
2001Slayer said:
I have a 1990 LJ78 Prado SX5, when I first start it in the morning I can push the hub button and the indicator light will come on, I then push the 4WD button and the indicator light comes on. I then switch the 4WD drive and hub button off and both indicator lights go out. So the vehicle is cold and everything works 100% of the time. I then drive the vehicle until it is up to temperature and do the same test. The hub light comes on 100% of the time but the 4WD light will not come on. This is not intermitten it is repeatable each time the vehicle is warmed up. I am pretty confident that everything is truly engaging when cold and I know for sure when it is warm and the 4WD light will not come on the 4WD system is not engaged.

If anyone has any insight into this problem I would greatly appreciate it.
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Will it engage if you move forward a little?
 
as Rosco is saying, the 4WD indicator will not come on till the t/case is actually in 4WD.
if the front output does not engage then the light will not come on.
 
crushers said:
as Rosco is saying, the 4WD indicator will not come on till the t/case is actually in 4WD.
if the front output does not engage then the light will not come on.
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What I cant understand is why it goes into 4wd when cold. Maybe its something to do with the oil viscosity:meh:
 
2001Slayer said:
The the light will not come on if the vehicle is moved forward or backward. Any ideas what could be in the 4WD system that is affected by heat?
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I would expect it to go into 4wd easier when warm. I suspect an electrical malfunction.
If it was mechanical the problem would be constant.
Im not too familiar with these but I would inspect the shift solenoid on the t/case.
Keep driving around from cold ,shifting in and out till it stops working.

If possible ,see if it can be shifted in/out by hand with the solenoid removed. That would definately isolate it between mechanical and electrical or test the voltage at the solenoid cold and warm
 
Well, here's this morning's test. I started; hubs in, 4WD in no problem. I then tested the system at least six times up to about 185F (I have a mechanical guage) then I continued to test at that temperature two or three times and everything worked just fine. After running at about 185F to 190F for 5 minutes it failed. I continued on to work running it up to about 200F and it was tested several more times and it failed each time. I haven't test the solenoid stuff yet, I'll need to get someone who actually knows what they're doing for that.
 
Losing vacuum when the truck warms up for some reason maybe??? Drive until the 4WD does not engage, then pop the hood and check for incoming and outgoing vacuum at the VSV valves per the pic. The hoses marked with red arrows are incoming. Check incoming vacuum by pulling both hoses, cap one with your finger, check for vacuum at the other. Check outgoing vacuum by pulling one hose at a time and checking for vacuum at the plastic nipple (blue arrows). If hub lock and 4WD are switched off at the dash, vacuum should be present coming out of the top VSV. If hub lock and 4WD are switched on at the dash, vacuum should be present coming out of the bottom VSV. If vaccum coming out of the VSV is not as it should be, could be indicative of a valve problem or electrical problem. If you reinstall the outgoing hoses backward, the 4WD dash switch will work backward. Shot in the dark...HTH.
VSV.webp
 
2001Slayer said:
Well, here's this morning's test. I started; hubs in, 4WD in no problem. I then tested the system at least six times up to about 185F (I have a mechanical guage) then I continued to test at that temperature two or three times and everything worked just fine. After running at about 185F to 190F for 5 minutes it failed. I continued on to work running it up to about 200F and it was tested several more times and it failed each time. I haven't test the solenoid stuff yet, I'll need to get someone who actually knows what they're doing for that.
Click to expand...

Just wanting to clarify a point. Are these electric shifting or vacuum shifting t/cases on the LJ78 ?
 
Well the nail seems to have been hit on the head with the VSV vacuum switch. The one labled with the lower blue arrow never has vacuum, if I switch the lines the 4WD light will come on when the switch is activated in reverse. All I know is that someone is a genius and it's not me. So now what? Replace the VSV valve? Any ideas where to source one if the answer is yes? Thanks for everyone's help.
 
Pull the electrical plug to the valve that isn't working and with a test light check that there's power when the 4WD button is pushed. If so and there's vacuum in but no vacuum out then the valve is bad. If no power to the plug then it's electrical. I have no idea why either of these issues would be temperature dependant but evidently one of them is. There's a post where a 60 series guy uses a VSV from a minitruck somewhere on the board....not sure if the prado being 24V would be an issue but the fix was under $20.
 
oldblue said:
Prado's are 12volt where as 70series Landcruisers are 24volt
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Depends on which market . Aust,African and S American models are 12V and so would NZ except for their imports.
 
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