So I've been attempting to diagnose my blinking 4x4 lights, ie my 4x4 not engaging. I have searched many threads and ended up doing a few different repairs already with no results, further help to diagnosis is needed, thanks for all suggestions.

2000 limited

Went to a trail recently and go to shift into 4x4, and it wouldn't engage the front end. I get the lurch from 4Lo like normal but I can tell front wheels aren't pulling. Recently changed front diff fluid, and also checked it, no metal at all. Started with the switches/sensors, pulled the one on the front diff, tested with multimeter and cleaned. Pulled both switches on t case actuator and tested/cleaned. No change. Then i replaced the whole actuator on the t case, and resulted with no change.

4x4 blinks all time now, from what I have read this should be electrical? Rather if it was mechanical I would think Id have a more obvious issue.

Also read into @TheDurk info about VSV system. I pulled the vac lines while in 2wd, only one vac nipple has pressure. In 4x4 the vsv does not switch. I switched the vac hoses around and truck still does not engage 4x4, lights continue to blink. Tried 4hi and 4lo with vac lines switched.

Where to next?

Any help is appreciated, thanks fellas!

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Well, assuming your reporting is accurate, the only explanation is two separate fails. Firt one I would tackle is why ADD won't engage with vacuum on either side.

When in doubt, RTFM:

First, make sure your vac lines are both intact all the way from the VSVs to the actuator on the front diff. If they are, you are going to have to get back under there and pull the actuator. This section shows how to test it. See item 3.

Attached Images

SA-49 Inspection (2000 4Runner) A.D.D. Control System.pdf (81.2 KB, 100 views)

The other thing you have to do is see why you get no vacuum on the 4wd VSV. You did not carry out the step where you verify if the electric signal is also not switching sides. Nothing more can be done on that until this test is done.

Thanks for jumping in on this Durk!

I did trace the vsv lines to the diff and found one disconnected! Must have happened when replacing the power steering return & pressure lines recently. Although when I re-connected the vac line and went back to testing the vsv it shows no difference.

Testing electrical connections on vsv with test light. I get response from the brown connection in 2wd and 4wd. No response from the blue vsv connection, in 2wd or in 4wd.

Curious if the disconnected line was my only issue, and maybe the t case actuator i replaced could be bad. I did get it used. I will throw the old actuator back in and see if my 4x4 comes back.

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To clarify,

Tested vsv connections.
Brown VSV shows good connection in 2wd & 4wd. Also has pressure.
Blue VSV no power, and no vac pressure.

Switching vac lines does not engage 4wd.


So the signal is not transferring in the vsv, and it seems I am also not getting pressure in the rear VSV module.

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I would check a few other things too.

1. Check if you transfer case is in 4wd. To do this, try to turn your front driveshaft. If it spins free, you are in 2wd. If it doesn't move at all, you are in 4wd. If it wiggles a bit, you are in either MM or have only the front diff engaged, but it doesnt seem like that is the problem.

2. Try using a vacuum pump on the front diff vacuum lines. They should hold at least 20lbs. If you can use this to switch into 4wd, thats good. If not, your front actuator may be stuck.

3. You already tested the sensors, but just to confirm, you got the one on the front diff, and the two on the transfer case near the actuator correct?

4. Make sure you installed the actuator in the proper position. There is a rotating gear inside of it with metal strips that the actuator uses to detect what position it has rotated to. Having this rotated into the wrong position will possibly cause damage or at least keep your 4wd from working. You can also test to see if your actuator motor is good by using 12v power on the pins

Lastly, here is a good resource for further diagnosis;

27. VF3AM TRANSFER - Google Drive

There are 5 sensors on the transfer case. 2 by the transfer case actuator motor, 2 on the left side in the center towards the top, and 1 sensor on the right side in the center towards the top.