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Discussion Starter #1
All,

This is a 1995 DX 3.4 Auto with Auto Disconnect Differential

Using the shift lever, 4WD will not engage. The dash light does not light up in any position except for "AWD Neutral". I have verified the following:

-Vacuum lines are in good order and routed properly
-VSVs fall within resistance specs per FSM
-Brown VSV (2WD) has power (12V) at its connector in all conditions (2HI, 4HI, 4LO)
-Blue VSV (4WD) has no voltage under any conditions. I did jump this to the battery and verified that it will provide vacuum when under power.
-The actuator and diff are in working order. If I apply vacuum to the 4WD side of the actuator, I can engage 4WD but I still get no light on the dash.
-I removed and tested the ADD relay and it met the specs per the FSM.

A couple other things to note - the fuel gauge does not function nor do some of the gear selector indicator bulbs. Could this be interrelated?

I am looking for any recommendations on what step(s) to take next. Any help is greatly appreciated.

WEDMAN
 

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1998 T100 SR5 2WD
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14,606 Posts
Welcome to the forums! :D

It's possible that the detection switches in the differential (located on the front left side of the differential) could be corroded or not making contact. You should be able to unscrew it from the differential case to remove it and test it's function. ;)

Has this T-100 been in a wreck previously? How many miles on the truck? Have you physically inspected all the fuses? It's possible that some of the bulbs have burned out. When you select a certain drive gear, does the selector indicate a different gear than what was selected? If so, it's possible that yer Neutral Safety Switch could be bad or gummed up. What gears are not lighting up when you select them?

As far as the fuel gauge not working, you will need to check the wiring harness and the fuel sender located on the top of the fuel tank. You can drop the tank but yer best bet will be to pull the bed so that way you will be able to inspect the top of the tank and their associated components. ;)
 
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Discussion Starter #3
Thanks for the quick reply Bam.

Truck has 140k miles and does not show any indication of a previous wreck. The drive selection bulb does not light up on dash.

I will check all fuses today and see if I can locate the switches on the diff.

Thanks,

WED
 

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1997 Toyota T100
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Thanks for the quick reply Bam.

Truck has 140k miles and does not show any indication of a previous wreck. The drive selection bulb does not light up on dash.

I will check all fuses today and see if I can locate the switches on the diff.

Thanks,

WED
Just as a precaution, are you sure there is a bulb back there, and that it's not burned out? Or can you tell that it won't engage 4WD?
 
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Discussion Starter #5
I know that the VSV that controls the 4wd does not power up under any selection with the shifter. I have 12V power on the 2WD side regardless of what selection I am in. Also, when in a 4WD selection, front driveshaft will not spin by hand.

I have verified that the VSVs both work but the signal to switch between the VSVs is not getting through.

Thanks,

WEDMAN
 

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Discussion Starter #6
Working on the 4wd again today.

Have tested all fuses and they checked out. Did find a wire on a transfer case switch that had been previously rigged up by PO with splicers so I removed those, cleaned and reconnected but that did not change anything.

Bam, the ADD indicator tested good per the FSM. Was this the differential switch you were referring too?

Thanks,

WEDMAN
 

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Discussion Starter #7 (Edited)
I just came across 2 seemingly cut wires on top of the transmission when I took off the rubber surrounding the J handle. With the key on, there is 12V power between these wires and the colors are white/blk and pink/green. It is tied in with the wire bundle that runs to my downstream O2 sensor.

I am now trying to figure out where they should connect.

Thanks,

WEDMAN
 

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Discussion Starter #8
I believe I found the problem. The wires that I found seem to lead to a transfer indicator switch on top of the transfer case.(its hard to identify when it has no wire leads, only nubs of what used to be). I will have to pull the trans to get this switch replaced. (or remove bench seat and cut a hole in the floor)

This seems to be a common problem with the T100. A couple things that I learned from diagnosing this particular issue, that may be useful for someone in the future:

- ADD Relay is labeled "Fuel Pump Relay". It is tucked up high under the instrument cluster on the driver's side.

- The 95' A/T has 3 "position" switches on the transfer case for the ADD function - transfer position 4HI (passenger's side), transfer position 4LO and transfer neutral (driver's side).

Thanks,

WED
 

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1998 T100 SR5 2WD
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It's possible that the previous owner replaced the ADD relay with a fuel pump relay as there isn't a relay labeled Fuel Pump relay on the T-100 in the original stock form. The relays that controls the fuel pump are the EFI relay and the Circuit Opening relay.

Here's what I show fer the ADD relay...from ToyoDIY.com. ;)
85917B
RELAY, AUTOMATIC DISCONNECTING DIFFERENTIAL
28380-44030
1
$87.22
 

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Discussion Starter #10
Solution

I wound up pulling out the interior for sound proofing, stereo upgrades, and cutting a hole in the floor to access the TCase Indicator Switch.

The switch was easy to get out of a hole i cut with a dremel on the side of the trans hump from inside the cab. Peeled 3 sides back like a tuna can. When I tested it per the FSM, its function was sporadic, but after a good cleaning with simple green it worked just fine. Rewired and now I have 4WD! JB Welded the tuna can-like flap back in place.

Seeing the 4WD light on the dash was like Christmas! Thanks for your input fellas.

WEDMAN
 
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