genuinegemini

Howdy, every one. The last time I posted on here it was regarding a wicked noise coming from my transfer case. Turns out, the chain in the case had stretched to the point of dragging across the bottom of the case making a gawd awful noise. The reason for that was because the previous owner (I'm assuming) had the vacuum lines wrongly connected and it was basically always in a 4WD mode. I have since replaced the differential and whatnot since the slack chain chewed up the inside of the old case, but now have a new issue.

While I was driving around town in 2WD mode the truck started making a bang type noise followed by a heavy jerking every few minutes. This was preceded by a clicking noise from under the dash. I pulled over into a parking lot and was able to replicate the noise with the help of my passenger and found the noise was coming from a relay labeled "Fuel Pump Relay". It seems the truck was basically shifting into 4WD mode all by itself causing the banging and jerking. I removed the relay and the truck did not die or stall so I'm guessing that relay isn't really for the fuel pump, but it did prevent the banging and jerking affect after driving it around the parking lot for several minutes. I drove it home and had no further issues.

I was told by a few people that the relay likely isn't the problem since it requires a ground to actuate the switch inside the relay. The general consensus seems to be that it is one of the modules on the side of the transmission which apparently controls the selection between 4WD modes and 2WD mode. However, I have not been able to locate the exact or specific name for this/these module(s), nor locate an image or identification of any sort. So, can any one on here please provide some insight into this issue and/or these modules on either side of the transmission? What do I call it in order to do a search for it? I have tried just about every conceivable combination of module, transmission, switch, selector, mode, transfer case, 4WD, etc., but have found absolutely nothing. Any help would be greatly and sincerely appreciated. Thank you in advance!

RJR

The relay is likely your ADD (Automatic Differential Disconnect) relay. The "fuel pump relay" name on it is just a red herring - Toyota uses the same relay for various functions, so you can't go by that.

It sounds like your ADD is randomly shifting into and out of gear. The relay controls two vacuum solenoids on the passenger side inside fender, which in turn shift the ADD system (on the front differential) into and out of lock. This relay in turn is controlled by the transmission/transfer case, and should switch when the transfer case goes into and out of 4wd.

You need a wiring diagram for the 4wd system for the automatic transmission. Unfortunately, my link to that is broken - perhaps someone else can point you in the right direction.

It could be as simple as a wire between the transfer case and the ADD relay making inadvertent contact to ground.

If the ADD shifts into gear while you are moving and before the transfer case shifts into 4wd, it will instantly try to start the front drive shaft turning, which would account for the clunk and jerk. It will also likely damage the ADD locking splines if you let this continue.

As a temporary fix, you can pull off the vacuum lines to the front diff. That will prevent it from shifting into the locked mode until you can figure out the root cause. Be aware this will also effectively disable 4wd.

genuinegemini

RJR, outstanding reply!

I just have some follow-up questions, though. Could the vacuum lines being hooked up wrong have damaged these vacuum solenoids? Can they be tested for fault(s)? If I'm unable to locate or identify a shorted wire, could replacing the relay and/or the solenoid(s) resolve the problem?

Also, does this mean that neither of the two modules on either side of the transmission have anything to do with the problem? And does either of those modules have anything to do with the 4WD or differential at all? I currently have the relay unplugged and have not had a repeat of the problem. is that enough or should I in fact disconnect the vacuum lines? I am so grateful for your input and willingness to share your knowledge on the matter, and so quickly, too. Any additional information will be equally appreciated and absorbed.

RJR

Here's a wiring diagram for the 4wd system with auto transmission. It's a bit complex, but you can find the ADD relay on the right hand side, and see the vacuum solenoids below that. The ADD relay is switched by switches in the transfer case that you see at the bottom center.

To answer your questions. No, you can't hurt the vacuum solenoids by hooking the lines up wrong. And yes, the modules could be involved, but I'd trace the wiring first and look for broken insulation that is allowing the ADD relay control pin (pin 2) to be grounded thus activating it.

scope103

Ask, and you shall receive: http://web.archive.org/web/201204242...26addcontr.pdf

This page from the 1993 FSM should give you all the diagnostic procedures you need. This page has the circuitry for both the manual and automatic. While I suspect the transfer position (or transfer indicator) switch, I would start with a multimeter hooked up to the relay socket while I jiggled the transfer shifter.

Note that the ADD is held either engaged or disengaged with vacuum all the time. When you removed the relay, both VSVs opened and the ADD has no vacuum on either side of the actuator. I believe that the ADD is "bistable" (it will stay either engaged or disengaged without vacuum), I wouldn't drive it full time that way. Also, it is possible that your ADD ended up engaged while you were working on the relay. This isn't a serious problem as long as the transfer case stays in 2wd, but you're spinning a lot of metal needlessly. This is easy to tell; jack up one wheel and spin it. If the front drive shaft moves with the wheel, the ADD may be engaged.

genuinegemini

Thank you so much RJR and Scope103! You guys have given some good solid info. Much more detailed than I ever expected to get. I seriously appreciate it to the fullest. I will get out there and do some serious TS with this information. The last thing I want/need is to wear out another chain or transfer case. I'll post back with my findings. Thanks again for the exceptional responses.