How to add a Center Differential Lock to a truck that doesn't have one

[kgrove]

I've also had the issue of the CDL not wanting to cycle when its cold.

So when you guys say "cold", you mean like really cold - Canada or at least Colorado ski town cold? Or do you only mean a car that hasn't broken a sweat yet but might be room temperature? Today, for instance, my tests were at probably 60F ambient temperatures, but on a car that hadn't been started in a few days.

 

[DSRTRDR]

to test whether the diff lock is actually engaged:

find a dirt lot, drive figure 8s; intensely notice how that feels - next: push diff lock button, drive the same figure 8s: you will notice a difference in the overall steering compared to figure 8s without the diff lock engaged

 

[mudgudgeon]

There should be an audible click coming from one of the kick panels when you shift into low or activate the added switch. It comes from the left kick panel on a left-hand drive truck. I do not know if it reverses sides in a right-hand drive. The click isn't loud so listen carefully. If there is no click there is a problem with the circuit.

The relay/ ECU for diffs is in the left side kick panel in RHD vehicles also

 

[cruiserdan]

So when you guys say "cold", you mean like really cold - Canada or at least Colorado ski town cold? Or do you only mean a car that hasn't broken a sweat yet but might be room temperature? Today, for instance, my tests were at probably 60F ambient temperatures, but on a car that hadn't been started in a few days.

Below freezing in my case.

 

[kgrove]

I might be zeroing in on the problem... maybe. I'm assuming the way this system functions is a relay gets a signal either from the Low position switch (sensor) that the transfer case is already in Low or from the dash mounted CDL button that the driver wants to lock the CD in H, then the relay tells the actuator to lock the CD? I'm thinking my relay is bad. Or am I missing the function and the relay is only for the indicator light, not telling the actuator to lock?

I've confirmed the switch works by putting it into a different '97 that also didn't have the CDL dash switch, and it works perfectly. The truck that is working - key in On position with engine off - the CDL will engage either by depressing the dash switch while in High or by shifting the transfer case into low gear. In both cases, locking center diff by CDL switch or transfer case gear shift, there is an audible "double click" coming from under the dash by the drivers side kick panel (right side in my case since this is a RHD). So.. me and the switch went back to the truck that isn't working...

This one... no sound at all with the dash CDL switch. When shifting the transfer case into Low range, there is a less pronounced single click from the kick panel, not the more defined "click pause click" on the working truck.

The relay/ ECU for diffs is in the left side kick panel in RHD vehicles also

ECU or relay? The relay sounds are coming from the right /drivers side kick panel, so removed the kick panel and found this bank of 8 relays and one jumper (at least I assume the round ones are also relays... I'm used to them being square). I went to the one that was working, but I have not been able to tell which one is making the noise when the button is pushed. I can't feel a vibration on any of the 8 positions and I can't get my head in there close enough to hear anything beyond that the sound is coming from somewhere in the area. I'd like to avoid removing them all one by one since sometimes that can cause problems on older cars.

Anyone have a diagram showing which relay is which?

 

[mudgudgeon]

I might be zeroing in on the problem... maybe. I'm assuming the way this system functions is a relay gets a signal either from the Low position switch (sensor) that the transfer case is already in Low or from the dash mounted CDL button that the driver wants to lock the CD in H, then the relay tells the actuator to lock the CD? I'm thinking my relay is bad. Or am I missing the function and the relay is only for the indicator light, not telling the actuator to lock?

I've confirmed the switch works by putting it into a different '97 that also didn't have the CDL dash switch, and it works perfectly. The truck that is working - key in On position with engine off - the CDL will engage either by depressing the dash switch while in High or by shifting the transfer case into low gear. In both cases, locking center diff by CDL switch or transfer case gear shift, there is an audible "double click" coming from under the dash by the drivers side kick panel (right side in my case since this is a RHD). So.. me and the switch went back to the truck that isn't working...

This one... no sound at all with the dash CDL switch. When shifting the transfer case into Low range, there is a less pronounced single click from the kick panel, not the more defined "click pause click" on the working truck.


ECU or relay? The relay sounds are coming from the right /drivers side kick panel, so removed the kick panel and found this bank of 8 relays and one jumper (at least I assume the round ones are also relays... I'm used to them being square). I went to the one that was working, but I have not been able to tell which one is making the noise when the button is pushed. I can't feel a vibration on any of the 8 positions and I can't get my head in there close enough to hear anything beyond that the sound is coming from somewhere in the area. I'd like to avoid removing them all one by one since sometimes that can cause problems on older cars.

Anyone have a diagram showing which relay is which?

View attachment 3231772

Those relays are for things like blinkers and, power windows, heaters, EFI circuits

There's other stuff in the left side kick panel

The locker/4wd/ t-case control module is called an ECU, or computer control. If you were to open it up, it has relays integrated on a printed circuit board.

From memory, if you pay attention, you should hear a relay click. They are a small relay, so it won't be a loud click.

 

[mudgudgeon]

I might be zeroing in on the problem... maybe. I'm assuming the way this system functions is a relay gets a signal either from the Low position switch (sensor) that the transfer case is already in Low or from the dash mounted CDL button that the driver wants to lock the CD in H, then the relay tells the actuator to lock the CD? I'm thinking my relay is bad. Or am I missing the function and the relay is only for the indicator light, not telling the actuator to lock?

The diff lock actuator is powered through a relay in the 4wd ECU in the left kick panel (RHD HDJ81).
That relay is triggered either by you shifting to Low range which activates the low range position switch in the transfer case. This completes the power circuit to trigger the relay to provide power to the centre diff lock actuator.

CDL switch circumvents the need to shift to low range. It triggers the relay directly to power the CDL actuator whether you are in low range or not.

The relay is small, inside a box, inside the kick panel. You won't really hear it unless you listen for it.

The CDL cannot lock unless the gears in the mechanism are aligned. Sitting stationary, this is random luck if they align. Rolling back and forth in a straight line is also not likely to move the diff gears enough for them to align.

Find somewhere that you can drive in a large circle, or figure 8s. The turning/steering will cause the centre diff gears to shift in relation to the locking mechanism. If the CDL doesn't lock doing this, then you have an issue. If it doesn't lock in your driveway, don't panic yet. Go out and test it.

 

[sunrk]

As a side-note, the hf1a transfer case is *fully* mechanical and has only one electrical part - the switch for the '4wd' light. When it's in 4d high or low drive is divided equally between front and rear. That doesn't constitute a centre diff lock in my view.

 

[kgrove]

The diff lock actuator is powered through a relay in the 4wd ECU in the left kick panel (RHD HDJ81).
That relay is triggered either by you shifting to Low range which activates the low range position switch in the transfer case. This completes the power circuit to trigger the relay to provide power to the centre diff lock actuator.

CDL switch circumvents the need to shift to low range. It triggers the relay directly to power the CDL actuator whether you are in low range or not.

The relay is small, inside a box, inside the kick panel. You won't really hear it unless you listen for it.

The CDL cannot lock unless the gears in the mechanism are aligned. Sitting stationary, this is random luck if they align. Rolling back and forth in a straight line is also not likely to move the diff gears enough for them to align.

Find somewhere that you can drive in a large circle, or figure 8s. The turning/steering will cause the centre diff gears to shift in relation to the locking mechanism. If the CDL doesn't lock doing this, then you have an issue. If it doesn't lock in your driveway, don't panic yet. Go out and test it.

I drove the car around in both Low and High ranges in both cases with the CDL button depressed and not depressed. The center diff is clearly not locking in either case, so I really don't think it's just a matter of getting all the gears to line up.

Sorry if this is long winded or overkill, but I always find it helpful to understand as thoroughly as possible how a system works so I can try to think of a logical series of tests to get directly to the problem or eliminate things that can't be the problem... Let me paraphrase what I think you're saying mixed with what I'm reading in other threads and make sure I understand how this should be working:

The relay (inside the 4wd ECU) should send power to the actuator to lock the center diff under one of two conditions: 1) the Low Range Position Switch is closed, telling the ECU the transfer case is in low, or 2) the CDL dash button is closed telling the ECU the driver has chosen the locked position (I presume the ECU is also looking for a simultaneous open signal from the Neutral Position Switch so it only will lock the center diff in either High or Low range... maybe that's not necessary). After that, the Center Lock Diff Switch closes when it senses the center diff is locked, which tells the ECU to illuminate the center diff lock indicator light. Does all that sound about right?

If so, the I see the following reasons the center diff wouldn't lock up:
A) Everything's working, but gears not aligned
B) CDL Actuator failed
C) 4wd ECU is not getting closed signal from Low Range Position Switch
D) 4wd ECU is not getting closed signal from dash CDL Switch
E) 4wd ECU failed... either the relay inside has failed or the board is not processing the signals correctly
F) CDL Actuator isn't getting enough or any power

I think I've eliminated A by driving several laps around the neighborhood and D by installing the switch in a working car. I think I can come close to eliminating C since the center diff also will not lock in High range with the dash CDL Switch, which I know works, selected to lock.

Could I be missing a fuse? I've checked everywhere I can think of, but none seem blown. At the same time, none are labeled as having anything to do with the transfer case, transmission, or even engine so that I can make sure I'm not missing something.

I was going to test the 4wd ECU by swapping the units from the good vs bad truck, but I'll be damned if I can find it even with the glove box removed. I thought I located it based on photos of the part number and approximate location from the diagram, but it was the power steering ECU. Is it easily visible? I've removed the glove box as well... nothing that looks like it is readily apparent to me. There's one box just inboard of the emissions ECU, but it doesn't look like the part and the connector is on top of the unit where the diagram suggests the connector plugs into the bottom.

Any corrections on my logic? Any other direct and not terribly invasive or difficult tests you can suggest to either specifically diagnose the problem or at least eliminate options? My suspicion is it's not the actuator but something to do with the 4wd ECU, relay inside that box, or power to the actuator. The fact it worked once but then stopped seconds later sounds more like some kind of electrical failure, plus the fact it makes a clicking noise when entering Low but not when selecting from the dash CDL button.

Open to ideas.

 

[cruiserdan]

B was the issue in my case.

 

[kgrove]

B was the issue in my case.

That's where I'm leaning at the moment, but it's an expensive enough part I don't want to replace it without being more sure. How did you confirm yours?

I ran two more easy tests just to check them off my list...

I used a paperclip to short the connector for the Low Range Position Switch... no effect. Then I used a paperclip to short the connector on the Center Diff Lock Switch. As soon as I did that, the dashboard indicator light would come on.

So I'm even more sure than before, unless I've got multiple failures at the same time, the ECU is getting the right information (dash CDL button is confirmed to work and the system is still not responding correctly when I manually tell the ECU the TC is in low gear). Either the actuator itself is bad, the actuator is not getting power to actuate (relay or fuse that I can't find), or the ECU is not processing information correctly.

Out of curiosity, did you do your own actuator replacement? I've read elsewhere it's a one-banana job, but I'm finding that difficult to believe given how difficult the area is to access.

 

[mudgudgeon]

I drove the car around in both Low and High ranges in both cases with the CDL button depressed and not depressed. The center diff is clearly not locking in either case, so I really don't think it's just a matter of getting all the gears to line up.

Sorry if this is long winded or overkill, but I always find it helpful to understand as thoroughly as possible how a system works so I can try to think of a logical series of tests to get directly to the problem or eliminate things that can't be the problem... Let me paraphrase what I think you're saying mixed with what I'm reading in other threads and make sure I understand how this should be working:

The relay (inside the 4wd ECU) should send power to the actuator to lock the center diff under one of two conditions: 1) the Low Range Position Switch is closed, telling the ECU the transfer case is in low, or 2) the CDL dash button is closed telling the ECU the driver has chosen the locked position (I presume the ECU is also looking for a simultaneous open signal from the Neutral Position Switch so it only will lock the center diff in either High or Low range... maybe that's not necessary). After that, the Center Lock Diff Switch closes when it senses the center diff is locked, which tells the ECU to illuminate the center diff lock indicator light. Does all that sound about right?

If so, the I see the following reasons the center diff wouldn't lock up:
A) Everything's working, but gears not aligned
B) CDL Actuator failed
C) 4wd ECU is not getting closed signal from Low Range Position Switch
D) 4wd ECU is not getting closed signal from dash CDL Switch
E) 4wd ECU failed... either the relay inside has failed or the board is not processing the signals correctly
F) CDL Actuator isn't getting enough or any power

I think I've eliminated A by driving several laps around the neighborhood and D by installing the switch in a working car. I think I can come close to eliminating C since the center diff also will not lock in High range with the dash CDL Switch, which I know works, selected to lock.

Could I be missing a fuse? I've checked everywhere I can think of, but none seem blown. At the same time, none are labeled as having anything to do with the transfer case, transmission, or even engine so that I can make sure I'm not missing something.

I was going to test the 4wd ECU by swapping the units from the good vs bad truck, but I'll be damned if I can find it even with the glove box removed. I thought I located it based on photos of the part number and approximate location from the diagram, but it was the power steering ECU. Is it easily visible? I've removed the glove box as well... nothing that looks like it is readily apparent to me. There's one box just inboard of the emissions ECU, but it doesn't look like the part and the connector is on top of the unit where the diagram suggests the connector plugs into the bottom.

Any corrections on my logic? Any other direct and not terribly invasive or difficult tests you can suggest to either specifically diagnose the problem or at least eliminate options? My suspicion is it's not the actuator but something to do with the 4wd ECU, relay inside that box, or power to the actuator. The fact it worked once but then stopped seconds later sounds more like some kind of electrical failure, plus the fact it makes a clicking noise when entering Low but not when selecting from the dash CDL button.

Open to ideas.

Yes, I think your logic is good.

From here, I would start by disconnecting the actuator and power it with some jumper wires from a 12volt power source. It may be cooked electrically, or siezed.
You can get at it by putting a jack under the gearbox cross member, unbolting the cross member at the frame, and lowering it with the jack.
It's tight, but accessible this way.
Clearance between the head, and firewall will limit how much you can lower the cross member.

 

[kgrove]

Yes, I think your logic is good.

From here, I would start by disconnecting the actuator and power it with some jumper wires from a 12volt power source. It may be cooked electrically, or siezed.
You can get at it by putting a jack under the gearbox cross member, unbolting the cross member at the frame, and lowering it with the jack.
It's tight, but accessible this way.
Clearance between the head, and firewall will limit how much you can lower the cross member.

I might try this. Any idea what kind of power the motor draws? I've got a DC power source for running tests on stuff, but it's limited to 10 amps. I'm not sure I'd be able to get to a test like this for a bit as it's one I'd want to be more careful with.

Alternate direction: Anyone have tips on finding the 4wd ECU? I know it's somewhere in the vicinity of the passenger side kick panel / glove box, but I haven't found it yet. If it's somewhat easy to remove, I could swap units with the car I know works and see if that was the problem. (I did earlier tonight finally locate the fuse I think powers the diff lock... I must have looked over the top of it a dozen times as it is *clearly* labeled "DIFF"). My fear since I haven't found it yet is it's behind a bunch of other stuff, so I'd have to dismantle lots of stuff under the dash on two cars to test this idea.

If anyone has a different idea on how to prove whether the 4wd ECU is working correctly, let me know. I thought about testing the voltage at the Diff fuse, but I think I'd just be testing the battery voltage (usually fuses are closer to the battery). I guess if I have already disconnected the actuator per the above post, I can simultaneously check the signals reaching the actuator... royal pain, however.

 

[mudgudgeon]

The CDL actuator is protected by a 30 Amp fuse labelled DIFF.

The CDL actuator control circuit (switches) is supplied with power through a 10 Amp fuse labelled GAUGE. this fuse is also for the instrument cluster.

The EWD I have for a 1990 HDJ80 labels the. CDL actuator control module as a TRANSFER CONTROL RELAY.

It is not the same as the front and rear diff lock ECU. My mistake. My first 80 was a HDJ81 (RHD) and it had diff locks, and I know the ECU was in the left kick panel (passenger side)
My hzj105 did not have diff locks, and the CDL, and Diff lock control was all in one module in the left kick panel. Sorry for the misdirect.


I found a full EWD for a 94 US market FZJ80. The key points will be correct, but reverse the side everything is on.

First up, the TRANSFER CONTROL RELAY is high up in the driver side kick panel, for you the right side. It will be above the main relay panel you photographed earlier.

You're looking for location C4 (but mirror reversed)

Wiring diagram (again, not the right model, but the critical circuit information is there)

And EWD notes

 

[mudgudgeon]

The DIFF fuse is in the fuse panel in the dash by the ignition key

 

[kgrove]

The CDL actuator is protected by a 30 Amp fuse labelled DIFF.

The CDL actuator control circuit (switches) is supplied with power through a 10 Amp fuse labelled GAUGE. this fuse is also for the instrument cluster.

The EWD I have for a 1990 HDJ80 labels the. CDL actuator control module as a TRANSFER CONTROL RELAY.

It is not the same as the front and rear diff lock ECU. My mistake. My first 80 was a HDJ81 (RHD) and it had diff locks, and I know the ECU was in the left kick panel (passenger side)
My hzj105 did not have diff locks, and the CDL, and Diff lock control was all in one module in the left kick panel. Sorry for the misdirect.


I found a full EWD for a 94 US market FZJ80. The key points will be correct, but reverse the side everything is on.

First up, the TRANSFER CONTROL RELAY is high up in the driver side kick panel, for you the right side. It will be above the main relay panel you photographed earlier.

You're looking for location C4 (but mirror reversed)

View attachment 3233683

View attachment 3233686

View attachment 3233687

Wiring diagram (again, not the right model, but the critical circuit information is there)
View attachment 3233684

And EWD notes
View attachment 3233685

Awesome! Thanks... This explains why I'm clearly hearing noise in the right kick panel area (double click on the car that functions correctly, single click entering low range and no click when using the dash button to override the "lock" command). The picture of the transmission relay helps a ton... Now to find the bastard. Cross my fingers it's easy to remove and I can swap the units in the good bad cars to see if that relay is the problem. That's my leading hypothesis at the moment given the sound differences between the working and broken cars. If it is that relay, the part is in stock... not super cheap at about $160 with shipping, but can be found.

I've confirmed the DIFF fuse is fine both visually and then to double check my work, swapped it with a new one. It's not the GAUGE fuse either... visual inspection plus all the other gauges work fine.

 

[kgrove]

More detail... for anyone going down the same path later. The Transmission Relay is indeed behind the drivers side kick panel (right side for my RHD) and it is for sure the piece that makes the clicking sounds when you lock the center diff. It's mounted behind the ABS box, so to access it, I had to remove the ABS brackets and lay the box to the side, then there's a single nut attaching the Transmission Relay to the sidewall. On one of the cars, the ABS box easily flopped to the side out of the way after nudging a couple cables around. The other had an extra set of wires blocking the path and I had to remove one of the triangle mounting brackets from the ABS box to allow me to maneuver the box out of the way. The first photo below is with the ABS box already out of the way and the second photo shows the ABS box's normal position sort of blocking the Transmission Relay (the mounting bracket on the right side hasn't been re-attached in the photo).

So that's the news helpful to others. The bad news for my case is I swapped the Transmission Relay box between my two cars, but the center diff lock problem didn't move with the box, so the problem is not the Transmission Relay.

I'm going to stew on it a bit longer and see if I have a genius idea for other solutions or tests, but I'm nearly convinced... maybe I already am convinced and just don't want to admit it... that I need to replace the CDL Actuator. I'm sure I'd end up taking it to my mechanic for that... uncomfortable access without a lift and not something I'm ready to tackle right now.

 

[landcrshr]

Referring to this diagram:

You should have +12V on pin #9, and GROUND on pin #10 of the control relay in order to unlock the CDL.