4x4 trouble shooting questions

[Dave85zxt]

I'll start off with a little history. My 97 4.0 SOHC control-trac has been experiencing a 4x4 bug. At first it showed up through the 4x4 only working when the truck was cold. After the truck would warm up, it would no longer work. I bought a service manual and followed the pinpoint test in order to identify the problem. Here is a short list of what I tested and the results:

- The brown wire that activates the clutch was reading only 4.5V when the selector switch was set to 4x4. (its supposed to read 12V according to the manual)

- The 4x4 and 4x4 Low lights work properly. No flashing or other indications of a problem.

- The 4x4 low actuator motor works properly. It shifts the T-case into low but only 2wd.

- The solid state clutch relay checks out ok per the manual's method.

- There is 12V to the solid state clutch relay.

- There are no shorts in the brown wire that delivers power to the T-case clutch.

This all led me to the conclusion that the GEM is faulty (per the manual) so I replaced the GEM and the 4x4 still does not work properly. I am about to bring it into the dealer and just suck it up. I have a few questions before I take the leap.

1. Could a faulty vehicle speed sensor (used for the control-trac) cause the GEM to not engage the T-case clutch?

2. If I were to bypass the GEM to engage the cluch would it cause any damage? I am thinking about installing a switch and standard automotive relay to supply 12V directly to the brown wire that activates the clutch.

Thanks for the help.

 

[gijoecam]

Admins: Please move to the trans and t-case forum....

Preface: I'm not a Control Trac expert and I don't play one on TV. But I've learned a few things on these boards in the last few years, and I've been wrong more than a few times.

OK, lets address the issues one at a time...

I'll start off with a little history. My 97 4.0 SOHC control-trac has been experiencing a 4x4 bug. At first it showed up through the 4x4 only working when the truck was cold. After the truck would warm up, it would no longer work.

Can you please describe the problem a little more in-depth? What do you mean by 'No longer work?' Does the light come on? Does it lock the t-case in 4high or not? Does 4auto function at all? Does it shift into/out of 4low?

- The brown wire that activates the clutch was reading only 4.5V when the selector switch was set to 4x4. (its supposed to read 12V according to the manual)

Normal if you're not moving in 4auto, 4high, or 4low. When the system is operating at minimum duty cycle (i.e. when stopped) the voltage read by a DMM is usually right around 4.5.

- The 4x4 and 4x4 Low lights work properly. No flashing or other indications of a problem.

Guess I should have read the whole thread first... LOL!

- The 4x4 low actuator motor works properly. It shifts the T-case into low but only 2wd.

Yup should have read the whole thing....

- The solid state clutch relay checks out ok per the manual's method.

Good.

- There is 12V to the solid state clutch relay.

Good. Means the fuse is OK.

- There are no shorts in the brown wire that delivers power to the T-case clutch.

Also good.

This all led me to the conclusion that the GEM is faulty (per the manual) so I replaced the GEM and the 4x4 still does not work properly. I am about to bring it into the dealer and just suck it up. I have a few questions before I take the leap.

I wouldn't have jumped to that conclusion without hooking it up to an NGS scanner and viewing the GEM inputs and outputs (specifically the duty cycle the system is calling for) first.

1. Could a faulty vehicle speed sensor (used for the control-trac) cause the GEM to not engage the T-case clutch?

I suppose it's possible, but typically a faulty OSS sensor causes the system to detect a problem and throw a trouble code (which causes the 'blinkenlights' problem). The other common failure mode I've read of is it fools the system into thinking you're getting wheel slippage and locks it in all the time. I've never heard of a bad sensor causing the system to NOT engage (although I'm no expert).

2. If I were to bypass the GEM to engage the cluch would it cause any damage? I am thinking about installing a switch and standard automotive relay to supply 12V directly to the brown wire that activates the clutch.

Nope. I've done just that. Read the 'Comprehensive Brown Wire Mod Thread' stickied in the Transmissions and Transfer Case Section. The short version is pull power from the cigarette lighter and send it down the pipe (brown wire) and (if everything is working) your t-case will lock.


Now, that being said, in your case, I would try doing just that as a part of the troubleshooting, but not right away.

First, I'd put the truck on stands, then engage the 4wd, and let it idle in park. Then, grab the front driveshaft and see if you can turn it. If all is working as it should, you should NOT be able to spin it. If you can, then we know the case is in need of surgery (since your checks pretty well covered all the electrical checks). If it does not turn, then we need to look farther forward at the front differential and axle.

Good job troubleshooting thus far! You've almost got it pinned down!

 

[Dave85zxt]

A little clarification:

- The 4x4 system works properly when I first start the vehicle. I can feel the difference in turning. I have also put the truck on jack stands and checked that the front drive shaft turns when its supposed to.

- After I drive the truck for a few miles the 4x4 no longer engages.

- I haven't measured the voltage to the T-case when the vehicle is warm yet. I will do that soon.

- I will also put in a switch to feed 12V directly into the brown wire in order to see if the clutch engages or not. Do I need to break the brown wire circuit or can I just T into it?

If the electrical system is not faulty then I suspect that the solenoid for the clutch is starting to wear out and heat is affecting its operation.

Is the solenoid exposed to T-case fluid? I don't think that the T-case fluid has ever been changed (i think its not needed per the manual). But in any case, maybe the fluid is burnt and not lubricating the clutch and or solenoid properly once its warm.

Thanks for all the help.
Dave

 

[Dave85zxt]

Oh yeah, I forgot, I don't have an NGS sensor available. I think I saw them in the $900 range and above. I guess I am better off bringing to the dealer if I need to hook it up to an NGS scanner.

 

[gijoecam]

OK, if it works at first, then there's something else going on.... after it warms up, if you flip the switch to 4high, does the light come on? Does the range selection still work at that point?

There is no solenoid for the clutch... it's controlled by both the GEM and the TOD relay which are in the dash. I suspect an issue with the transfer case clutch coil, but what specifically, I don't honestly know. To my knowledge there is no thermal overtemp switch or anything like that built into the system, but I can't say for sure. That's why an NGS tester is so useful in this case... you could view the GEM inputs and outputs and see where the failure is... if the GEM is calling for 100% from the TOD relay and you're getting full 12V at the TCCC (brown wire at the t-case) then we know for sure it's the t-case.

Wish I could be more help here.... sorry.

-Joe

 

[Glacier991]

The control trac aspect of the 4x4 engages when the coil activates the ball ramp clutch apply mechanism. There are pictures in the 4405 Transfer case rebuild diary.

A bad electromagnet coil could cause a problem.... but that seems an unlikely candidate.

I do not have enough expertise on these to know if the clutches routinely get burned, but that could also be the case....

I am guessing that you are not getting a 12V feed to the electromagnet and hence not getting lockup in the clutch/chain.

 

[Dave85zxt]

I doubt its a bad electromagnet or clutch because I can really feel the drive train "bind up" when the 4x4 does work. This tells me that it is transferring a good deal of torque and the mechanicals should be fine.

I measured 4.5V on the brown wire at the transfer case under the following conditions. Key in the "on" position but the vehicle not running. Switch in the 4x4 position. Should I be measuring 12V in this condition?

After the truck warms up and the 4x4 no longer engages the 4x4 light still comes on and the low range still works properly but only with 2wd.

 

[Dave85zxt]

Another update:

The 4x4 no longer works at all. No flashing lights.

I measured 6.5V on the brown wire at the T-case with the truck running in Park and the selector switch set to 4x4 high.

I then applied 12V directly to the brown wire and still no 4x4. Am I correct in assuming that something (clutch, magnet, etc.) is shot in the T-case?

Why in the heck am I only measuring 6.5V in the brown wire?

 

[gijoecam]

I then applied 12V directly to the brown wire and still no 4x4. Am I correct in assuming that something (clutch, magnet, etc.) is shot in the T-case?

I would concur. Check something else: Disconnect the battery, both positive and negative leads. Then, unplug the connector at the transfer case. Measure for continuity to the ground cable connection through the brown wire in the connector that leads into the transfer case. You should have a path to ground. If you don't, there may be an open in the circuit inside the t-case. I suspect you'll have continuity to ground and there's something mechanically wrong, but you never know.

Why in the heck am I only measuring 6.5V in the brown wire?

The system varies the duty cycle of the clutch through the TOD relay. Picture it like a faucet that throttles the water flow coming into a hose. Even though you have 50 PSI up-stream of the valve, if you only allow a little bit of flow into the hose, you're only going to have a little bit of pressure in the hose. If you open that valve all the way, you'll have more pressure in the hose.

What you're reading in that wire is the hose pressure, not the system pressure. Even though the battery is reading 12V, the TOD relay only allows a little bit of juice into the wire, so it essentially throttles the current into the Transfer Case Clutch Coil. The result is that your meter reads a reduced voltage when it's at the minimum duty cycle. This is normal. If you put it in drive while measuring that voltage, you'll see it change when you start moving. Those of us that have installed lighted switches or LEDs on that brown wire observe a changing intensity of the light with speed, throttle position, and switch selection. Completely normal as the system varies the duty cycle over its range of programming. Regardless of the switch position, the system operates at a minimum duty cycle when stopped. No sure why.

The real measurement one would want to take would be the measurement of the current through that brown wire, not the voltage, but that's much more difficult to measure. I've been meaning to try that for months, just never got around to it. The current is a true measure of the power consumed by the system.

The bottom line is that as long as you have *some* voltage down there, and it increases when the vehicle is moving, it would appear that the TOD relay is working. That voltage should jump up to 12V when it tries to lock the t-case.

-Joe

 

[Dave85zxt]

Thanks for the explanation. I am a mechanical engineer so I understand how a PWM signal works and how the VOM is averaging the input. I guess I should have stated my question a little better but you did answer it anyways. I thought that the brown wire was supposed to see 12V 100% duty cycle all the time in 4x4 high. I guess I was wrong. I'll pull the battery cables and check the continuity to ground tomorrow. If that's not it I guess its time for a T-case rebuild.

I took a look at the T-case rebuild sticky (awesome resource!). Can anyone recommend a source for parts?

 

[Glacier991]

www.tbtrans.com is about the least expensive I have found

 

[Glacier991]

The ball ramp clutch apply assembly has been known to break.... it could be something mechanical inside. I'm thinking you might want to go ahead and split the case and check it out. It really is not that difficult a job surprisingly enough. The Diary is a good start for disassembly, and I hope to maybe finish up the reassembly in the Diary next week....with the new Omega case halves.

 

[Dave85zxt]

Now this is getting ridiculous. Last night I was playing with the 4x4 again and it worked! It worked in 4x4 high and when I delivered 12V directly to the T-case.

The problem is obviously in the T-case because bypassing the GEM didn't make a difference. What could be causing an intermittent problem like this in the T-case?

I can only think of electrical causes (bad ground, intermittent wire contact, etc). I would think that any mechanical failure would be permanent in nature and wouldn't work part of the time.