Have you been driving in 4wd-auto for most of the vehicle life, Maybe the clutch is just worn out. I don't suppose you have access to a hand held fluke scope-meter. you would be able to view the square wave pulse and see if it is actually correct. it would have a frequency of 33hz.

the period of the wave should be 30mSec and the On-Time should be variable, but timed as described below.

There is a lot of talk about why this PWM method is used. Most people think it it to maximize current in the coil. But I believe the more correct explanation is as follows.

If a straight DC signal is applied to a coil then after a short time period the coil would appear as a electrical short to the device supplying the DC signal, which would mean the current through the coil would be as high as possible and the DC voltage source would either be loaded down and not supplying voltage anymore. or if the DC supply was strong enough it would burn out the coil.

If a AC voltage is supplied to a coil then the coil would be producing a magnetic field (inductance) that would mimic the voltage applied but be 90degrees out of phase (the magenetic field would be pulling then pushing) the net magnetic force would be zero and the clutch would never engage

If a Pulsating DC signal (square wave) was applied, and timed so that the max on time was equal to the time constant of the coil (the Time Constant is a function of the internal resistance of the coil and the inductance of the coil and is defined as the time it takes the current to build up to 86% of it's maximum.) then the magnetic effect of the coil would be maximized without the power supply being loaded down and without burning out the coil. as the on time is reduced the magnetic effect would also be reduced thereby lowering the holding power of the clutch.

This explains why there is a diode in the circuitry . to make sure any negative voltage gets shunted to ground so the signal to the coil is really a modulated DC.

this also explains what the TOD relay is. it is really a Filed Effect Transitor (FET) amplifier that boosts the low power signal coming from the GEM.
The GEM supply a low power, variable frequency signal that is proportionate to the desired holding power of the clutch.
The TOD boosts the power of the signal so that it is srong enough to operate the clutch coil (I imagine the coil must be powerful enough to hold the clutch in. this would take a lot of juice).
the diode in the transfer case shift relay is for rectification purposes (making sure no AC gets to the clutch coil)

i think everyone who is telling us that no voltage means the clutch is engaged is incorrect. I was very confused when I first started looking at this, but as the 33hz thing kept popping up I started believing it more and more.

the above description is only my opinion of how it works, but I really believe it is correct. my background is 4 years of technical electronics in high school and 4 years of electrical engineering in college.

 

CNTHOR:

The 33 Hz info came from the U.S. patent #5485894. You can access the patent office online, I’m not sure the address but just searching for “U.S. patent” or some version thereof will get you there thru’ MSN search engine. When you get to this website, just enter the number in the search field. Given your educational background, you’ll probably find this a lot more stimulating than I did.I believe you're right with the current to engage, no current to disengage theory.

Thanks for checking the resistance for me, I really appreciate that. I don't think I have to check the inductance knowing that there should at least be some continuity across the coil. Since you got such a low resistance value, you must concur that I’ve either got a ground problem or the clutch coil is kaput. I changed the fluid last week and it worked, then quit again the next time I tried it. Hopefully just a bad ground, the coils are pricey units.

Tomorrow, the unit’s going to a trans. shop. I’m going to stick around & have look. I would normally do it myself, but I don't have the time (too busy emailing, I guess).

 

Clutch coil replaced and all is well again. Thanks for all the input.

 

Just out of curiosity, How much did it cost to get the coil replaced?

My dealer told me that they consider the clutch coil to be a non-serviceable item and wanted $1700 for a ne transfer case.

Of course, I never bought into there line of bull. And I later found out it was only a bad speed sensor

 

Clutch coil was $195 Canadian (around $120 U.S.). It's easy to replace once the t-case is out of the unit. The transmission shop that did the work didn't have much experience with these units, so I I helped them diagnose. Shops aren't always receptive to customers looking over their shoulder... they also wanted to charge me a bench charge (to take the unit totally apart, inspect it, clean it, and install new seals). Since all I wanted was to have the coil replaced, it took some negotiating (do this before they have the case on the bench), but I got them to reduce the labor charge considerably and they installed new seals (3) while they were at it.

The new coil has 3 ohms of resistance, CNTHOR. Before I did any any further investigating, I should have asked you what resistance you got and it would have saved alot of hassle. Maybe all these posts will help others in diagnosing their 4405 issues.

 

Fellas I garantee your seemingly endless relentless Investigateing and Info will be priceless tomorrow when I attempt to diagnose my 4wd with the same symptoms. Hopefully I can see if it is the same problem with a simple resistance test (thanks to you guys). The only thing that I did not see was what the resistance was when the clutch coil was bad. I relize that probly this will not be the same but it will give me more of a guideline.

 

In the first post on this thread IAMSAMMY stated that he had no continuity. most meters define no continuity as anything above 50 or 75 ohms

 

Most of the 4wd problems have been due to bad driveshaft speed sensors. are you getting the 6 blinks every minute on the 4wd and 4wd-low lights?

 

When I said "no continuity", I meant a resistance so high that I didn't even get a reading with the meter on the highest resistance range. I'm guessing the coil should have somewhere below 10 ohms resistance if it's good.

 

here it is

 

to the top

 

I have a 98 with a 4405 it just stoped working the lights say its in 4x4 but its not,and there is no indication that it is not working other than the back wheels spinning. I there anything I can check or change before I take it to the dealer and get reamed!! I called them and said I would take out the transfer case and bring it in they said they had to have the whole thing. I just don't trust these guys

J.D.

 

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