4x4 operation?

[topo4u2]

This may be a short question to a long answer but if someone has some time could they explain the 4x4 operation in my 95 control trac. I would like to know when the electric motor on the side of the transfer case comes into play(damm thing looks like a wiper motor) and what about that vacuum deal on the front axle.
You can make the explanation as short as possible.

Thanks.

 

[Crankcase]

Basically, when you are driving down the street in 2WD, the front driveshaft is spinning, as it is connected to the front wheels all the time.

When you turn the little switch on the dash to 4X4, the motor on the side of the transfer case pushes the case in to 4X4 mode, locking the front driveshaft to the motor power. There are many more details, but you wanted short and sweet.

 

[Robert]

Not even close. His driveshaft does not turn when driving down the road since the '95 & '96 Explorers have a vacuum disconnect on the front differential. When in 2WD mode, the front differential is disconnected from the driveshaft. When he puts it into 4x4 Auto the front differential is engaged. There are electric clutches inside the transfer case that are energized when the two speed sensors inside the transfer case detect a difference in speed. When a difference in speed is detected, power is diverted from the rear driveshaft to the front electrically. After several seconds of both driveshafts turning at the same speed, the power will be lessened from the front until it is all going to the rear driveshaft. When he puts it into Low, the transfer case shift solenoid is activated which turns a worm gear that switches from the standard 1:1 gear to the low range gear. The only time the shift solenoid comes into play is when shifting from low to high or from high to low.

 

[Robb]

Robert,
I hate to ask such a question, but if you have a little time, could you type out exactly what is happening at each point with the control-trac. I would love to read a very elementary explanation of the whole thing. Everything from what is happening with the driveshaft to the auto hubs. A simple, yet complete explanation of the entire 4x4 system would be greatly appreciated.

Anything would be nice,

Robb

 

[Robert]

Tom Wilk already wrote up a very detailed explanation. His explanation is the second post in this link:

http://www.explorerforum.com/forums/showthread.php?threadid=10166

OK, I just read through Tom's post, he didn't write the explanation, he found it through a search. There is one thing I noticed that was incorrect thoughl. The GEM doesn't get its speed input from the ABS sensors. There are two separate sensors mounted in the transfer case that measure the rotational speed of the driveshafts. When it detects a difference in speed it reacts. The Auto mode works the same for the '95-'01 Explorers. The '95 & '96 Explorers have 2WD, Auto and 4x4 Low.

On a '95 & '96 when in 2WD, the front differential is disconnected via the vacuum disconnect and no input is taken from the speed sensors. When the switch is placed in either Auto or Low the front differential is activated. When the switch is put in Auto, then the system reacts as Tom posted. When the switch is put in Low the clutches are set so there is a 50/50 torque split between the front and rear driveshafts. In other words the speed sensors are taken out of the loop again, but the transfer case is locked in. Also when in Low, the transfer case shift solenoid is engaged which switches to a lower ratio gear (I think it is about 2.3:1).

On a '97 & up they have Auto, 4x4 High and 4x4 Low. The Auto is the standard driving mode and works the same as above as far as the transfer case is concerned. One difference between the '95 & '96s though is that Ford did away with the vacuum disconnect on the front differential. The front driveshaft is permanently connected to the front differential. It will spin any time the front wheels turn. When a '97 and up is put in High or Low the transfer case works the same as it does in just Low on the '95 & '96s except that when in High the transfer case shift solenoid isn't engaged.

 

[Robb]

Thanks Robert!
That was exactly what I was looking for.
Robb

EDIT:

When things get a little more slippery, the system reacts. The wheel speed sensors that are part of the ABS system (anti-lock brakes) are used to detect rear wheel spin. If the computer senses that the rear wheels are turning faster than the front wheels, the computer automatically tells the clutch to be engaged for a longer portion of the time

Does this mean that if I lost my ABS, my control-trac would not perform properly? I am researching a SAS and was thinking of just losing the ABS all together, but this makes it seem I am heading for disaster taking that route.

 

[Robert]

No read my edit post above yours The Control Trac doesn't use the ABS sensors, it uses the transfer case speed sensors. They are completely different sensors. The ABS sensors are located on each front wheel and on the rear differential. The transfer case speed sensors are located in (you guessed it) the transfer case.

 

[2001ExpSport]

Both front a rear axels are live. Meaning they are always turning, including the driveshafts. The rear shaft has continuous power while the front shaft gets power only if selected manually or by C-Trac sensers. The front drive shaft and axel free spin under 'normal' driving conditions without power. With C-Trac sensors determine the driveshaft speed and relay info to the electric clutch to engage power to the front drive shaft. This system uses a GEM(Generic Electronic Module) to control automatic shifting. The automatic shifting occurs only in 4WD high. When the shift selector is in 4Lo a cam selects the proper gear.

1997+ models with 5.0L engines were available with the AWD system. This system has no driver controls and uses full time 4WD.

1997+ V6 models had the A4WD option. This did have 3 driver controlled selections(AWD,4WD,4LO) and operated the same as C-Trac in the AWD mode.

 

[2001ExpSport]

Originally posted by Robert
Not even close. His driveshaft does not turn when driving down the road since the '95 & '96 Explorers have a vacuum disconnect on the front differential. When in 2WD mode, the front differential is disconnected from the driveshaft. When he puts it into 4x4 Auto the front differential is engaged. There are electric clutches inside the transfer case that are energized when the two speed sensors inside the transfer case detect a difference in speed. When a difference in speed is detected, power is diverted from the rear driveshaft to the front electrically. After several seconds of both driveshafts turning at the same speed, the power will be lessened from the front until it is all going to the rear driveshaft. When he puts it into Low, the transfer case shift solenoid is activated which turns a worm gear that switches from the standard 1:1 gear to the low range gear. The only time the shift solenoid comes into play is when shifting from low to high or from high to low.

That is true on the Ranger, not the Explorer.

 

[Robert]

That is also true on the '95 & '96 Explorers which have a 2WD, Auto and Low setting.

And, it is not true on the Rangers. They have a manual switching transfer case which has 2WD, 4x4 High and 4x4 Low. The Rangers do not have an Auto mode.

 

[dogfriend]

another question

And I HAVE searched for this, so don't tell me to search. (Last time I asked about control trac I got thrashed)

I think that Robert's description is correct, from what I have read and from the description in the factory manual. If you look at the EVTM for 97, the schematic shows how the power is supplied to the electronic clutches and shift motor. All of the logic for this is controlled by the GEM. However, on the C-Trac schematic, they also show a signal to the GEM supplied by the ABS system. This would imply that the GEM uses info from the ABS to determine the logic for C-Trac, but it isn't explained how that info is used.


Does anyone know if/how the ABS signal is used? It isn't to determine if rear wheels are slipping, because the driveshaft speed sensors do that. I don't think that it is used to determine vehicle speed (you have to be moving less than 5 mph to engage 4low) because the VSS supplies that info.

 

[Robert]

The reason the ABS signal goes into the GEM is because the GEM also acts as a diagnostics monitor for sending DTCs out. When there is a fault in the ABS, the GEM will send out one of two brake related codes through the GEM. The ABS and Control Trac have nothing to do with each other. The GEM controls a lot more than just the Control Trac. It also controls the following functions:

1) Heated backlight and mirror timer
2) Windshield wiper control and interval timer
3) Rear interval wiper control
4) Chime warning
5) Battery saver
6) Courtesy lamps
7) Interior lamp relay
8) Panel dimmer switch
9) Remote anti-theft personality (RAP) module
10) Delayed accessorry
11) One-touch window
12) Two-step unlock
13) Diagnostics monitor

 

[dogfriend]

Quote: The ABS and Control Trac have nothing to do with each other.

I don't agree with this statement. Bear with me for a second ok?

If you look at the schematic for 34-3 Electric Shift Control you can see CKT 539 which is labeled "ABS Input". It strikes me odd that they would include this circuit, because they don't include all of the other unrelated GEM circuits (windshield wiper controls, heated mirror, etc). They only include the circuits that have impact on the Electric Shift Control. If you go to 42-2 Anti Lock Brakes, you can see that CKT 539 connects to the ABS Pump Motor Relay output. My interpretation is that this circuit sends a signal to the GEM every time that the ABS Pump Motor is energized.

Also, I read the original patent (5485894) for the Contol Trac system. In the description, they make this statement:

"The system also includes a two position brake switch which is activated when the vehicle braking system is activated and provides a two state logic signal indicating that the vehicle brake pedal is depressed. A signal may be provided from the ABS braking system of the vehicle, if so equipped, which indicates that the ABS is active. Such an ABS signal may replace or be in addition to the signal from the brake switch."

The brake switch is the BOO switch which has an input to the GEM. They don't give any additional information about how the ABS signal is used though.

Also, there is a separate circuit from the ABS module that supplies the Data Link Connector, so there is no reason to relay the signal thru the GEM.

Another interesting topic discussed in the patent is the use of a steering angle sensor to help modify the modulation of the clutch engagement to account for different wheel speeds when turning. It appears that Ford chose not to implement this feature.

 

[Robert]

I remember seeing a couple of years ago that the ABS was disabled when the Contol Trac was put into 4x4 Low. I wonder if it is a part of that. I haven't tried hard enough to lock up my tires when driving in Low so I don't know if that happens or not. When I looked up the GEM capabilities earlier there were two brake parameters that do go to the GEM but they had nothing to do with the Control Trac operation. I couldn't find any information linking the ABS and Control Trac together. It's been a few years since I have read the patent documents on the Control Trac so I can't say for certain if the ABS and Control Trac are related or not. I just couldn't find any link between them in the GEM or Control Trac documentation from the Ford Service CD.

 

[dogfriend]

The signal from the ABS that is on the Electric Shift Control schematic is from the ABS pump relay, so it is only on when the pump is on.

I'm just curious how the logic works and why it would matter to the Control Trac if the ABS is active. The patent description refers to this, but doesn't explain why the ABS signal is needed or desireable.