How Dangerous Could This Be?

[imp]

May not be perfect place to ask this, but late '90s Explorers started showing up with "4X4 AUTO" as the normal running mode.

I have not heard this scenerio addressed: Driving on a freeway at 70mph a tire, front or rear, suddenly blows out..... The much-smaller flat tire must be interpreted as "slip" between front and rear axles, so the PCM would instantly lock the electronic clutch, tying the front and rear axles together.

Any reports on what happens? Does it help or make worse the already serious problem of possible loss of control of the vehicle? Any ideas, or facts, would be appreciated! imp

 

[IZwack]

Not that dangerous at all - because the flat tire would easily "slip" which would allow the driver to steer the vehicle.

 

[blakshukvw]

Sounds pretty f-n dangerous to me. LOL Man i can just picture it happening. That would definitely be impressive to see. Who volunteers their X to this little experiment?

 

[waskly]

May not be perfect place to ask this, but late '90s Explorers started showing up with "4X4 AUTO" as the normal running mode.

I have not heard this scenerio addressed: Driving on a freeway at 70mph a tire, front or rear, suddenly blows out..... The much-smaller flat tire must be interpreted as "slip" between front and rear axles, so the PCM would instantly lock the electronic clutch, tying the front and rear axles together.

Any reports on what happens? Does it help or make worse the already serious problem of possible loss of control of the vehicle? Any ideas, or facts, would be appreciated! imp

Shifts between 4x4 AUTO mode and 4x4 HIGH modes can be made at any speed. Listed below are the inputs and outputs needed by the 4X4 control module to execute a change between these modes.

Feature inputs:

Accelerator pedal position information (transmitted from the PCM)
MSS
Transfer case shift motor encoder contact plate position inputs A, B, C, D
Wheel speed (transmitted from ABS module)

i would think the car would take the blow out as a abs action first. same way when an abs sensor goes bad 4x4 don't engage on these trucks.

same as a car with a flat, when you drive it the 4x4 dont kick in.

 

[my pos mounty]

i would think the car would take the blow out as a abs action first. same way when an abs sensor goes bad 4x4 don't engage on these trucks.

same as a car with a flat, when you drive it the 4x4 dont kick in.

I disagree. I have been in this situation (although it was an AWD truck). It was not as "white knuckle scary" as you may think. Mine happened on the front, which made it easier to control than if it had happened on the rear. You can compensate for it by using the steering if it blows-out on the front, you can't compensate for it as easily if it happens on the rear of the truck.

The reason why you can not engage the 4 wheel drive when you have a bad ABS sensor is because the truck can't read the bad one, thus can not detect the differential in wheel speed.
If you have a blow-out, the truck can still read the sensors. I would say that your instinct would be to take your foot off the accelerator, so since the PCM uses throttle position to engage/disengage the 4 wd, I would hazard a guess that the auto 4WD would not be engaged. You would also (most likely) apply the brakes (even lightly) so the ABS sensors might activate the ABS pump to whichever end of the truck had the blow-out, attempting to equalize the wheel speeds in deceleration.

 

[waskly]

I disagree. I have been in this situation (although it was an AWD truck). It was not as "white knuckle scary" as you may think. Mine happened on the front, which made it easier to control than if it had happened on the rear. You can compensate for it by using the steering if it blows-out on the front, you can't compensate for it as easily if it happens on the rear of the truck.

The reason why you can not engage the 4 wheel drive when you have a bad ABS sensor is because the truck can't read the bad one, thus can not detect the differential in wheel speed.
If you have a blow-out, the truck can still read the sensors. I would say that your instinct would be to take your foot off the accelerator, so since the PCM uses throttle position to engage/disengage the 4 wd, I would hazard a guess that the auto 4WD would not be engaged. You would also (most likely) apply the brakes (even lightly) so the ABS sensors might activate the ABS pump to whichever end of the truck had the blow-out, attempting to equalize the wheel speeds in deceleration.

i was more thinking of 4dw and 4wd auto mode i didnt think of awd since the car would engage and disengage it.

i'm also guessing it would be worst in the rear blow out then front since the vehicle is rwd.

if it has traction control the blow out would cause an abs event first before the 4x4 event.

 

[Rudy602]

I've only noticed the 4wd engaging in "Auto" mode when I was giving it throttle but I never had that high speed blow-out scenario happen so who knows. Did I understand correctly that you can't engage 4wd if you have a bad ABS sensor? I'm sure my 4wd works and my ABS doesn't.

 

[imp]

Lots of additional questions arise here- hope I did not open a "can of worms". Still think it's worthwhile knowing the answers, if possible.

I assume to begin with all ABS sensors are working, as well as the 4X4 system. The ABS system "tells" the ABS hydraulic pump when to run only if the brake pedal is depressed while "slippage" is detected. This should happen regardless of which 4X4 mode happens to be "on".

I have never heard of, nor think it logical, that 4X4 operation only exists when the throttle is depressed, or that it "drops out" when the throttle is closed. If it did, closed throttle engine braking downhill could suddenly become dangerous.

Any other thoughts come up along these lines? imp

 

[snoranger]

I have not heard this scenerio addressed: Driving on a freeway at 70mph a tire, front or rear, suddenly blows out..... The much-smaller flat tire must be interpreted as "slip" between front and rear axles, so the PCM would instantly lock the electronic clutch, tying the front and rear axles together.

OK, So the axles are "locked" together. BUT, the differential will allow the smaller tire (flat) to spin at a different speed then the fully inflated tire. So, 3 tires are spinning at the same speed and on is spinning faster (since its smaller), whats the problem?

Yes, It will pull, but you should be able to safely stop.

 

[Rudy602]

I have never heard of, nor think it logical, that 4X4 operation only exists when the throttle is depressed, or that it "drops out" when the throttle is closed. If it did, closed throttle engine braking downhill could suddenly become dangerous.

In 4WD-AUTO, It seems like the 4WD only kicks in when there is slippage on a drive wheel which only happens when I'm giving it throttle, when I back off the gas, it goes back to 2WD. Is that not how it's supposed to work? I think the key word here is "AUTO", in 4HI, it's real 4WD on or off the throttle.

 

[IZwack]

Thats because of the design of the transfer case. The "ball-ramp" design uses the difference in RPM speed to actually squeeze the clutch together and "lock" the transfer case. The electromagnetic clutch only provisions for 4wd "lock", the actual difference in RPM between the two driveshafts is what forces the "lock".

 

[SoNic67]

My opinion is that there will NOT be a difference between the axels speed, it will be a difference between the wheels on the same axel (thats the front diff job to "solve").
The good front tire will still rotate at the proper speed, therefore forcing the front shaft to rotate at the same speed as the rear. There will be no locking of the central.

 

[imp]

....In 4WD-AUTO, It seems like the 4WD only kicks in when there is slippage on a drive wheel which only happens when I'm giving it throttle, when I back off the gas, it goes back to 2WD. Is that not how it's supposed to work? I think the key word here is "AUTO", in 4HI, it's real 4WD on or off the throttle.

You hit it "right-on", as I see things happening.

 

[imp]

OK, So the axles are "locked" together. BUT, the differential will allow the smaller tire (flat) to spin at a different speed then the fully inflated tire. So, 3 tires are spinning at the same speed and on is spinning faster (since its smaller), whats the problem?....

The "problem" is, the PCM "looks" at not the relative speeds of the 2 driveshafts to decide if there is slippage between front & rear, but rather 4 ABS sensors, which are reporting in the speeds of ALL 4 wheels. The flat tire, spinning faster than the other one on the same drive axle, represents a slippage to the PCM between front and rear wheels, does it not?

Am I missing something that's happening here? imp

 

[imp]

i would think the car would take the blow out as a abs action first. It would, IF the brake pedal were being depressed, but it's not. Vehicle is cruisin' along under power, ABS can't do nuthin' with brakes until brakes are applied.
same way when an abs sensor goes bad 4x4 don't engage on these trucks. Sensor failing to give "slippage" info, sure, 4X4 AUTO don't know it should turn on.
same as a car with a flat, when you drive it the 4x4 dont kick in.

Wait now, you saying if you drive a 4X4 vehicle with a flat tire, there is never 4-Wheel drive available? imp

 

[imp]

Thats because of the design of the transfer case. The "ball-ramp" design uses the difference in RPM speed to actually squeeze the clutch together and "lock" the transfer case. The electromagnetic clutch only provisions for 4wd "lock", the actual difference in RPM between the two driveshafts is what forces the "lock".

Maybe a key to the puzzle here. What you're sayin' is that the electromagnetic clutch ONLY activates in 4X4 HIGH and 4X4 LOW, and the "automatic" feature is completely mechanical as far as locking the front & rear drivelines together in 4X4 AUTO. That would mean the ABS input has NO EFFECT when driving in 4X4 AUTO.

Can you verify that the clutching is actually 2-part, mechanical (ball-ramp), and electronic? This is a new one on me, and seems entirely plausible. Unfortunately, the Shop Manual does not really show the clutch in detail. imp

 

[waskly]

Wait now, you saying if you drive a 4X4 vehicle with a flat tire, there is never 4-Wheel drive available? imp

you can put a car in 4x4, will it go into 4x4 auto with a blow out, no. unless you nail the gas pedal instead of the brakes and then it probably still wont.

now the abs sees 1 wheel spinning 3 not. it throws 4x4 on to see a front and rear spin. since 4x4 auto is rwd. take a 4x4 up in the air, and accelerate it, it can engage 4x4 or it wont depends how hard you are on the gas pedal. how much wheel speed differs in the rear of the car,

at 70 mph cruise, with tip in on gas, and a blow out, frontal 4x4 wont go on.
on a rear tire, i doubt it less you gun the accelerator after the blow out.

 

[iwannabamedic]

May not be perfect place to ask this, but late '90s Explorers started showing up with "4X4 AUTO" as the normal running mode.

I have not heard this scenerio addressed: Driving on a freeway at 70mph a tire, front or rear, suddenly blows out..... The much-smaller flat tire must be interpreted as "slip" between front and rear axles, so the PCM would instantly lock the electronic clutch, tying the front and rear axles together.

Any reports on what happens? Does it help or make worse the already serious problem of possible loss of control of the vehicle? Any ideas, or facts, would be appreciated! imp

the bold is the question, have spare that is the same size as rest of tires

 

[Rudy602]

I think he meant the tire was smaller because it was flat - not because it was a different size. I think the biggest problem with a high speed blowout it one of these is the high center of gravity and the narrow footprint. The next generation came through lower and wider and were much less prone to rolling over. ---"but we were all tooled up so we figured we might as well run a few million, we'll make the next ones safer." I know they blamed it all on the firestones but if I had a choice of which one to be in during that scenario, I'd pick lower and wider. [I'm talking all SUV"S here, foreign and domestic not just Ex's - they all had a too high CG those years and they all changed]