Four Wheel Drive

[Rawhead]

Hello:

We have had 2 weeks of freezing rain and snow. I have a long drive to work. It is a real pain to drive 10 miles per hour for 75 miles to work because my 4-wheel drive disengages around 10 to 20 miles per hour.

Anyone know of a 4-Wheel Drive selector switch bypass that can be done so that I make my RAv 4 do what I need it to do, rather than it disengaging at a predetermined time? Maybe one of the pin's on the back of the switch being grounded to override the vehichles computer etc. I don't know , I'm no mechanic/electrician. It just seems logical to me that if something tells the vehichle to disengage 4-wheel drive at a certain speed, it must not be to hard to override it or simply eliminate it. I don't plan to drive any higher than 35 to 45 miles per hour in inclement weather , but 10 to 20 miles per hour is rediculous when all I need is a little better controle on a wet or slick surface for the highway. It's not like I'm gonna be mountain climbing in low range or anything like that.

Any information would be appreciated: Switch schematics , write ups on this ; Anyone else that has thought of or has done it before?

Thank You;

Jim Pennell

[anchorman]

Hello Jim and welcome to the club.

I understand what you are saying but I don't fully understand how it manifests itself as a problem to you.

The 4WD system essentially comprises of a variable clutch in the nose of the rear axle. It can transmit anything up to 45% of the drive to the rear axle and is completely variable down to disengaged (no drive). When and how much it takes up the drive is dependant on what is happening to the car in terms of conering/braking and how hard you are pressing the accelerator. If you floor the pedal at 70mph it will engage to some extent (maybe fully if you are going uphill) but if you are off power at 20mph it will be disengaged. Are you suggesting it seems to disengage at 20mph when under power? If this is the case then I think what you might be experiencing is something different. Although the car doesn't have a traction control button as in the normal sense, i.e. a FWD car starting might transfer some weight to the back and cause wheelspin, this is eliminated with the RAV by it automatically taking up the necessary amount of RWD to compensate. It does however monitor left/right wheelspin both at the front and the rear and in detecting any will back off the power as a normal one axle system does - this cannot normally be over-ridden without pressing the 4WD lock button on the facia. On the odd occasion it has snowed while I have had mine it has not been a problem but there is a difference between driving through snow which has some friction and on ice which has little or non.

In the first instance you can press your 4X4 lock button which will work up to 25mph. If you want to know how to fudge that function to work at higher speeds then you need to talk to one of our electronics guys - shcm or JHRC which you can pm or sit tight and they will look in anyway. My feeling is that you would need to interfere with the speed signal recieved by the ECU but that might be more of a reprogramming issue than for example earthing a sensor.

The other thought that springs to mind is that you can disable the active drive system. It is a fiddle but you can do it and it might be worth a try. You must remember Jim that in doing so you take away a system that has been added for your safety and if you lose control of the car you will get no assistance from those systems. The other consideration is that I don't know exactly to what extent the systems are disabled and you might find that the traction control still operates. The instructions on how to do it are here;

http://toyotaownersclub.com/forums/index.p...mp;#entry707773

To my knowledge there is no quicker way as they probably don't want you to do it but again one of the electronics guys will give you their thoughts on it.

Good luck and please let us know how you get on.

Regards

[shcm]

Hello Jim,

OK, I've had a look at the 4WD system wiring and description and the short answer is it's probably not impossible, but maybe not that easy.

Here's the long answer. I don't know how technical or electronically minded you are, so forgive me if this goes over your head.

The 4WD ECU would appear to get all its information about vehicle speed, yaw rate etc, over the CAN bus. (The CAN bus is a vehicle network - a vehicle "internet" if you like - where ECUs can communicate with one another and share info. It's been around for years. Some manufacturers are beginning to look at something faster with improved data integrity - called "Flexray" - anyway, I digress).

So, it's not just a simple matter of disconnecting a speed signal input. To do that, you would have to intercept the CAN data and send modified data to the 4WD ECU. Maybe not an unsolvable for me (I work with this stuff), but (without wanting to sound big-headed) probably close to impossible for most people.

The only real output from the 4WD ECU is the linear solenoid, which would appear to control the amount of coupling from the prop to the rear wheels via the diff. Now, perhaps Anchorman can check on the mechanical side of things, but it would appear the amount of coupling is proportional to the amount of current through the solenoid. This current is controlled by the 4WD ECU.

One solution might be to disconnect the solenoid from the ECU and inject current into the solenoid separately. What I don't know (or I haven't found it yet) is how much current is required for "full 4WD coupling". It may be that for full coupling 12V is effectively applied across the solenoid. That would make it easy. If it's not designed for that, then connecting 12V directly may well end up damaging it (heat). Simple current limiting would be with a resistor, but that too may get hot. Also, the 4WD ECU is probably going to detect that the solenoid has been disconnected and you are likely to get a fault flagged. It may well also affect other handling characteristics of the vehicle if the solenoid is fully on and not being controlled.

Personally, I wouldn't mess with it, but I appreciate you get "proper" winters that side of the pond, not like our "limey Mickey Mouse" ones :D Experiencing winter in Detroit is more than enough for me! :D

Hope that helps (well probably not!)

Cheers

[Rawhead]

Hello Jim,

OK, I've had a look at the 4WD system wiring and description and the short answer is it's probably not impossible, but maybe not that easy.

Here's the long answer. I don't know how technical or electronically minded you are, so forgive me if this goes over your head.

The 4WD ECU would appear to get all its information about vehicle speed, yaw rate etc, over the CAN bus. (The CAN bus is a vehicle network - a vehicle "internet" if you like - where ECUs can communicate with one another and share info. It's been around for years. Some manufacturers are beginning to look at something faster with improved data integrity - called "Flexray" - anyway, I digress).

So, it's not just a simple matter of disconnecting a speed signal input. To do that, you would have to intercept the CAN data and send modified data to the 4WD ECU. Maybe not an unsolvable for me (I work with this stuff), but (without wanting to sound big-headed) probably close to impossible for most people.

The only real output from the 4WD ECU is the linear solenoid, which would appear to control the amount of coupling from the prop to the rear wheels via the diff. Now, perhaps Anchorman can check on the mechanical side of things, but it would appear the amount of coupling is proportional to the amount of current through the solenoid. This current is controlled by the 4WD ECU.

One solution might be to disconnect the solenoid from the ECU and inject current into the solenoid separately. What I don't know (or I haven't found it yet) is how much current is required for "full 4WD coupling". It may be that for full coupling 12V is effectively applied across the solenoid. That would make it easy. If it's not designed for that, then connecting 12V directly may well end up damaging it (heat). Simple current limiting would be with a resistor, but that too may get hot. Also, the 4WD ECU is probably going to detect that the solenoid has been disconnected and you are likely to get a fault flagged. It may well also affect other handling characteristics of the vehicle if the solenoid is fully on and not being controlled.

Personally, I wouldn't mess with it, but I appreciate you get "proper" winters that side of the pond, not like our "limey Mickey Mouse" ones :D Experiencing winter in Detroit is more than enough for me! :D

Hope that helps (well probably not!)

Cheers

[anchorman]

The diagnostics call for measuring the resistance of the actuation windings. I suppose the easy way is to switch the 4WD lock on and measure the voltage at the connection.

My feeling is that Toyota will have limited the amount of actuation and the operating speed to limit potential damage to the coupling/transmission.

Understandably they would throw out any warranty claim as a result of bypassing it.

[shcm]

My feeling is that Toyota will have limited the amount of actuation and the operating speed to limit potential damage to the coupling/transmission.

I did wonder too if that would be the case, as you infer, whether the coupling is designed to operate a high(er) speed for long periods, fully actuated, without letting out the magic smoke

[Plutus]

As a newbie to Ravs, whilst we are on this topic can anyone explain to me the difference between Vehicle Stability Control and Traction Control, and do all Ravs have both?

Thanks in advance.

Peter

[anchorman]

As a newbie to Ravs, whilst we are on this topic can anyone explain to me the difference between Vehicle Stability Control and Traction Control, and do all Ravs have both?

Thanks in advance.

Peter

Peter

Not all have both but yours does. Stability control uses a yaw rate sensor under the passenger seat to determin how much g force is being generated by comparison with inputs from the vehicle speed and steering angle. For example if you enter a curve at 40mph on a dry road the computer knows that with a given steering angle (picked up from the angle sensor on the steering column) it can expect a certain amount of sideways g force. If the yaw rate sensor receives that it will be happy and remain dormant. If however it feels less than expected it will litteraly take the car off you by releasing the accelerator and applying certain brakes (but not all) to stabilise the skid. How much it will intervene and which brakes it will apply will depend on lots of input to the ECU from different sensors. It is a very clever and very effective system.

Traction control will use the ABS sensors to determin that certain wheels are slipping under acceleration. When slip is determined it will apply the effected brake (while the car remains under acceleration) in order that the drive will be transfered through the relevent differential to another wheel. It may well brake both front wheels in order to transfer drive to the rear axle. It may also back off the acceleration (power) until the condition is stabilised. I have some very good pdf files explaing these systems if you would like to know more but you will need to give me your email address as I cant send pdfs via pms.

Regards

[anchorman]

This is quite good;

[Plutus]

Thanks Anchorman

Great info.

Peter