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The automatics have an electrical solenoid in the transfer box called "transfer control solenoid". (Same principle to the automatics 3 solenoids)

I guess it has to be there so the ECU can switch something in the transfer box

If it's switching the power to the rear off - when does it do it and why - I have noticed the RAV does handle more like a FWD than a rear at speed (but maybe that's my imagination)

If not - what is it for?
 
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Thanks for the standard reply

I am aware of 3.1.3 and am aware how the RAV4 four wheel drive 'system' works -

But thas DOES NOT answer my question
 
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*** I removed this cos I was grumpy ****

I removed my reply because it isn't worth the hastle - some people find in easier to snipe than answer a simple question

If "When does the transfer solenoid work and how" is to difficult I don't think I'll come back here - had my fill of "Why does my RAV seem slow"
 

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ag0w said:
Guys - accept the faq doesn't answer my question. I know you all love to scald people when they don't look in the faqs but the fact is the faq does NOT answer my question!

I know it has some vagary about automatically locking the center diff - but no detail as to what I am asking which is what does the ECU achieve by switching this solenoid and when

I suspect it can't be reactive as the ECU does not have sufficient speed input (I haven't got ABS sensors) so it must be more to do with driving patterns

Please can someone answer --- If you are just going to say read the faq - just move on to the next post and save all our time
There are front and rear speed sensors on the outputs of the transmission whether you have ABS or not. That solenoid probably does what we are talking about but what you are calling it is not in the service manual.
 

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It is a digest from Mitchell manual refer to 1997-1998 RAV4 transfer case for automatic transmission.

Transfer Case:
"The A-540H transfer case is an electronically controlled unit which uses a center differential control solenoid with an internal valve body. Transfer control solenoid is operated by duty cycle signals from the Electronic Control Unit (ECU). Duty cycle signals modulate fluid pressure of center differential clutch control valve. For additional information on electronic control components, see TOYOTA A-540H ELECTRONIC CONTROLS article in AUTOMATIC TRANSMISSIONS."

Automatic Transmission:
"SLD solenoid is used to control hydraulic pressure acting on center differential clutch and maintains limited slip differential operation. ST solenoid is used to control transaxle line pressure."

"ST Solenoid
When difference in speed between front and rear wheels is extreme, and condition takes place during low speed operation in "D", "2" or "L" position, ST solenoid is turned on by ECT ECU to adjust transaxle line pressure. ST solenoid is located on transaxle valve body.

SLD Solenoid
SLD solenoid controls hydralic pressure applied to center differential clutch and maintains limited slip differential level. ECT ECU determines optimum control pressure according to signals from TP sensor, front and rear speed sensors, and volume of current flow to SLD solenoid. Amount of current flow to SLD solenoid is controlled by duty ratio of ECT ECU output signal. The higher the duty ratio becomes, the higher center differential control pressure becomes druing center differential operation. SLD solenoid is located on transaxle valve body."
 
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Thanks 4VAR - Toyota (At least in the UK) call the SLD the "TCS: Transfer control solenoid"

I allways thought the magnetic pick-up speed sensor next to the proper speed sensor was just for back-up. However, now I think about it the magnetic one is on the front and the proper one is on the connection to the transfer box

So at slow speeds if the front or rear slip the speed will differ on the sensors and the ECU frantically modulates the solenoid to lock the axle

Very clever!
 

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Honda CR-V takes a quite different approch - No electronic control, purely mechanical and hydraulic. interesting. Here is a brief description of Honda RealTime 4WD system from a Honda Australian website.

"Honda's RealTime 4WD system - which automatically engages all-wheel drive only when required - now delivers better acceleration and hill climbing performance on slippery (particularly) snow covered) surfaces, as well as a swifter response when accelerating around corners or turning out of slippery intersections. The new system uses a one-way ball cam and pilot clutch to transmit torque instantaneously and responds to even the slightest amount of wheel spin. Thicker stabiliser bars also contribute to greater stability.

The RealTime 4WD system on the CR-V is designed to overcome the traditional drawbacks of a full-time four-wheel drive system - notably lower fuel economy, higher noise and vibration levels, higher weight. The CR-V's RealTime 4WD operates automatically and only when needed. It requires absolutely no intervention on behalf of the driver for it to be engaged or disengaged.

The CR-V's RealTime 4WD system sends power only to the rear wheels when there is insufficient traction for the front-wheel drive system. The system consists of the conventional front-wheel drive system - a compact transfer system that distributes drive to a propeller shaft running the length of the vehicle, a dual-pump system, the rear differential, and left and right rear wheel driveshafts.

The heart of the system is the dual-pump unit. It consists of two hydraulic pumps - one driven by the front wheels via the propeller shaft and one driven by the rear wheels via the rear differential.

A hydraulically actuated, multi-plate clutch - similar to the clutches used in Honda automatic transmissions - connects the propeller shaft to the rear differential.

When the CR-V is operating with the front and rear wheels turning at the same speed - for example, on dry bitumen - the front and rear hydraulic pumps operate at the same speed.

Hydraulic fluid circulates between the two pumps; however, no pressure is generated. In effect, the fluid fed by the front pump is absorbed by the rear pump.

If the front wheels begin to turn faster than the rear wheels - as would be the case if they were spinning on snow or ice - the two hydraulic pumps would turn at a different rate and hydraulic pressure proportional to the difference in their speeds of rotation would be generated. The resulting hydraulic pressure opens a valve body and feeds pressure to the multi-plate clutch, which engages the front propeller shaft to the rear differential. The rear differential feeds the drive torque to the right and left rear wheels.

System operation is completely automatic - no electronics or driver action is involved. The greater the degree of front-wheel slippage, the greater the amount of torque fed to the rear wheels. RealTime 4WD is also practically maintenance- free, requiring only a scheduled fluid change at 120,000 km and every 60,000 km thereafter. "
 

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The marking dept. left out a couple of other facts, the first one being that the CRV will never give you 50/50 torque split, more like 75/25, and the rear wheels engage with a noticeable delay. For example when I drop he clutch on a CRV or Element, the front wheels spin and screech for about a half rotation, then the rear wheels engage, then the engine bogs down, then you look like an idiot limping away from the light. Also the rear drive shaft is not serviceable on the CRV, no grease fittings on the U-joint and the U-Joint is not replaceable. So when the time comes, and it does around 120,000km, you need a whole new drive shaft. Fuel economy and getting up hills in winter is about all it's good for. It's no fun to drive at all.
 

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I agree with that "the rear wheels engage with a noticeable delay" for CR-V, although I do not have chances to drive CR-V. From 4WD mechanism described by Honda, I can image one front wheel has to spin fast enough and long enough to build up hydralic pressure that opens the valve body to engage the clutch. It can't be more sensitive than electronic speed sensors.

Why does Honda use this technology instead of more advanced one? There must be some advantages that make Honda to think it is worthy, low production cost, durability, stabablity or something Honda does not want other competitors to know?
 

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4VAR said:
I agree with that "the rear wheels engage with a noticeable delay" for CR-V, although I do not have chances to drive CR-V. From 4WD mechanism described by Honda, I can image one front wheel has to spin fast enough and long enough to build up hydralic pressure that opens the valve body to engage the clutch. It can't be more sensitive than electronic speed sensors.

Why does Honda use this technology instead of more advanced one? There must be some advantages that make Honda to think it is worthy, low production cost, durability, stabablity or something Honda does not want other competitors to know?
Honda prides itself on fuel efficiency and that is the most fuel efficient automatic system in existance, even when fitted to the Element which is shaped like a toaster :) They figure the only reason anyone would want 4wd on one of their vehicles is to get up hills in winter anyway, if you wanted performance you'd buy one of their proper AWD Acura products which has active differentials not unlike the Evo or Impreza.
 

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Thanks for bringing back this 2005 thread, guys!!!.....I musta missed it first time......and it's the stuff I wanted to know for a long time........ :wall
.....and thanks to the original posters for the great detail!!!!....and comparisons!!!! :cheers:
 
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