4H Button. What does it do?

Joined
Sep 17, 2021
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15
Location
Calgary, Alberta
New owner of a 1990 Prado SX5. New to the site as well but have got lots of very helpful information so far on a variety of subjects.

No owners manual came with which is just fine because the vehicle is from Japan and I don't speak or read Japanize.

I am working my way through a myriad of small items that need repair or don't work at all at all so thanks for your patience. I am certain this has been covered off before but my search is turned up nothing that answers the question.

When I bought the vehicle the guy I bought it from was older and very nice but his operational knowledge, short of driving the truck was, well, shall we say, rudimentary at best.

When I pushed the "HUB LOCK" button both the "HUB LOCK" and the "4H" buttons illuminated and the vehicle switched to four wheel drive with the front hubs locking up and the Tcase engaging. Some what confusingly, with the HUB LOCK button "not" pushed, I could push the 4H button till the cows came home and nothing happened nor did it illuminate. When I just pushed the 4H button, the back light on the 4H button did not illuminate, the Tcase did not engage and as such the drive shaft from the Tcase begin to turn.

Clearly, both processes of hub lock and T case engagement are working because the vehicle does switch in and out four wheel drive but "only" by activating the "HUB LOCK" button.

Just to save you from asking, yes, I can tell when it switches in and out. The driveline feels and sounds different in four wheel drive and on dry pavement in a sharp turn, the front wheels hop as they are being pushed by the rears. I did this test to ensure that the front CV joints on the front axles were not worn out which can cause them to click or grind in a tight turn if they are toast.

When I asked the old fella I bought it from why it was doing what it was doing, he said the 4H button is for vehicles that have "manual" locking front hubs as compared to the electric ones that are on my vehicle. He said the manual hub vehicle owners typically leave there hubs engaged 24/7 when the weather is inclement which then allows them to just simply push the "4H" when they want to engage four wheel drive while on the fly. He said the "HUB LOCK" button was there just to remind the drivers of the manual locking hub vehicles that their hubs are locked so they don't forget that they have them engaged.

See what I mean by kind of made sense but not really??????

If his story was accurate, then the HUB LOCK button would not be a button at all. It would just be an annunciator light that would come on when the manual hubs are locked and the manufacture would not have installed a button in the first place. My guess is he never really knew why it did what it did but the reason he came up with kind of made sense at the time so he stuck with it. Besides, the vehicle switches in and out of four drive with out a hitch so no problem, right???? WRONG! Sorry old fella but I don't agree!

I believe the HUB LOCK button and the 4H/four wheel drive Tcase engagement function are "supposed to work" independently. I think the HUB LOCK is supposed to engage the front hubs "only" and not simultaneously engage the Tcase as it is doing now. My thoughts are that this process, if working properly, would allow me while driving on crappy roads, to keep the hubs locked up (HUB LOCK button turned on) and when I have a need for four wheel drive mode, I can then push the 4H button which would then engage the Tcase sending power to the front wheels.

So after all this typing, I am now taken back to the title of this post. What is the 4H button supposed to do? Am I correct in my assumption as described above, OR , was the old guy really correct in how he thought it was supposed to work?

If it is supposed to work the way I think it is supposed to work then I have to start digging into how and why that HUB LOCK button is activating both the hub locking and Tcase engagement simultaneously and as equally as mystifying, why does the 4H seem to do nothing.

I can kind of see an advantage to the way it is working now but only as an optional choice of how to activate four wheel drive. Just conjuring up a scenario, I can envision driving with the roads unexpectedly turning to crap. Only having one button to push would reduce the terror of the somewhat holly crap, I need four wheel drive right freeking now moment.

Having said that, it still doesn't answer my original question of what does the 4H button really do.

My only answer is that the 4H is not working despite the back light coming on when the HUB LOCK button is pushed. If it is inoperative (not switching the Tcase on independently) then I just have to pull the switch and take a look as to why.

YOUR THOUGHTS PLEASE!
 
Joined
Sep 17, 2021
Messages
15
Location
Calgary, Alberta
I could be wrong, but without the hubs locked there would still be no traction to the front wheels even if the transfer case is engaged to 4wd . That's how mine works anyhow.
You are not wrong but but with just the 4H button ebgaged my thoughts are that the Tcase would then engage independent of the hubs being locked in there by allowing the hubs to be engaged independently as well though not advisable I would think because if you were spinning your wheels with just the Tcase engaged and then tried to lock in the front hubs while still spinning the rears, I could see where you could easily damage the front hub locking cogs that are in those electric locking hubs.

With the way it is working now the 4H button does nothing. I would think that as compared to how it works now, again, based on there being two separate buttons for the two separate functions, one should be able to lock the hubs and not have the Tcase engaged. Though driving in the mode would reduce fuel economy due to the front turning the front drive shaft it does afford a person the option of rapidly (comparative word) turning 4H on and off as the road conditions dictate.

Most serious off road 4X4 vehicles have this function so that they can access the power of the engaged Tcase through a PTO output shaft on the Tcase. These shafts are often used for engine driven winches that require the engine to be running and the Tcase engaged with the vehicle in neutral and the hubs not locked in.

Are you saying that your 4H button works the same way mine does as in it does nothing if pushed by itself and that when you push your HUB LOCK button that both the HUB LOCK and 4H buttons then illuminate and your vehicle then both locks the hubs and engages the Tcase?

If that is the case, then what is your understanding of the 4H button's function and it's reason for existence?

Thanks for taking the time to help me figure this out!
 
Joined
Jul 3, 2004
Messages
4,414
Location
Anchorage, AK
H4 shifts transfer into 4 Hi (4x4). Independent on what’s going on with your hubs.
Don’t know if there is a computer logic for those vehicles with the electric lock hubs, that only 4H works after the electric hubs are locked.
 
Joined
May 16, 2008
Messages
1,608
Hello,

In the early 1990s 70 Series and Prados had electric hubs. A small electric motor on each hub locked it.

This device was intended to replace manual lacking hubs.

Electric hubs are not entirely reliable, especially if the front axle is under mud or water at or above the hub level. The motors are very sensitive to mud or water, and tend to fail.

Winter driving can be tricky, and it is wise to have the hubs locked just in case more traction is necessary. Yes, it is possible to have the electric hubs locked for everyday driving, and then press the H4 button to engage 4WD, provided your electric motors are working properly. Otherwise, there is no traction in the front axle.

It is possible to change the electric hubs to manual ones. If I remember correctly, the control circuit needs some tweaking to work properly without the electric hubs.

If winter driving with reliable hubs is a concern for you, you may want to switch to manual locking hubs.






Juan
 
Joined
Oct 22, 2006
Messages
452
Location
Nelson,New Zealand.
When you select H4 or L4 the ECU engages the electric hubs automatically with out the Hub lock button being pushed. Hence the 2 green lights glowing. There is a detent switch on the T/F Box that send a signal to the hub ECU.
 
Joined
Sep 17, 2021
Messages
15
Location
Calgary, Alberta
Hello,

In the early 1990s 70 Series and Prados had electric hubs. A small electric motor on each hub locked it.

This device was intended to replace manual lacking hubs.

Electric hubs are not entirely reliable, especially if the front axle is under mud or water at or above the hub level. The motors are very sensitive to mud or water, and tend to fail.

Winter driving can be tricky, and it is wise to have the hubs locked just in case more traction is necessary. Yes, it is possible to have the electric hubs locked for everyday driving, and then press the H4 button to engage 4WD, provided your electric motors are working properly. Otherwise, there is no traction in the front axle.

It is possible to change the electric hubs to manual ones. If I remember correctly, the control circuit needs some tweaking to work properly without the electric hubs.

If winter driving with reliable hubs is a concern for you, you may want to switch to manual locking hubs.






Juan
Juan, thanks for your reply but as I said, I can not just engage the transfer case nor can I lock the hubs as independent actions.

My 4H button does nothing. Full stop. It does not independantly engage the Tcase when pushed. It does nothing.

Comparitively the Hub Lock button activation does not just simply and independently lock the hubs. So respectfully Oldblue my truck does not work the way you indicated that it should.

Again my 4H button does nothing. It does not engage the Tcase.

Conversely, the HubLock performs both functions namely locking the hubs and (with no ability to do it differently) engages the Tcase in four wheel drive high and the 4H light coming independanr of the 4H button being pushed.

So what I am getting from the replies to my question is that the 4H is not supposed to do anything because I have the electric front hub option and that the Hub Lock button locking up the front hubs AND simultaneously engaging the Tcase is how the system is supposed to work the way it is working.

Simply put, is my truck working the way Toyota designed it to and they just failed to remove the 4H button and replace it with just a light?
 
Joined
Oct 22, 2006
Messages
452
Location
Nelson,New Zealand.
The H4 Button is only to lock the electric hubs, ie H2 to H4, when you pull back on the T/C lever to L4 the ECU activates the electric hubs so you are locked up in low range.
Check the plugs that are beside the right side of the T/F Box, if these are not connected then there is no signal from the T/F Box to the hub lock ECU.
 
Joined
May 21, 2006
Messages
468
Location
Rotoiti, New Zealand
On my 1990 SX5 Prado both the transfer case 4wd/2wd & the electric hubs were controlled by a '4wd control' ECU under the dash. IIRC you had to have the hub lock button in (& they would only engage under a certain slow speed, otherwise the light would just flash) before the H4 button would engage the t-case. Sounds like your 4wd control ECU may be flaky, or more likely, something has previously crapped out then been bodged so you can get 4wd.

Have you confirmed the t-case is disengaging 4wd without the hub lock pushed? Without separate control you may have to unplug the electric hubs or unbolt the front driveshaft to check that.

Good advice above re replacing the electric hubs with manual. I did this soon after buying mine. I still had to push the hub lock button before H4 would engage. Eventually my 4wd control ECU crapped out & I rewired & replaced it with a changeover relay to switch the vacuum solenoids that shift the t-case H4 selector. Now I have just one H4 button that works the same as in a non electric hub Cruiser.

Cheers
Clint
 
Joined
Jan 4, 2019
Messages
160
Location
Wilmington, North Carolina
My hub lock switch only locked the hubs and did not engage 4WD. H4 switch engages the 4WD.
I replaced the electric hubs with manual. I do not have to use the hub lock switch to activate 4WD. Just manually turn the hubs to LOCK, and then engage the H4 switch.
 
Joined
Sep 17, 2021
Messages
15
Location
Calgary, Alberta
On my 1990 SX5 Prado both the transfer case 4wd/2wd & the electric hubs were controlled by a '4wd control' ECU under the dash. IIRC you had to have the hub lock button in (& they would only engage under a certain slow speed, otherwise the light would just flash) before the H4 button would engage the t-case. Sounds like your 4wd control ECU may be flaky, or more likely, something has previously crapped out then been bodged so you can get 4wd.

Have you confirmed the t-case is disengaging 4wd without the hub lock pushed? Without separate control you may have to unplug the electric hubs or unbolt the front driveshaft to check that.

Good advice above re replacing the electric hubs with manual. I did this soon after buying mine. I still had to push the hub lock button before H4 would engage. Eventually my 4wd control ECU crapped out & I rewired & replaced it with a changeover relay to switch the vacuum solenoids that shift the t-case H4 selector. Now I have just one H4 button that works the same as in a non electric hub Cruiser.

Cheers
Clint
Clint thanks for your reply. Your explanation makes perfect sense. I have been able to confirm that the Tcase is disengaging. I put the truck up on jack stands and watched the engagement and disengagement of it as my wife pushed and released the hub lock button. Additionally, with the Hub Lock button disengaged, I jacked up the front end and confirmed that both the hubs were unlocked (wheels turned by hand with no restriction and the front drive shaft turned freely as well with no indication that the Tcase was engaged. And as if that wasn't enough, I had my wife drive slowly drive by me while I was laying on the ground and I was able to see that the front drive shaft was not turning with my Hub Lock button not engaged. I will begin looking for the ECU that you spoke of and see is has been bypassed or perhaps physically removed in it's entirety. Your explanation of the ECU's algorithm also makes perfect sense in that it would not attempt to lock the hubs until the truck was at or below a certain speed. When I took off one of those electric locking hubs I was surprised to see how simplistic they were. I was also just as surprised to see how flimsy the locking cog was. I could definitely see how they could break if the rotation speed of the axle and the rotation speed of the tire were not real close. A significant differential in the speed of those two components and a request to engage could result in a costly repair not to mention the resultant metal fragments that would be floating around.

 
Joined
Sep 17, 2021
Messages
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Location
Calgary, Alberta
When you select H4 or L4 the ECU engages the electric hubs automatically with out the Hub lock button being pushed. Hence the 2 green lights glowing. There is a detent switch on the T/F Box that send a signal to the hub ECU.
Thanks Oldblue but as I said, my 4H button does nothing. Pushing it does not engage the Tcase in high range (though I think it should) and it certainly does not lock up the front hubs.
Pushing the button doesn't even turn on the back light.
 
Joined
Sep 17, 2021
Messages
15
Location
Calgary, Alberta
So everyone, this new owner (me) of this old vehicle has been brain dead throughout these posts and even my original post was out to lunch.

To clear things up, neither the HUB LOCK nor the 4H buttons are back lit. I have no idea where I got the idea that they are. The two "lights" that I was somehow confusing the buttons for are located on the upper left hand corner of the instrument cluster and are labelled Hub lock and "4WD" not 4H. DA!

Here is how the system works and I now think that everything is working fine.

Scenario #1: Press the HUB LOCK button with the Tcase selector lever pushed forward to the 2H position and the Hub lock and 4WD dash lights illuminate. The hubs are electrically triggered to mechanically lock and the Tcase engages into 4WD high range.

Scenario #2: Press the 4H button with the HUB LOCK button not pushed and the Tcase electric over vacuum relay activates presumably engaging the Tcase and here is the Perry Mason moment, NO lights illuminate on the dash. This now all makes sense because without the hubs locked up, the vehicle is not in 4WD though the Tcase is presumably engaged spinning the axles just waiting for the hubs to be locked in.

Scenario #3: Is with the 4H button disengaged, the HUB LOCK engaged and the Tcase selector level pulled back to the Nuetral position, the hub lock light on the dash illuminates but the 4WD light does not. Again, this makes perfect sense as well because with the Tcase in Neutral, the vehicle is neither in 4WD or 2WD. The hub lock light on the dash presumably is illuminated to remind/tell you that the hubs are locked in and that the HUB LOCK button is engaged.

Scenario #4 Is the HUB LOCK button activated with the Tcase lever all the way back into the 4L position. At this point the truck is in 4WD low range with the Tcase engaged and hubs locked . Resultantly both the hub lock and 4WD lights are then illuminated.

I presently have the truck up on jack stands doing the brakes but I am very anxious to see if the 4WD ECU is working in the way you described it is suppose to. I plan to do a test by driving the vehicle above the 5klms hub lock speed to see if in fact the hub lock light just flashes. If it does then the entire system will then have be proven to be working as designed.

Thanks again to all who weighted in on this subject trying to help when once I turned my brain on, I realized that no help was required.
 
Joined
Dec 22, 2015
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Whitefish/Bozeman, MT
Scenario #1 is incorrect if I'm reading it right. With the Hub Lock button pressed and the T-case lever in the H position but the 4H button not pressed, the hubs will lock but the transfer case should not shift into 4WD. This allows one to drive around with the hubs locked and the vehicle in 2WD, but with the ability to engage 4WD at the touch of a button and at any speed. This is generally how I have it in the winter.

I have never tried to engage the hub locks when moving, as I always assumed it would cause damage to the relatively delicate internal components (somewhat like shifting into 4WD while moving with the hubs unlocked).
 
Joined
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So I finally dug into my Tcase engage/dis-engage mystery and more specifically why the 4H button on the dash did not seem to be doing anything. I consulted a very knowledgeable tech at a shop here in Calgary that has a great deal of experience working on a wide variety of these types of vintage Toyota vehicles.

What I found out was that the Tcase not only requires vacuum being applied to the activation diaphragm that is located on the Tcase to engage, it but it also requires vacuum being applied to that same diaphragm to dis-engage the Tcase. The difference being that vacuum is applied to one side of the diaphragm to engage and then once engaged, it requires vacuum being applied to the other side to dis-engage the Tcase. Additionally the 4X4 light on the dash is turned on and off by a switch in the Tcase and not the 4H button on the dash. Once the Tcase is engaged, that switch will then provide 12V to that light on the dash which is the confirmation that the Tcase has actually been, engaged.

So, armed with that information, I then started to look at the 12V electric over vacuum switches/valves that are located on the firewall directly behind and above the brake master cylinder. One is blue in color and the other is light brown in color. Both are mounted to a secondary bracket and that bracket is mounted to the fire wall.

I first checked to ensure that I had a good source of vacuum being supplied to those valves. Checked and confirmed while the engine was running and found 14 in/hg on the supply side of both of the valves.

Then I checked the operation of the valves by putting my vacuum gauge on the Tcase side of each of them. With the button on the dash not pushed in, I found no vacuum on the Tcase side of either the blue or light brown valve. Hmm? That made no sense! Why was my Tcase enganed with not vacuum out of either valve. With the 4H button on the dash pushed in I had vacuum on the Tcase side of the blue valve. Then I released the 4H button on the dash and the vacuum on the Tcase side of that same blue valve was gone. So, with that, I was able to confirm that the button was serving to provide 12V to the blue valve when the button was pushed in. I also new that the Tcase was being engaged, that the valve tasked with that engagement process was working properly and finally that it is the blue colored valve the performs the Tcase engagement process.

So armed with the information that in order for the Tcase to disengage, it needed vacuum applied to the dis-engage side of the Tcase diaphragm, I first checked to ensure that the button on the dash when not pushed in, was not inadvertently/incorrectly still providing 12V to the blue valve. I confirmed it was not by putting my multi-meter on the connector for the blue valve and found no voltage at the connector with the button not pushed in. Of course, I had sort of already confirmed this by checking for vacuum on the Tcase side of that blue valve with the button both pushed in and then not pushed in. I had Tcase side vacuum with the button in and no vacuum with the button not pushed in. So the blue valve was confirmed to be working properly but why was the Tcase remaining engaged with that blue valve closed and not sending vacuum to the Tcase diaphragm when the 4H button on the dash was not pushed in?

It was this point that I finally realized what was going on. It was now apparent to me that my Tcase was always engaged and that was why the 4X4 light on the dash always came on when I pushed the Hub Lock button in on the dash.

So now that I confirmed that the Tcase engagement process was working properly, I then focused in on why it was constantly engaged and more to the point, why was it not disengaging when the 4H button on the dash was not pushed in. I then made a presumption that the purpose of the light brown valve was to provide the required vacuum to dis-engage the Tcase. To check to make certain that the disengagement function of the Tcase diaphragm was working properly, I took a vacuum source and connected it to the Tcase side of the light brown valve. Low and behold the 4X4 light on the dash then when out and was no longer coming on by my only pushing the Hub Lock button like it had been doing before. I then drove the truck slowly around the block. I pushed the Hub Lock button on and off a number of times while not having the 4H button pushed in. The 4X4 light did not come on and most importantly, the Tcase was not being engaged either and as such, I was not engaging four wheel drive each time I pushed the Hub Lock button like it had been doing before.

I removed the test vacuum source from the Tcase side of the light brown valve and then started to check the operation of that valve. I checked to see if 12V was being sent to it and more importantly, if it was, when was it being sent. What I discovered was that when the 4H button on the dash is not pushed in, consistent 12V is being sent to that light brown valve. When the 4H button was pushed in, that 12V was turned off. Now it all made sense as to how the system was supposed to work. When the 4H button is not pushed in, a consistent 12V is sent to the light brown valve which in turn provides vacuum to the dis-engage side of the Tcase diaphragm. Conversely, when the 4H button is pushed in, a consistent 12V is being sent to the blue valve not the light brown one. That blue valve then provides vacuum to the engage side of the Tcase diaphragm and four wheel drive can then be engaged when the Hub Lock button is pushed in. Additionally, the Tcase can be engaged and dis-engaged independent of the hubs being locked. This is how other posters to this forum have been able to drive in the winter with their hubs locked with the Hub Lock light on the dash illuminated and then activate four wheel drive on the fly by pushing the 4H button.

So back to the first question I had asked. Why was my truck not working that way? The answer was simple. I found the light brown colored valve to be defective. Even though the switch on the dash was providing the required 12V to power that light brown valve, I found that the valve was not opening with the 12V applied. As such, it was not providing vacuum to the dis-engage side of the Tcase diaphragm. The resultant effect was that my Tcase was consistently engaged even with the 4H button not being pushed in. This was evidenced by the 4X4 light coming on and four wheel drive being activated each time I pushed the Hub Lock button in no matter what position the 4H button was in.

I replaced that light brown valve and now everything works the way Mr. Toyota designed it.

So a long and protracted story to get to the root of my problem but I hope in my having explained each of my diagnostic thoughts and processes, that readers of this with similar problems can resolve their challenges more rapidly.
 

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