I had an interesting experience yesterday with my wife's H2. She went to the Schools to pick up the kids and when she went to come home the truck would not come out of park. She called me and I came down and looked at it and then came home with the kids and her and looked it up and found that it is a common "problem".
So I went back and hit the reset/bypass switch and the car would then shift BUT the Brakes felt really spongy so I climbed out and sure enough there was a good puddle of brake fluid under the truck.
From my reading how this thing works is that it senses brake line pressure and it it is not right it prevents the truck from shifting out of park.
So thanks GM your engineering actually prevented my wife from driving a truck with no rear brakes yesterday.
Never knew that. It would have prevented me from getting home multiple times but for the general driving public it's probably a good thing.
NickD
Dork
12/9/16 8:05 a.m.
I'm a GM tech and I never knew that. Pretty good thinking on their part.
It is such a simple thing. I wonder if other vehicles have it.
As for bypassing it that is actually rather simple. There is a small mushroom shaped button on the side of the shifter mechanism in the center console that you push in and it will let you shift it but if you put it back in park it is locked again. That is how I got it home.
This system is also tied to the ignition switch. There is an electronically controlled lock out that prevents you from removing they key from the Ignition. I am not sure what the reason for this is other than it will keep you with your car until you get it fixed especially if it happens in a public location. There is a way around this as well. You have to take the plastic cover off the steering columb (two allen head screws) and there is a button on the ignition switch that you can press to reset it and let the ignition switch come back to the off position and you can then get the key out.
Vigo
PowerDork
12/9/16 10:37 a.m.
My old Magnum turned itself off after a few seconds with no oil pressure when my wife ran it out of oil. I always appreciated that.
Isn't it required by law that there has to be an interlock between the brakes and the shifter to get it out of park? The clever thing GM did was use brake pressure for this interlock instead of the brake lights.
NOHOME
PowerDork
12/9/16 11:12 a.m.
I don't disagree with this kind of tech, but really, in today's world, the car should be able to communicate in English why it is locked out?
When the security system tripped on the Versa, there was no indicator that that was the issue. Would have been good to know.
speaking of the Versa, that piece of E36 M3 failed to start today until I went and berkeleyed with it for ten minutes. Acted like it was flooded. I am afraid to go home in case there is a new Mazda in the driveway.
Opti
HalfDork
12/9/16 11:46 a.m.
In reply to Vigo:
Gmt400 trucks will do the same thing if you pull a relay, wanna say fuel pump relay. Power will pass to the pump via the oil pressure switch, but when oil pressure drops the switch opens and pump loses power.
Do the H2s have brake-by-wire rear brakes like the H3? Be super easy to implement if so.
If not, I'd be just as inclined to believe someone stomped the brakes ultra-hard while trying to get the car out of Park AFAIK the interlock goes straight from the brake pedal switch to the shift mechanism.
NOHOME wrote:
I don't disagree with this kind of tech, but really, in today's world, the car should be able to communicate in English why it is locked out?
An H2 doesn't live in today's world. They started production 15 years ago.
The brakes in the H2 are your "standard" GM hydraulic truck brakes. A quick look and they look exactly the same as what is in my 03 Sierra Denali.
My Wife's H2 is a 03 so it is going on 14 years old now. It is an early GM Can Bus system. Crude by today's standards.
Knurled wrote:
Do the H2s have brake-by-wire rear brakes like the H3? Be super easy to implement if so.
If not, I'd be just as inclined to believe someone stomped the brakes ultra-hard while trying to get the car out of Park AFAIK the interlock goes straight from the brake pedal switch to the shift mechanism.
Not really it uses brake line pressure sensor to determine if it will let you out of park. Hence forth you have to have your foot on the brake to shift out of park. If the pressure sensor in the brake hydraulic system does not sense pressure in the brake system and you don't get out of park. It is not dependant on the switch associated with the brake lights.
In this vehicle the ignition switch is also part of the circuit as is the shifter. I have to look at it closer. It is not like the early versions of the systems that actually had a manual lever thing that would retract a pin in the shifter and allow it to shift when you stepped on the brake peddle.
Another weird one I had in my 75 Formula 400 was a lever that went from the shifter (manual 4 speed) to another ring on the steering column that then rotated a ring on the column that would prevent the car from starting unless I had the car in reverse. This also activated the backup lights. I disabled that but had to manually twist the ring on the steering wheel below the ignition switch to turn the backup light on and off. I eventually defeated that system and bypassed the ignition lockout and then reattached the lever of the shifter to automate the backup lights and or prevent me from driving down the road with them on.
Keith Tanner wrote:
Isn't it required by law that there has to be an interlock between the brakes and the shifter to get it out of park? The clever thing GM did was use brake pressure for this interlock instead of the brake lights.
Yes it is a law. I just thaught it was intresting that GM decided to do it this way. Because they did it this way it is in affect testing the brake systems functionality each time you try to drive the car and that is what prevented my wife from driving a car with a bad brake line.
Most manufacturers just have an electrical switch on the brake peddle that just has to be activated. I think some even use the brake light circit. The drawback to this is that you really don't have to have your brakes on. You just have to have the brake lights on. By GM doing it this way you actually have to have line pressure and I assume the sensor is set so that the pressure is such that it will keep the vehicle from moving when you shift out of park.
I just thought it was a cool system. I am not sure that GM intended for it to also serve as a brake system test each time you drive the vehicle.
dean1484 wrote:
Not really it uses brake line pressure sensor to determine if it will let you out of park. Hence forth you have to have your foot on the brake to shift out of park. If the pressure sensor in the brake hydraulic system does not sense pressure in the brake system and you don't get out of park. It is not dependant on the switch associated with the brake lights.
I dunno what year you have, but I guesstimated a 2006. Shift interlock circuit goes from the backup lamp fuse, through the range position sensor on the trans, back into the fusebox to the BTSI fuse, through pins E and F in the brake pedal switch, then through the shift interlock solenoid and ground.
Interestingly, they run it through the range position sensor probably so you don't hear it going clunk every time you hit the brakes while driving