rear defroster connection to remote start?

Anyone ever done this? The diagrams from alldata are a little vague. I'm not sure if I can just send a pulse ground into the PPL wire connected at C13 on the HVAC control and let the clock in the HVAC programmer handle timing the defroster or if I have to access the rear defrost relays directly with a latched (-) output from the remote start.

I would do a pulse ground via a relay for isolation.

Albertj

Thanks Albertj, it's good to know I'm on the right track. That's what I would prefer to do too but I can't tell if the momentary switch in the HVAC control sends to the HVAC programmer. I didn't ask my original question very well.

Do you know what happens on the purple wire at C13 on the HVAC control when I press the defroster button?

No, but if you have the wiring diagram, it does not matter if I know what happes on the purple wire.

What you do depends on your remote start's control method.

A SPST relay will provide the isolation you need no matter how you wire this thing. The switched wire of the relay has to run in perfect parallel to the switch connected via c13 on the HVAC control. The coil wire gets the pulse from the remote start control. SInce I do not have the wiring diagram or control chart from your remote start control, I can't help you further. It's just that the basic point is that you wire another switch in perfect parallel to that switch if you can.

And if you can not - then... what I'd consider is using a latched output from the remote start. As it is, I do not remember whether the rear defroster wires are common hot or common ground. If they are common ground then it ought to be easy enough to do the following:

1) protect your other control circuits by clamping the current INPUT to the defroster grid with a diode. That way you do not screw up the current control circuit. what this means is just before the point where you're adding this circuit you put a fairly hefty diode in to keep the feedback out of your current control circuits. I suspect a 1N4004 ought to do it, but you need to check the spec sheets and decide for yourself.

2) after the diode, put the switched side of a relay between the defroster grid and + battery voltage thru an appropriate fuse. The coil of that relay is connected to the latched output of the remote start. So that the remote start can control the on-off of the rear defroster.

3) If the remote start is not smart enough to turn off the defroster grid after so many minutes, you'll need to build a delay device or rely on the remote start's self-timer (most turn themselves off after so many minutes).

Good luck.

Albertj

That's some good info, I appreciate it. I think my question is even more basic than that. The alldata diagram for the Riviera I am using just treats the HVAC control as a black box. I can see there is a connection on C13, but I don't know if this is a - pulse directly from the momentary switch in the HVAC control. I assume it is and you're right - isolating the factory circuitry from the addons is a very smart move.

I'm really just looking for a way of simulating the defroster button being pushed, not entirely sure what the right method is.

Or have I misread your post?

No.. Your answer is just paragraph 3.

Albertj

OK, do you happen to know for sure if this is a (-) pulse?

No, I would double check using a meter. The HVAC control latches the pilot LED "on" when the button is pused. I would suspect the button press is the "pulse" you're looking for.

What this does is put you into using a latched output from the remote start and isolating it from the defroster with a relay.

Albertj

Sounds about right. I was hoping to avoid the latched output, hoping I could just use a pulse to initiate the HVAC programmer to do the rest.

I'll check it with a meter, thanks for the info!

I don't recommend that you do this - but one thing I would consider is that the defroster switch is a little rubber pad that gets pressed across a grid.

If I was considering this then I could get a 'perfect parallel' connection by soldering a 2-conductor wire to the circuit board in parallel to the printed contacts for the defroster button. I would then bring the wires out thru the back of the HVAC controller and terminate them using (dangling) spade or pin connectors.

Then I could simply connect the 2 switched terminals of a 2-terminal normally-open relay to the (dangling) spade or pin connectors. To run the defrpst I'd "send a pulse ground" to a relay coil, to close it, (hot side would be ACC+). I think it'd be the same as sitting in the drivers seat hitting the button.

Albertj

Holy crap did Buick make this way overly complicated in 1998 or what?

I just did the same thing on my 1995 last night and it was a no brainer as I expected. After reading all of this I got scared, and took apart the Defogger switch - as I figured there's only discrete components in the module - no logic.

Here's a quick hand drawn schematic of the connector face on the back of the switch module:



Here's a shot from my 95 manual:



So, yes it's pretty simple. Pressing the switch just sends a logic low or 'ground pulse' to the HVAC unit. Hook up your (-) pulse to the light blue wire... which may be the PPL wire you speak of?

Good luck - it worked for me and boy was I glad to have it tonight!

...good job.

however, you might want relay isolation in order to avoid problems that are caused if a related circuit blows a fuse - the Riv's ground floats.

The '95 has a different looking heater control. I so not know if it's actually different and suspect that it is not. BUT if it is different that's an issue.

Albertj