In sci.electronics.repair, on Thu, 05 Oct 2017 20:34:47 -0700, mike
wrote:

On 10/5/2017 2:06 PM, micky wrote:
Same new-used car as in the other thread, a 2005 Toyota Solara.

Question about possible reverse voltage problem into an electronic
control module. Is there any chance that trying to put 12 volts into
what at other times is a 12 volt output can hurt anything? ***.

Car has a mechanical trunk release, plus the remote fob opens the trunk,
but it only works when the car is not running or maybe when the key is
not in the ignition.

But there is no keyhole for the trunk and if the fob breaks (as it did
for my prior car, a 2000 Toyota, same model) the only way into the trunk
will be the lever on the floor near the door in front of the driver's
seat. The problem with that is that I have to keep that locked, or
when the convertible top is down, someone can climb over the door or
crawl over the trunk and then open the trunk and take everything in it,
including my tools etc.

***So I want to install an electric release with a button on the dash
that works when the car is running, like many cars have (especially
convertibles whose designers were paying attention). I can't manage to
buy the shop manual except for the Elecric Wiring manual, and there is a
wire, Wire T, from the Body Control Module processor that goes back to
the trunk and powers the trunk release. I can run a wire from a hot
location to a push-button switch in the dash and from there to Wire T.

When the fob energizes Wire T, it won't hurt the switch, which is open
and not being pushed anyhow.

But what happens in the other situation, when the button is pushed and
12V+ heads to Wire T and part goes to the trunk and part goes to the
output of the Body Control Module? Is there any chance that trying to
put 12 volts into an output can hurt anything? I'm guessing it's at 0
volts when the fob is not being used.

Guessing is how you break things.
What is the trunk release?

Yes, a solenoid.

If it's a solenoid CONNECTED DIRECTLY TO THE T-WIRE WITH THE OTHER
END OF THE SOLENOID AT GROUND AND THE T-WIRE GOES TO +12 WHEN
THE RELEASE IS ACTIVATED (MEASURE IT, DON'T ASSUME) that releases the
latch, I'd put a spdt pushbutton
that powers the solenoid only when pushed and hooks it to the control
signal when it isn't pushed.

I see your point. It's taken me a while to post back because I've been
looking for one big enough electrically. I don't know how much the
solenoid uses, and because everything, lots of things, go through that
control module, I can't judge from the fuse that supplies it. I found
an add-on trunk-release solenoid that it says uses 14 amps, but havent'
found an SPDT switch nearly that big that would look decent on the
dashboard.

Maybe I could put some diodes in parallel and use it to block the 12V
from the pushbutton from going to the control module.

Of course the module only sends current to the solenoid for a short
time. An add-on alarm I had one time allowed me to change the time from
iirc 1/10th of a second to 4/10ths of a second. Something like that.

I suppose the diodes woudl block the reverse current but the intended
current would blow out a set of 8 1-amp diodes if it sends out say 12
amps, even for a split second? I'd have to use 12 or 14 1-amp diodes
-- what a pile -- or find something bigger.

If there's a control module in the back that sits between the T-wire
and the solenoid, you're gonna need a lot more info.

No, the solenoid and control module are at opposite ends of the
T(runk)-wire