Electrical Engineering - Auto key resistor pellet changed value

Author

Auto key resistor pellet changed value

Peter

2006-03-22, 1:21 am

Can someone assist me with this very strange problem? My car came with a
security feature where it includes a resistor pellet on the key. Obviously
this is linked to a comparator that determines if I have the right key or
not.

A few months ago my car's security system gave me problems and in the
process of repairing it they needed to change the key cylinder and give me
a new key (and a spare key).

The new key was a different cut (because of the new cylinder) but contained
the same value resistor pellet as my old one. I know the resistor values
were identical because I measured the old one, the new one, and the spare
key.

Last night my car wouldn't start. After having it towed, the repair guy
asked if I could bring my spare key to him. Then the car started!

He took the key I've been using (that didn't start the car) and measured it
to be 6k while my spare key (that started the car) was the 7.5k my car's
security system likes. He did some research and heard the key can become
"de-magnitized". I'm in electronics and know resistors burn and open, they
never change values especially becoming less resistive. Assuming the "de-
magnitizing" theory to be true, I thought about what I did from when the
key worked till it failed. The only thing I did was put the keys in my
pocket along with my cell phone.

If you look at the list of resistor values for keys, the 6k is the value
before my 7.5k. I find this fact to be facinating but yet I have NO idea
how a resistor pellet could become less resistive. I also inspected the
pellet for cracks or chips and didn't see anything.

Is the resistor pellet made from some other material that can become less
resistive? If anyone can answer this, you'd allow me to sleep better.

Thanks in advance!

TimPerry

2006-03-22, 9:21 am

"Peter" <private@private.com> wrote in message
news:Ocydnb9nOfe9RL3ZnZ2dnUVZ_umdnZ2d@comcast.com...
> Can someone assist me with this very strange problem? My car came with a
> security feature where it includes a resistor pellet on the key. Obviously
> this is linked to a comparator that determines if I have the right key or
> not.
>
> A few months ago my car's security system gave me problems and in the
> process of repairing it they needed to change the key cylinder and give me
> a new key (and a spare key).
>
> The new key was a different cut (because of the new cylinder) but
contained
> the same value resistor pellet as my old one. I know the resistor values
> were identical because I measured the old one, the new one, and the spare
> key.
>
> Last night my car wouldn't start. After having it towed, the repair guy
> asked if I could bring my spare key to him. Then the car started!
>
> He took the key I've been using (that didn't start the car) and measured
it
> to be 6k while my spare key (that started the car) was the 7.5k my car's
> security system likes. He did some research and heard the key can become
> "de-magnitized".

find a new mechanic.

> I'm in electronics and know resistors burn and open, they
> never change values especially becoming less resistive.

you havent been in electronics long enough.

>Assuming the "de-
> magnitizing" theory to be true, I thought about what I did from when the
> key worked till it failed. The only thing I did was put the keys in my
> pocket along with my cell phone.
>
> If you look at the list of resistor values for keys, the 6k is the value
> before my 7.5k. I find this fact to be facinating but yet I have NO idea
> how a resistor pellet could become less resistive. I also inspected the
> pellet for cracks or chips and didn't see anything.
>
> Is the resistor pellet made from some other material that can become less
> resistive? If anyone can answer this, you'd allow me to sleep better.
>
> Thanks in advance!

make new key, destroy old key, carry spare key in wallet, sleep well

Bill Kaszeta / Photovoltaic Resources

2006-03-23, 12:21 am

On Tue, 21 Mar 2006 22:57:04 -0600, Peter <private@private.com> wrote:

>Can someone assist me with this very strange problem? My car came with a
>security feature where it includes a resistor pellet on the key. Obviously
>this is linked to a comparator that determines if I have the right key or
>not.
>
>A few months ago my car's security system gave me problems and in the
>process of repairing it they needed to change the key cylinder and give me
>a new key (and a spare key).
>
>The new key was a different cut (because of the new cylinder) but contained
>the same value resistor pellet as my old one. I know the resistor values
>were identical because I measured the old one, the new one, and the spare
>key.
>
>Last night my car wouldn't start. After having it towed, the repair guy
>asked if I could bring my spare key to him. Then the car started!
>
>He took the key I've been using (that didn't start the car) and measured it
>to be 6k while my spare key (that started the car) was the 7.5k my car's
>security system likes. He did some research and heard the key can become
>"de-magnitized". I'm in electronics and know resistors burn and open, they
>never change values especially becoming less resistive. Assuming the "de-
>magnitizing" theory to be true, I thought about what I did from when the
>key worked till it failed. The only thing I did was put the keys in my
>pocket along with my cell phone.
>
>If you look at the list of resistor values for keys, the 6k is the value
>before my 7.5k. I find this fact to be facinating but yet I have NO idea
>how a resistor pellet could become less resistive. I also inspected the
>pellet for cracks or chips and didn't see anything.
>
>Is the resistor pellet made from some other material that can become less
>resistive? If anyone can answer this, you'd allow me to sleep better.
>
>Thanks in advance!
I had similar problems, turned out in my case that the connection to the
key contacts wore out and the result drifted.

The system will accept resistance about +/- 10% from nominal, so you
may have had a 6K on the high side and it was accepted. The system
works in most GM cars by having the resistor control a small and
rather hidden module. This module sends a squarewave of specific
frequency to the engine computer and also controls an interlock
relay on the starter circuit. Hard to troubleshoot because the
Mitchel manual does not detail it for 'security' reasons. The companies
that sell remote start controls have instructions on bypassing the
feature so a remote start will work.
Bill Kaszeta
Photovoltaic Resources Int'l
Tempe Arizona USA
bill@pvri-removethis.biz

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