Posted to Technical Tips Forum on 3/23/2011
What Does a PassKey-II 'Fuel Enable' Signal Look Like?!
We know when a PassKey-II Decoder Module does recognize the
key pellet and finds it to be the right one (meaning the
match up is OK), it will enable the starter relay and send
an enable signal to PCM to power up the fuel pump relay and
the injectors. We also know enabling the starter relay is
accomplished simply by pulling down one side of the relay's
control coil (Figure-1) [1996 Chevrolet Monte Carlo Z34,
ECM/Inputs/Outputs Drawing] . But I never knew what the
fuel enable signal to the PCM looked like. This post is
about that…if you know it, no need to read on any
We had this 1996 basket case Monte Carlo (3.4L) that came on
the hook for a no-crank, no-start problem. A little more
questioning of the owner revealed that a security system
that might or might not have had a remote start feature was
installed on this car for years, and since they have no key
fob of any sort, best is, he suggested, to have it
disconnected to see what else was wrong...if any! We were
also informed about the customer's budgetary restrictions,
pending divorce, lost dog, leaking roof, ailing
The fact that the car has been sitting for a year and a half
in their driveway was not something that was mentioned at
all, that came out a few days later as an added bonus when I
demanded additional information and/or a little more truth.
About a bucket load of wires, modules, relays and
Scotch-lock connectors that constituted the bulk of the
add-on system were removed and the car was wired up to it
original shape as best as possible. The battery was charged,
and then the engine cranked but would not start. Fuel
pressure was checked to be zero, and when the fuel pump
relay was manually activated, the fuel pressure came up
reasonably well. So the PCM was not enabling the fuel pump
relay. The security light was of no help because it was
out…either taken out or had a blown bulb. So the
question now was do we have or have not a security issue?
Well, to figure this out quickly, we disconnected the 2-wire
connector under the column (the one that goes to ignition
lock cylinder) and then, the starter did not crank!
I concluded this means the Decoder Module is recognizing the
key's resistance, that's why it does not crank without
it...but...do we have a fuel enable signal? What it is and
how it looks like?
Well, MOD (in my case...the worn 1996 Mitchell Manual) said
to check for a 5V reference voltage at the Pin #A3 of the
Decoder Module's plug, with the plug disconnected and
ignition switch on. There was no 5V to speak of; best value
read was 0.55V...not enough.
So...we pulled the PCM out from the cavity around right
front wheel well by clearing a few odds and ends like a
junction block, radiator overflow bottle, body braces,
covers...etc. A mouse nest larger than the size of a
baseball was also found under the PCM but no sign of
critters chewing out any wires.
With the connectors still on PCM, Pin #55 of the C-1 was
checked along with the continuity from this pin to A-3 of
the Decoder Module plug, still no sign of 5V, but continuity
was intact. The conclusion was that we had a PCM!
Luckily, my friendly local salvage yard had a PCM that was
close enough (!), meaning the service number was within a
digit or two of the original. I know this is not good enough
and not exact...but for the car on hand, it seemed good
enough. So I gave it a try.
Our used PCM started the engine! After a few nerve-wracking
knocks, it smoothed out and surprisingly run well for a car
that had sat for year and a half. All warning lights went
off…and it seemed to be running fine anytime we
started it and allowed it to warm up.
Since the PCM was still hanging there by the wires, I
decided to see what actually the fuel enable signal looks
like. Well, on key up, it shoots up to 5V and once module
starts enabling (!), it changes into a pulse-train as shown
on Figure 2. [1996 Chevrolet Monte Carlo Z34,
While the engine is running, it maintains the pulse-train of
the previous capture and remains as such (Figure-3) [1996
Chevrolet Monte Carlo Z34, ECM/Inputs/Outputs Waveform]
...and I assume (watch out!) as long as it stays in that
state, the fuel pump/injectors are activated and the engine
keeps running. I do believe this is the 2.5 V that is the
step Mitchell says to check for if the 5V is present.
I am not sure whether the specific of this signal
(pulse-width, frequency…etc.) is/was particular to
this car/engine, VIN, or is a Decoder Module's
feature,…or is something general to all cars of this
vintage Pass-key-II. I am assuming (here I go again) the
latter, and am thinking perhaps this is what the add-on
remote start people do to fool the PCM. That is put a signal
generator device in this line which makes the PCM think the
Decoder means "Enable". But I am not certain…and I
So the car was delivered with the used PCM and all the
related caveats, and the owner said he is taking it to DMV
right then to get a license plate for it! Go figure.
Hope this help someone somewhere.
Mohammad from Illinois