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What Does a PassKey-II 'Fuel Enable' Signal Look Like?!
Posted to Technical Tips Forum on 3/23/2011 5 Replies

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 further.

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 grandma…etc….!!!

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 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, ECM/Inputs/Outputs Waveform]

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 don't know.

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

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car Vehicle Data

1996 Chevrolet Monte Carlo Z34 3.4L

Engine3.4 L
Trans4-speed Automatic (Electronic)