42. Émulateur SKIM / SKREEM pour Chrysler, Jeep et Dodge (J1850 VPW)

[Abrara31]

Hello,

I’m currently working on a Jeep that originally arrived with a wiring fault on the gateway. The initial problem was simply a broken wire in the gateway harness.

During previous repair attempts, the SKIM module was removed and replaced with one of your SKIM emulators in order to bypass the immobilizer issue.

Since that modification, the vehicle will only start when a diagnostic tool is connected to the VPW network.

Here is what I observe:

* When the emulator is installed, it appears to pollute or saturate the VPW bus. Communication between modules becomes unstable.
* As soon as I connect a diagnostic tool and initiate communication, the VPW network becomes stable and all modules communicate correctly. In this condition, the emulator works and the vehicle can start.
* If the diagnostic communication stops (even if the tool remains plugged in), the network becomes unstable again.
* If I reconnect an original SKIM module, the VPW network behaves normally and communication is clean, but the vehicle cannot start because I no longer have the correct SKIM data file.

For reference, the vehicle uses an **EDC16C2 ECU**, which communicates on **CAN**.
The immobilizer path is therefore:

SKIM → VPW network → Gateway → CAN → EDC16C2 ECU

Unfortunately, I no longer have the original SKIM data. I attempted to recreate a valid file using the ISK / PIN / VIN, but I was not able to generate a working configuration.

At this point I am stuck. The original fault was only a broken wire, but now I no longer have a valid SKIM file and the emulator does not appear to behave correctly on this network architecture unless a diagnostic tool is actively communicating.

Could one of the developers of this emulator please contact me or provide guidance to understand what might be happening?

Thank you for your help.

vpw-not-ok.jpeg

vpw-ok.jpeg

[Girts]

If the diagnostic communication stops (even if the tool remains plugged in), the network becomes unstable again.

Try to play with some pull-down between VPW and ground (about 10 kohm).
Test voltage between different ground points (ECU(GW) and SKIM).
Anyway, there must be visible difference on scope if bus is OK or faulty.

On your scope screens looks like some ECU (or possibly emulator) is trying to send something but can't do that because feedback from VPW line is not as expected. Or - simply something on car is living on their own...

[Abrara31]

Hello,

First of all, thank you for your interest and for taking the time to help me investigate this issue. I really appreciate it.

I have already spent about four full days working on this vehicle trying to understand what is happening on the VPW network.

Regarding the points you suggested:

I have already checked the ground points multiple times between the ECU (gateway area) and the SKIM location. I did not find any abnormal voltage difference between the grounds.

I also tried adding a pull-down resistor between VPW and ground (around 10 kΩ) as suggested, but this did not resolve the problem.

Unfortunately I did not document every test step with pictures or scope captures while I was doing the initial diagnostics. However, after your message I will repeat the tests again and document them properly with oscilloscope screenshots and measurements so everything is clearly visible.

That being said, from what I have already observed, the behavior is always the same:

When the SKIM emulator is connected, the VPW bus becomes unstable and it looks like something is repeatedly trying to transmit but failing because the feedback on the VPW line is not what the device expects.

When I reconnect an original SKIM module instead of the emulator, the VPW network becomes clean and stable immediately. The “feedback / retry” behavior completely disappears.

So far this phenomenon only happens when the emulator is present on the network.

Another very interesting observation is the following:

The vehicle will only start when a diagnostic tool is connected and actively communicating on the VPW network.

As soon as the scan tool initiates communication, the VPW bus becomes stable and all modules communicate normally. In this condition the emulator also behaves correctly and the vehicle starts.

If the diagnostic communication stops, the network becomes unstable again.

This makes me wonder if the scan tool is effectively acting as some form of network “master” or providing periodic traffic that keeps the bus synchronized (possibly something similar to a keep-alive or arbitration effect).

I will repeat the tests again and provide oscilloscope captures for each configuration so that the differences on the bus can be clearly seen.

However, based on the current behavior, how would the emulator developers explain the fact that the vehicle starts only when an active diagnostic communication is present on the VPW network?

Thank you again for your help.

[Girts]

What is not clear - what happens on "faulty bus" when you disconnect emulator? Bus normalizes?

[Abrara31]

Yes, I can confirm the following behavior.

Without the emulator connected, the VPW bus is normal and stable.

When the emulator is connected, the bus becomes unstable.

If I connect a diagnostic tool but do not start any communication, the bus is still unstable.

However, as soon as I start a diagnostic request on the VPW line (EOBD pin 2), the network immediately becomes stable.

This happens regardless of which module I try to communicate with. For example:

PCM (communication goes through the gateway to CAN)

BCM

Instrument cluster

Attempting to connect to SKIM (even though the original SKIM is no longer present)

As soon as the diagnostic communication starts on the VPW network, all modules communicate normally and the vehicle is able to start.

If the diagnostic communication stops, the network becomes unstable again.

I will also attach oscilloscope screenshots to show the difference between these states.

See attached image.
thank

no-strat-no-diag-no-emul.JPG

emul-pluged-no-diag3.JPG

emul-pluged-diag-pluged.JPG

emul-pluged-diag-tryconnected.JPG

start-after-diag-emul.JPG

[Abrara31]

start-after-diag-emul.JPG

Sorry, I made a mistake when naming the images. Images 2 and 3 are reversed. The last one shows “start and emulator removed.”

Here is the image with the emulator installed: the diagnostic communication tool is established, the bus is stable, and start authorization is enabled.

To properly understand the images, you need to check:

the engine RPM,

whether the terminal block connection is visible or not,

whether the emulator is placed on the oscilloscope or connected (hanging from the wires under the steering wheel).

I sincerely apologize for the mistakes. I am trying to be as clear and precise as possible
.

start-after-diag-emul.JPG

[Abrara31]

Sorry, I lost the photo, but if it started, it was thanks to the emulator. Sorry for the confusion.

[Girts]

OK. No ideas.
1) add resistor between VPW and ground, 2.7...3.3k;
2) add additional 22pF capacitor to filter noises - bus to ground.

Make scope screens where upfront is visible (one bit width on scope screen).