CM5 RPM Signal Source: VAG Applications

CM5 and CM5-LT require a clean (i.e. free of electrical noise) square RPM signal source that is synchronized to engine speed and that alternates between 0V (GND) and 5V or 0V (GND) and 12V.

If a signal that swings higher than 12V or significantly lower than 0V (i.e. a negative voltage) is fed into the CM5, the unit could get damaged. If the signal is noisy the RPM values the unit works out will not be accurate - they may jump around, drop out or periodically register as very high RPM values.

Here is an example of a noisy RPM signal:

Noisy RPM Signal

Here is an example of a clean RPM signal:

Clean RPM Signal

The unit and its firmware include some RPM filtering and hardware protection, but since the unit uses the calculated RPM value as the basis for duty control in nearly all of its output modes, a good and reliable RPM signal is a MUST to ensure trouble-free unit operation.

Option 1 - the preferred option !!!

RPM Coil On Plug Pickup

A good quality 0-5V signal can be obtained up by teeing into one of the wires between the ECU and the Coil On Plug modules found on newer models (N70,N127,N291,N292 aka Ignition Coils with Power Output Stage) or Ignition Control modules that were used on some older models (N122, N192 aka Power Output Stage Module).

It's best to tee into the wire as close to the ECU as possible since the ECU is the source of the signal and the coil is the sink, but either location will work. If the ECU wiring is not accessible the signal can be picked up at any of the Coils On Plug as shown.

Coil On Plug RPM Pickup

Here are the recommended Option 1 RPM signal pickup locations for several common VAG engines. Only one of the locations marked with the red "RPM arrow" below should be used.

RS7 RPM Pickup 

Option 2 - "ok" if no other option is available.

Injector RPM Pickup

A 0-12V signal can be picked up by teeing into one of the wires between the ECU and an injector.

Option 1 is preferred because these signals are generated by push-pull logic inside the ECU, which means that the RPM signal will be very clean. In Option 2, the ECU is switching an inductive load (the injector coil) on-off. Inductive load switching is generally a source of significant electrical noise. Depending on the particular ECU's internal electronics, on injector coil characteristics, as well as the length and state of the vehicle's wiring this signal may or may not be good enough to use. Unfortunately, the only true way to tell if this option will work is to look at the signal with an oscilloscope. Since most users won't have access to such equipment, it is strongly recommended that only Option 1 be considered whenever possible.

Wire Routing (IMPORTANT !!!)

The RPM wire tee should be made as close to the ECU as possible and it should be routed so that it is NEVER running alongside (i.e. parallel) or bundled with any other wires. This is ESPECIALLY true for noisy wires like the CM5's power wires, pump output wires, J538 output wires (if that option is used), etc. The reason is that any of these noisy wires will couple their electrical noise onto the RPM wire and corrupt the RPM signal . This will cause the CM5-LT to read false RPM values which will result in erratic and unexpected operation.

Don't bundle the RPM Signal with these signals

The CM5 or CM5-LT User Manuals shows a table showing different wire groups that should or must not be bundled together and allowed to run next to each other for long distances.

The CM5 and CM5-LT manuals state:

The diagram below shows how the unit wiring should be grouped. Some wires carry switching signals which are very noisy and can corrupt other sensitive signals and cause the unit to interpret false pressure readings or derive erroneous RPM values. The unit provides extensive hardware and software signal conditioning mechanisms to minimize the effects of noise-induced problems on its sensitive signal lines, but the user is cautioned to NEVER bundle together or run parallel to each other for any considerable distance, any wires from different Wire Groups shown below – especially anything from Group A together with or parallel to anything from Groups E or F.

CM5 Wire Groups

Posted by Torq Byter_ on 07 November, 2015
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