I’m starting work on my ’85 VW GTI project, which includes installing VEMS. This will be complicated by moving the battery to the rear of the car. It is the first time I’ve attempted anything this ambitious. In an attempt to get as much as possible right on the first try I’ve started studying the VEMS installation manual. I’ve read over the “Ground Rules’ section a few times and am left with some questions about the rules in general and how they should be applied in my specific case. If any of the experts on here could answer these questions it would be appreciated. Some of them may be minor points but I’d like to understand as much as possible.
It seems that it is permissible to run a wire or wires from the common ground point (the junction of the five ground wires from EC36) to grounding point on the chassis or engine.
In my case if I mount the ECU in a convenient location inside the passenger compartment, this wire or wires would be approximately two to three meters long.
-Is 2-3M length acceptable?
-Is there a length which can be considered the maximum allowable length?
If it is vital to reduce this length I have two options: move the ECU to a less accessible location inside the passenger compartment. This would make the wire(s) closer to 2 meters in length. Or I could move the ECU under the hood to the rain tray. This is where the factory ECU is mounted and is separated from the engine by sheet metal. This would shorten the wires a little more, to 2 meters or possibly less. I’d feel safer locating the ECU inside the passenger compartment.
If it’s critical to shorten the wire length to the ground point:
-Should I mount the ECU in an difficult to access location inside the passenger compartment in order to shorten the wires to close to 2 meters length.
-Should I mount the ECU under the hood in order to shorten the wires to 2 meters or less in length?
-If the ECU is located under the hood, should it be installed in an enclosure of some sort?
In the “ground rules” text it is specified that the wire(s) from the common ground point to grounding point should be 4mm2, total. But the drawing under the heading "Sensor Ground" shows this area as 6mm2. This is a significant difference. Considering the length required by my installation, I plan to use 6mm2 wire. Or:
-Is 4mm2 acceptable is all cases?
It is suggested that the wire(s) from the common ground point could be terminated at the battery, chassis or engine. But is it seems that the battery (large current pulsations) and chassis (large current fluctuations) are basically ruled out.
- Is the engine really the only point that can be recommended as the ground point?
It seems there are two options for connecting the sensor grounds. They can be connected directly to the common ground point. Or they can be connected to a point along the wire that joins the sensor ground pin to the common ground point. The drawing under the heading "Sensor Ground" shows the sensors connecting to the sensor ground branch wire at a point "~20mm" from the common ground point. This dimension is not mentioned in the text. I believe that I read a forum post read that stated this 20mm distance was not necessary. Another post seemed to suggest that this 20mm distance was the maximum distance up the sensor ground wire from the common ground point at which the sensor grounds can be connected.
-Can this ‘rule’ be summed up as:
The sensor grounds may be connected either directly to the common ground point or to the sensor ground wire (the wire that runs from the sensor ground pin to the common ground point) at any point no further than 20mm from the common ground point.
It seems that the rules call for any sensors with shielded wire the shield should be grounded near the sensor mounting point, not at/near the ECU. My concern is that the shield connections near the sensor mounting point will amount to single, small gauge, loose wires connected to the engine block and as such will be much more prone to damage than if they were grounded at the other end near the ECU.
-Is it possible to ground shields near the ECU?
I don’t see mention of using shielded cable for the communications wiring from the ECU to the PC serial port.
-Would shielded cable be a good idea here?
There is a line in the ground rules which reads: “Connect Power Grounds to the Common Ground point, or anywhere you like (except the sensor-ground branch).” Maybe I am misinterpreting this but I find it completely perplexing. A major part of the discussion is that the power grounds must be connected at a specific, single point (the common ground point) certainly not “anywhere you like.” Is this sentence referring to some power grounds other than the five wires from EC36? If someone can explain what this means I’d like to know.
Thanks for your help.
