The stock vanagon tach (and many aftermarket tachs/devices) signal is taken from the negative terminal of a single ignition coil. Engine swaps that include more modern distributorless ignition systems do not have this kind of ignition. They use multiple coils, each with their own signal controlled by the ECU.
There are several methods of getting the vanagon tachometer (and ones like it) working after an engine conversion. The first method to get the vanagon tach working with this type of ignition is to use diodes on the coil wires, which allows the vanagon tach to read the combined signal without crossfiring the coils. However, this method can be very dangerous as the ECU was not designed for this and isn't protected from potential problems. A short to ground of the tach signal wire can cause damage to the coil drivers on the ECU resulting in a dead ECU. Another common method is to modify the tach itself to read the type of signal generated by the ECU. This involves removing the instrument cluster, removal of the tach itself, replacing components in the circuit, or sending the tach to someone else to modify. All of these complications and potential sources of problems are eliminated with the Tach signal converter. Simple 4 wire hookup makes it non-invasive, completely safe for the ECU, and entirely reversible.
HOOKUP:
(old style)
Red- switched or constant power
Black- Ground
Yellow- tach signal from ECU (you could intercept the tach wire on the van side so you don't have to cut the bostig harness)
Green - tach wire to the dash
(new style)
WHITE- Ground
BLACK- Tachometer Signal IN
RED- Tachometer Signal OUT
BLUE - Ignition Power
