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Well, that was quite a discussion. A few points (pun intended).
The car is an 89 with Bosch coil.
It is reasonable to assume the tach is powered, as it gets it signal form the primary side of the coil. Whether it will move is probably a function of how sensitive the tach is, I'd use another instrument if one is available to check the coil input. Question is, does the signal for the tach come off the ignition ECU, before the power stage, or directly from the coil, after the powere stage?
Seems to me the OD relay functions quite differently than the Suppression relay. The OD relay responds to pulses to flip the relay on and off. The suppression relay is just a simple on/off, more like the function of the fog light or headlight relays (providing battery power direct to the load).
Hope to get a spare at the junky and take a look inside it.
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744 & 745 16v 4+OD, 245 SE auto, 242Ti 4+OD, 245DL auto parts car
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