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Old 09-16-2020, 11:48 PM   #1
940 16 valve
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Join Date: Oct 2005
Location: Calgary, Alberta
Default How to drive tach in whitebkock swap?

Gotta admit, the mechanics don't intimidate with the whitebkock swap, even wiring up the motronic 4.4 was logical enough. But I've realized I don't know crap about how tachs work and how to get mine running (now that the car drives).

Car in question is a 89 760, so I'm looking to drive the 6 cyl tach. The 3.4 is converted to run COP if that makes any difference. Noticed M4.4 wire b21 supplies the tach in the S70/850, but I'm unsure if it's a usable signal (maybe with an adapter).

Was eyeing this as a potential solution but I'm not sure if I need a booster per what the seller is saying.

This is the response I got "For an '89 it's a crapshoot whether you'll need a booster, but you can check it yourself with a tap-tap test. Bounce the tach signal wire on battery+ or another 12V source and see if the needle responds in any way. Any response at all means the adaptor can probably drive it without help. If it stays dead then the tacho is a back-EMF type (high-voltage) and you'll need a booster along with the adaptor (see listing# 142171581596). In low-voltage mode (switch 7 on) the adaptor will work fine with the Motronic ECU output, whether it's 5V or 12V. Driving it off a COP is also an option since all COP's are essentially 1-cyl signals, but if the COPs are 2-wire you'll probably need to wire one of the filter resistors included in the kit. If they're 3-wire then you treat them just like an ECU signal (low-voltage). You'll use a 5:6 ratio for the Motronic, 1:6 ratio for a COP"

1991 940 - stock b234+t -retired
1989 760 - T5 (B5254T2 with a 6 speed!)
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