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Old 09-16-2020, 11:48 PM   #1
940 16 valve
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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 http://forums.turbobricks.com/showthread.php?t=243308
1989 760 - T5 +T5 swapped http://forums.turbobricks.com/showthread.php?t=341191
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Old 09-17-2020, 01:53 AM   #2
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m4.4 outputs a standard tach signal in the 4 cyl variation
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Old 09-17-2020, 09:59 AM   #3
940 16 valve
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Interesting. I cam across an old post saying that volvo outputted tach signals as 4 cylinder for some time. Any idea if the signal is the same voltage/frequency as what the stock 7xx/9xx would be expecting?
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Old 09-17-2020, 03:50 PM   #4
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Join Date: May 2015
Location: Finland

A standard 4-cyl tach signal means it works with 200/700/900 and most aftermarket tachs for four cylinders. Can't say what a PRV tach needs if you have that.
444 -55,
242 DL -80 (was 4.6l)sold, 245 -88 "B6284T" sold, 245 -92 B6294,
245 -90 "B6284T"
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Old 09-17-2020, 04:25 PM   #5
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Location: Boulder CO

The response you got from the ebay seller is amazingly good. About the only thing that might be incorrect is that the ebay seller was expecting the 5cyl ECU to put out 5 pulses per engine cycle, instead of 4 (per RvolvoR). For 4, you'd use a 4:6 setting.

Try the suggested tap-tap test to see if you need the booster. The TBricks/MegaSquirt version of the booster is here: http://forums.turbobricks.com/showthread.php?t=311116 Some folks need it, some don't, when doing a 240 to MegaSquirt conversion.
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Old 09-17-2020, 04:57 PM   #6
James M
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B21 outputs a signal that works fine with a 240 tac, so I assume would work with a 7/9 4 cylinder tac that would have been triggered from the coil just like 240.
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Old 09-20-2020, 07:44 AM   #7
Mats 960t16
Join Date: Feb 2010
Location: The Netherlands

On my t5 swap in (88 volvo 740 with b234f) i just connected the motronic tach output directly to the dash tach and it worked fine. The instument panel was a VDO.
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