Tach replacement options for MD11D

[BYCKarma]

My Westerly has a 1983 Volvo MD11D. Last summer a backflow (?) short circuit in the engine wiring fried out most of my instruments, including the original VDO inductive tach.
Replacement options seem to include 1) replacing it with the original or upgraded tach (23715874 I believe, appx $500) or installing one from tinytach.com. My alternator has an unused W lead, and alternator driven tachs seem to be a whole lot cheaper than a replacement inductive tach. Is it as simple as wiring up a new tach from my alternator? None of the other boards seem to discuss that option.

[DougR]

No need to buy a VP tach, there are loads of replacement tachs available, either inductive pickup driven or alternator driven. VDO replacement tachs are available on the internet for $100 and up as well as Stewart Warner, Faria, and others. Parts4 Engines in the U.K. has MD11 replacement tachs under $100.

Your inductive pickup puts out 28 pulses/ crank rev and the alternator W terminal frequency is 466.7 HZ @1000RPM. (assuming standard alternator and standard pulley diameters)

[BYCKarma]

Doug, thanks for the P4E link, $88+ship for MD11D compatible alternator driven tach. Great option. It's funny that in all the boards I've looked at re: replacing inductive tachs on MD11's (at very high prices), I've never come across one that says to replace with alt-driven tach. Several on tiny-tach though. You sure it'll work? I have a cheap manual laser rpm reader to calibrate

[DougR]

No, I’m not sure it will work, I’m just saying that other options are available besides VP product. Check with Parts 4 Engines to be sure it will work with the alternator that you have.

If it were me I would stay with an inductive driven tach, as there are fewer variables that can affect the tach operation, such as loose or broken alternator belt, marginal alternator output,etc.