volvo penta tacho conversion - Page 1
Forum section: electronics / projects

author: m3vuv
author_rank: Super Contributor
timestamp: March 15, 2020, 10:31:48 am
content: Hi all,ive been asked to fit a new engine control panel to my friends yacht,its a volvo penta 2003 3 cylinder diesel,the old tacho has been lost,its a alternator driven tacho/hour meter,the one he has is a vdo unit,on the back is a switch to select the number of cylinders marked 4cyl 6 cyl or 8 cyl,its not got a 3cyl option,could it be moddified to work with a 3 cyl engine,new ones are in the £600 ish price reigon,cheers m3vuv

author: georges80
author_rank: Frequent Contributor
timestamp: March 15, 2020, 08:25:56 pm
content: The VDO unit sounds like it's for a petrol engine and picking up one of the spark plug 'pulses'.The lost alternator tacho would have likely connected to the alternator 'W' terminal (one of the phase outputs). It would need a way to calibrate it to deal with the number of alternator poles and also the pulley ratio between the crank pulley and the alternator pulley.cheers,george.

author: m3vuv
author_rank: Super Contributor
timestamp: March 16, 2020, 12:13:42 am
content: the new tacho is for a diesel,how about telling me something i dont know,what you have said is no help,we already know this!!

author: james_s
author_rank: Super Contributor
timestamp: March 16, 2020, 12:28:05 am
content: Well if you already know this then just say so, being rude to people trying to help you is not a terribly effective way to get help. We can't read your mind and we're not being paid to help you.What happens if you wire up the tach you have? If you set it to 6 cylinder does it read double the true RPM? I'm not actually sure why an alternator driven tach would care how many cylinders the engine has, I agree that it sounds like it's for a spark ignition engine but since you say it's not I'll have to take your word for it. Do you have the model number of the tach?

author: m3vuv
author_rank: Super Contributor
timestamp: March 16, 2020, 06:01:51 am
content: ive yet to wire it up,i will do that later today as ive only just got whe wiring diagram for it,not going to be easy as the engine has been resprayed wires and all and the space is dark and cramped.

author: Ian.M
author_rank: Super Contributor
timestamp: March 16, 2020, 07:46:48 am
content: Diesel tachos don't *care* about the number of cylinders because either the engine is mechanically injected, in which case there is no electrical pulse from the injection pump to run a tacho off, or in the case of common rail systems with solenoid injectors, the ECU generates individual injection pulses for each cylinder and whatever tacho signal the manufacturer desires (often nowadays just a CANBUS message the instrument cluster interprets and displays).Petrol engines with conventional (Kettering) ignition tachos need to be set for the number of cylinders as they tap off the points, which open half as many times as the number of cylinders per crankshaft turn.Due to the almost infinite variation in alternator pully ratios, and possible number of alternator poles, aftermarket alternator driven tachos almost invariably have a continuously variable calibration preset rather than a switch for number of cylinders. OEM alternator driven tachos may not - as the correct calibration for the specific alternator and pulley ratios can be designed in or they may have a switch for different OEM alternator options, again with all settings precalibrated.If the same case moulding is used for the Diesel and Petrol versions of an aftermarket tacho, you'll probably find that on the diesel version its *NOT* a cylinder switch, doesn't 'click' into positions but is a continuously variable adjustment.