bnamunds Posted March 8, 2017 Share Posted March 8, 2017 I have a 1988 Defender 90 with a 2.5 petrol motor that I would like to add a tachometer/RPM gauge to - I have already purchased the gauge and plan to fit it where the clock is in the dash. Has anyone ever fit a tachometer to an engine before? I can figure out how to power it electrically, but I'm not sure how to mechanically connect it to the motor so it reads the correct RPM. Thanks! Quote Link to comment Share on other sites More sharing options...
timc1967 Posted March 8, 2017 Share Posted March 8, 2017 Depending on type of rev counter, most modern ones connect to the coil on a petrol engine. Quote Link to comment Share on other sites More sharing options...
dangerous doug Posted March 8, 2017 Share Posted March 8, 2017 It can work off of the coil or alternator Quote Link to comment Share on other sites More sharing options...
Puffernutter Posted March 8, 2017 Share Posted March 8, 2017 With a diesel its usually the alternator (which isn't a function on earlier Landies). With a petrol it depends on the tacho. Some tachos use a wire from the coil looped around the tacho (forms a single turn), others are a direct connection. What instructions came with it? Cheers Peter Quote Link to comment Share on other sites More sharing options...
Peaklander Posted March 8, 2017 Share Posted March 8, 2017 Do they need to be calibrated to the actual engine? I mean, if they pick up a signal on say the alternator (W terminal) doesn't that mean it shows alternator revs? If it's the coil doesn't that mean four impulses per engine rev? I might be being dim ? Quote Link to comment Share on other sites More sharing options...
Puffernutter Posted March 8, 2017 Share Posted March 8, 2017 Can of worms! If a tacho for a petrol engine that takes a feed from the ignition, then generally all you set up is the number of cylinders. If you're taking pulses from an alternator it depends on how many pulses/revolution from the alternator and the pulley sizes from the crank pulley to the alternator pulley! I'd buy a tacho for a petrol engine that is driven off the coil if you could - by far and away the easiest solution! Cheers Peter Quote Link to comment Share on other sites More sharing options...
=jon= Posted March 8, 2017 Share Posted March 8, 2017 The VDO tach I have (unconnected) in the 110 has a setting for being connected to an alternator (as well as #cyls settings) with a calibration pot so you can tune it to read correctly if using the alternator... Quote Link to comment Share on other sites More sharing options...
Jocklandjohn Posted March 8, 2017 Share Posted March 8, 2017 You can calibrate with one of these, I bought one and works perfectly. Stick reflective tape on the crank/fan belt pulley, point device at it and it reads out the rpm. (taking into account what puffernutter mentions about cranks revolutions etc etc). Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted March 8, 2017 Share Posted March 8, 2017 I also have one of those tachos, work great Quote Link to comment Share on other sites More sharing options...
Peaklander Posted March 8, 2017 Share Posted March 8, 2017 ^^^^ that all makes sense! Interesting Thanks Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted March 8, 2017 Share Posted March 8, 2017 Jeez you guys have headed off into the weeds there, OP did say it's a petrol engine and he's already bought the gauge! 8 hours ago, bnamunds said: I have a 1988 Defender 90 with a 2.5 petrol motor that I would like to add a tachometer/RPM gauge to - I have already purchased the gauge and plan to fit it where the clock is in the dash. Has anyone ever fit a tachometer to an engine before? I can figure out how to power it electrically, but I'm not sure how to mechanically connect it to the motor so it reads the correct RPM. Thanks! To answer the original question, albeit with another question: A tacho is usually electrically triggered from the points-side connection of the ignition coil, so it's just counting pulses-per-second. They usually have a little switch to select how many cylinders your engine has, so it knows that number of pulses equals one rev, easy! Mmechanically connected ones would be very old or very rare. What tacho do you have (make / model / any markings)? Can you post a picture? Quote Link to comment Share on other sites More sharing options...
Peaklander Posted March 8, 2017 Share Posted March 8, 2017 To be fair he said I can figure out how to power it electrically, but I'm not sure how to mechanically connect it to the motor so it reads the correct RPM I read that as "I can get power to it but don't know how to get a signal to it" and it was that that got me wondering about calibrating (assuming it's electrical input of course). Quote Link to comment Share on other sites More sharing options...
bnamunds Posted March 14, 2017 Author Share Posted March 14, 2017 Thanks fellas - I think this is exactly the information I needed. If I still can't figure it out based on the responses then I'll post a picture of the gauge and all the details. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.