 | Home > Technical > Getting an accurate reading for a tachometer? |
 
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| donmacn Member Since: 06 Nov 2017 Location: Nth Scotland Posts: 1845  
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Hi folks,
I started wandering into this topic in this thread https://www.defender2.net/forum/topic68082.html but I think it's moved far anough away to be worth a fresh go. I bought a handheld laser tachometer. Using this on the crank pulley, at idle (hard to do otherwise!) I get a reading of about 670rpm. I doubled up the reflective marks for the gauge to pick up on and got 1340 - so happy that the gauge is consistent. But isn't this low for a 300tdi? I thought they were supposed to idle around 800/850rpm? On the alternator pulley, with just one reflective mark, I'm getting a reading of 953rpm. This is on a standard, original factory fit, 65amp alternator. The VDO tacho in the car, just fitted today, is giving a reading of about 550rpm, so it needs tweaking anyway, but I'm just wondering if 670rpm would be an accurate figure? Ideally I'd try the gauge on something else with known revs, but I can't think that I have anything to hand. EDIT - just occurred to me that I took a reading with the engine cold. Maybe if I'd taken it for a wee run first it would have idled faster? Thanks Donald 1994 Defender 300Tdi 110 SW - owned since 2002 - 230k miles and going strong (The 'rolling restoration' or tinkering thread: http://www.defender2.net/forum/topic58538.html ) 2000 Range Rover P38 4.0L V8 in the past.. RR classic - fitted with 200Tdi 1984 RR classic - V8 with ZF auto box 1993 Discovery 300Tdi not to mention the minis and the Type 2 VW camper... |
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22nd Jun 2019 7:45pm |
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| donmacn Member Since: 06 Nov 2017 Location: Nth Scotland Posts: 1845
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Ok, I've been doing a little googling about. Probably should have done that first...
Seems the workshop manual has 720 RPM plus/minus 20... so mine isn't as far out as I thought, but maybe a little low. I'll have a look at tweaking it up, but before I do, does anyone have any more experienced comment to make? Thanks as always. Donald 1994 Defender 300Tdi 110 SW - owned since 2002 - 230k miles and going strong (The 'rolling restoration' or tinkering thread: http://www.defender2.net/forum/topic58538.html ) 2000 Range Rover P38 4.0L V8 in the past.. RR classic - fitted with 200Tdi 1984 RR classic - V8 with ZF auto box 1993 Discovery 300Tdi not to mention the minis and the Type 2 VW camper... |
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22nd Jun 2019 8:01pm |
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| donmacn Member Since: 06 Nov 2017 Location: Nth Scotland Posts: 1845
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Eh?! Really...? I didn't get a chance to do anything about this today. Found something else Defender related to do.... 'cos there's always something.
I have noticed that when I start the car, the tacho stays at zero, even with the engine running, until I blip the throttle and then it comes up to 550. I'll try putting the headlights on and see what happens! Donald 1994 Defender 300Tdi 110 SW - owned since 2002 - 230k miles and going strong (The 'rolling restoration' or tinkering thread: http://www.defender2.net/forum/topic58538.html ) 2000 Range Rover P38 4.0L V8 in the past.. RR classic - fitted with 200Tdi 1984 RR classic - V8 with ZF auto box 1993 Discovery 300Tdi not to mention the minis and the Type 2 VW camper... |
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23rd Jun 2019 6:43pm |
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| jfh Member Since: 08 Jan 2014 Location: West Coast Posts: 359  
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If it does that it the alternator that hasn't kicked in yet.
What I mean is without headlights the needle is a bit jumpy, but with headlights on it's very stable. This is for idle only. |
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23rd Jun 2019 7:58pm |
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| donmacn Member Since: 06 Nov 2017 Location: Nth Scotland Posts: 1845
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Quoting myself is probably the first sign of madness.... or something like that. But this is still the simple question I need confirmed. Is the crank pulley on a 1:1 ratio with engine revs, or is there some sort of multiplier to calculate? On my handheld laser thing, I get idle at about 680rpm now. This seems just a little low according to the WSM which says 720 +/- 20. So I'm not miles off, and could adjust, but before I do I just want to idiot-check this simple thing - that the crank pulley is 1:1 with the engine revs. Ta Donald 1994 Defender 300Tdi 110 SW - owned since 2002 - 230k miles and going strong (The 'rolling restoration' or tinkering thread: http://www.defender2.net/forum/topic58538.html ) 2000 Range Rover P38 4.0L V8 in the past.. RR classic - fitted with 200Tdi 1984 RR classic - V8 with ZF auto box 1993 Discovery 300Tdi not to mention the minis and the Type 2 VW camper... |
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31st May 2020 11:38am |
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| blackwolf Member Since: 03 Nov 2009 Location: South West England Posts: 17417  
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Yes, the crankshaft pulley is fixed directly to the crankshaft (traditionally located with a Woodruff key) and rotates at the same speed. I am not aware of any vehicle engines where this is not the case (but there will be aircraft engines and other specialist engines where it may not be). There are some engines which have a rubber damper built into the pulley, and it is conceivable depending upon the specific design that one of these could 'slip' if the damper has failed allowing the pulley to rotate at a lower speed.
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31st May 2020 12:02pm |
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| donmacn Member Since: 06 Nov 2017 Location: Nth Scotland Posts: 1845
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Thanks very much. I was confident I had it right.... but I've been confident before..
I have a VDO 4000 rpm tacho, and I had this idea it was reading wrong, and that the micro switches at the back weren't set properly, but now that I've confirmed the crank pulley is 'datum' then I can work from that. Ta Donald 1994 Defender 300Tdi 110 SW - owned since 2002 - 230k miles and going strong (The 'rolling restoration' or tinkering thread: http://www.defender2.net/forum/topic58538.html ) 2000 Range Rover P38 4.0L V8 in the past.. RR classic - fitted with 200Tdi 1984 RR classic - V8 with ZF auto box 1993 Discovery 300Tdi not to mention the minis and the Type 2 VW camper... |
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31st May 2020 6:32pm |
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| sako243 Member Since: 08 Jul 2014 Location: Wales Posts: 1219  
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Most diesels have this - particularly 4-pots. And if your damper has failed the pulley is unlikely to be slipping for long because crank failure is experienced shortly after. Ask me how I know  Ed
82 Hotspur Sandringham 6x6 95 Defender 110 300Tdi |
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31st May 2020 7:03pm |
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