Thread: Dumb question...tachos and timing lights

Hi Jeff
Most diesel tachos recieve thier input from the alternator.
Andrew

Originally Posted by LandyAndy

Hi Jeff
Most diesel tachos recieve thier input from the alternator.
Andrew

Yeah, sorry, maybe I wasn't clear...I have the tacho connected to the alternator just fine, but I need to calibrate it, which needs a timing light or something else to accurately read the engine speed so I can set the tacho to match...and as I've never done this on a diesel I'm wondering whether I can use a petrol engine timing light to do this by taking the timing light's pulse feed from the alternator...

Interesting concept,sorry cant help!!!
Andrew

Originally Posted by LandyAndy

Interesting concept,sorry cant help!!!
Andrew

First time I've come across it too...but this is the tacho recommened on here and as far as I know the OEM model from VDO for the non-TD5 diesels...

Will have a further play in daylight.

I'm sure that others will comment in the morning - I'm of the opinion that the timing light will not be any use in calibrating your tacho. The pulse from the alt will probably cause it to fire allright - but that is all. Remember in a diesel the timing of the injections stays the same - it's the amount of fuel that changes the RPM/load. Your other petrol engine tune up equipment is generally useless too. However, you know the spec idle speed of your engine (720 warm +/- 20) - your tacho should be close to that. A couple of revs either way won't matter. If it's a couple of hundred out, then it may be set up wrong. If you are a electronic wiz, then using the tacho output measured via a frequency meter might work if you calculate the difference in diameter of the crank pulley to alt pulley, or there exists a vibration tachometer - drop by a diesel engine workshop, they may have one. Otherwise, if the tacho says idle is around that the right mark, you shift around 3000 RPM, and you do 2250 RPM in 5th @100k - then all is OK.

Hi Jeff,
You can get an optical Tacho on ebay for $39.00 which reads a piece of reflective tape placed on the crank pulley. That will give you an accurate count for calibration.
I'm working on a low cost tacho for my 3.9 Isuzu at the moment which will use piezo sensing on the fuel line, for those of us that can't be bothered with the alternator setup (which is subject to belt slippage error). And stepper motor movement for the display.
PM me if you want any info
Danny

Originally Posted by langy

The pulse from the alt will probably cause it to fire allright - but that is all. Remember in a diesel the timing of the injections stays the same -

.


sorry, not quite they advance and retard, its just harder to change and most dont do vacume advance.

Remember if your reading from the alternator your reading alternator rpms.

to read engine rpms you need a strobe meter that reads a return light signal from a bit of tape on the front crank pully or one of the adaptors that puts a piezo sensor on an injector line and drives a normal spark lead type timing light/tacho.

OK, thanks all...seems nowhere near as simple as I thought. Think I'll set it up so it's roughly right at idle, and then wait till the thing is on a dyno having the pump played with, and use their test jig to adjust it if necessary!