Tacho question for the electronics gurus

[Turbofan]

I recently acquired a VDO tach put of a Land Rover Defender, nice little 2 1/16" 6000rpm gauge, that runs directly from the W terminal. The info off the gauge is:
Vdo 333.230/105/26
12v/1800hz

I've been playing around with it, and can get it to read within a couple hundred rpm's of what it should, but I'd line to try to get it a bit more accurate. I played with the pot on it, on the highway, and at 70 it reads a hair over 3k, but at lower rpm's it's a lot further off (maybe 200 at ranges between 1000 and 2500 rpm). I'm thinking that I'll measure the pulley ratios at work (crank and alt) of the Defender, then try to match that on my mk2, possibly by machining a custom alternator pulley. Here's my question, is there a frequency issue (as the gauge says 1800hz) using our alternators? I have no idea what frequency my 1.6 alt puts out. The money hasn't exchanged hands yet so if this isn't going to work, I can still back out.
Anyone have a thought or two? Thanks for reading all this!

[92EcoDiesel Jetta]

Spend $12 and buy a photo tach to calibrate yours.

http://www.ebay.com/itm/New-Digital-Laser-Photo-Tachometer-Non-Contact-RPM-Tach-/260833548849?pt=LH_DefaultDomain_0&hash=item3cbae41a31

[RabbitJockey]

the frequency is what the tach actually measures for the rpm, the faster the alternator spins the higher the frequency, so its nothing to be concerned with.  i think if u can get the tach perfectly accurate at one rpm it should stay accurate. through out the rpm range, unfortunately tho, these types of things do not always work linearly, it would be nice if there was a zero pot and a span pot, instead of just a span pot.  perhaps one could be added, but someone like regcheeseman would have to figure that one out

[Turbofan]

Thanks for the thoughts. That $12 calibration tool looks neat, so I'll probably pick that up if for nothing other than setting idle speed correctly... I suppose it's possible I miscalculated my gear ratios too, which is why it's off at lower rpms, but I really don't think I did. I'll have to check my tranny code today at work and make sure at least that's right. I'd absolutely welcome Regcheeseman's input. Hopefully I'll get it!

[Mark(The Miser)UK]

Every gauge  is set by span and zero.
Zero could be by lifting and moving needle, and span by a pot, or a resistor change.

Adjust span until constant error, then lift needle.

 Another factor is the number of poles on the alternator. They vary somewhat and are matched to the gauge.

Key word searching should find Reg's circuit

[regcheeseman]

I can't remember that far back!

I did have a test frequency to cal the revcounter on the bench but that wouldn't help a great deal.

I'm guessing that the defender alt has less/more poles than the VW one and the W signal is of a slightly different magnitude.

It depends how much it bothers you, but I'd set it to read correct at idle and then live with any error, an output from the alt is never going to be very accurate.

All most people want is a whizzy cool gauge on the dash so they can pretend at playing race car driver (or is that just me)

Another option would be to make a custom clock graphic and adjust the markings on it to suit.

I played with the pot on it, on the highway, and at 70 it reads a hair over 3k

I don't understand what should it read at 70? your comparison can only be against engine speed not road speed.

[Vitwagen]

As said, it should be a linear relationship.

But you can use this to fine tune it:

Dakota Digital LINK

 

[Mark(The Miser)UK]

I can't remember that far back!

I did have a test frequency to cal the revcounter on the bench but that wouldn't help a great deal.

I'm guessing that the defender alt has less/more poles than the VW one and the W signal is of a slightly different magnitude.

It depends how much it bothers you, but I'd set it to read correct at idle and then live with any error, an output from the alt is never going to be very accurate.All most people want is a whizzy cool gauge on the dash so they can pretend at playing race car driver (or is that just me)

Another option would be to make a custom clock graphic and adjust the markings on it to suit.

I played with the pot on it, on the highway, and at 70 it reads a hair over 3k

I don't understand what should it read at 70? your comparison can only be against engine speed not road speed.

Interesting you should say that Reg, except this tach from a van [stupidly I didn't pay attention as to what it was when I took it out] is completely linear with the alt,  and matches the diesel tach in the car and the digital frequency counter on my strobe. Yet it's FSD is only 3500.  I got this before  I imported a vw tach. I could take it under the bonnet for rev checks. Talk about lucky

[Mark(The Miser)UK]

The Quantum reads about 70mph in 5th @ 3000rpm.  Some G/b's are better but most are there, or 65mph to 70mph, so the reading is not far off.
Do as I said before and you will be fine. [I was actually an instrumentation engineer 30 years ago]

I worked in a research station where the experimenters were forever adjusting the gauges to suit their  expected results, so it was my job to recalibrate them

[Turbofan]

Thanks or all the help! I don't remember why I decided to calibrate it at 70, probably because I spend a bit of time on the highway and I have no idea what I'm idling at. I used a gear calculator to compensate for the 195-60-14's (slightly over-sized) and just plugged in what the rpm's should read at 70. I think I actually over-adjusted it a bit because I realized I'm a hair over 3K. At idle it sits around 1K, which is a little high too (though I know my car idle's a bit high). I ended up ordering that suggested digital laser tach from china, just to compare what I've actually got going on. The only reason I put it in really is because I want to gov-mod my car and I thought that was a suggested gauge. Though it does look cool next to my "turbocator" gauge. http://www.isspro.com/proddetail.php?prod=R609

Speaking of that, do these things really make peak hp at 4800 rpms? You couldn't pay me to rev this thing up that far. I've only had the guts to bring it to 4500 or so. What's the suggested redline?

[RabbitJockey]

its odd how they supposedly make peak power up there, seems like that where it all falls off.

[Turbofan]

I completely agree. I can't figure it out.

[RabbitJockey]

i guess it has to do with  how hp is calculated, it is torque at speed, torque and hp are always equal at 5250 rpms or else the dyno is a liar.  torque is always higher below 5250, and lower after 5250.