a real tach

[92EcoDiesel Jetta]

I wanted to check my idle rpm but that's kinda hard to do w/o a tach. I have all the Tach parts from a Mercedes 300D so decided to hook it up to the Jetta as test equipment. Circled in red is the magnetic pickup, in green is the tooth to generate the signal for the pickup. It's just a small magnet with a dab of crazy glue.


Here's a video of it running. I will see if it is possible to trim the tach  down and fit it in place of the clock. If it won't fit, I'll neaten it up and mount everything in a box and leave it as test equipment and look for another tach from another car that will fit.  

[92EcoDiesel Jetta]

ps. the pickup is mounted to a bracket mounted to one of the power steering pump bolts. The "tooth" magnet is on the crankshaft pulley.

[Rabbit on Roids]

the most important part...




DOES IT WORK?

[92EcoDiesel Jetta]

In case you don't know, click on the pic below and the video will play.

[Powered by Spearco]

Nice

[burn_your_money]

Sweet that's pretty cool. Why not just use the stock tach though?

[92EcoDiesel Jetta]

What stock tach? Mine didn't come with one. I've read about using the alternator with the W terminal. I don't really like that idea because it needs to be calibrated for pulley size and I don't have any of those parts and I'm not about to look for them and buy them. I know the Mercedes tach is pretty accurate and no calibration is necessary.

[maxfax]

Very nice!! I wonder what the signal is from Merc box to the gauge itself...    I used a similar setup using 2 teeth on the crank pulley, magnetic pickup, and an aftermarket gasser tach..  Needed some way of converting the trigger pulses to a tach signal..  I used a GM HEI ignition module but it isn;t so accurate unless there is something wired in to simulate the coil....  For now it's an 1156 light bulb, but they burn out frequently...

[burn_your_money]

All you need is to find the cluster from a mk2 diesel with a tach. Match up the wiring style, CE1 or CE2 and that's about it. I've always thought that no calibration was needed but I could be wrong.

Either way though, I like what you've done and my truck will probably get something like that

[92EcoDiesel Jetta]

maxfax, the Merc box puts out a square wave pulse signal to the gauge, which also has a chip in it.

[maxfax]

Thanks for that info..  Might just be easier to stick with my a/c pulses than try to calibrate everything to work with anything..   Not to mention I have get to find a complete working Benz diesel tach in the boneyards around here...


Burn, take a look at the housing for the IP gear on your truck..   There should be a plug either at the 12 o clock or 2 oclock postion..   That's where they screwed in a magnetic pickup for the factory tach....Although I can;t remeber for sure if there were specfic teeth to trigger it, or if it just used the teeth on the IP gear... If it used the teeth on the gear it would probably be easier to trigger it off the crank for calibration sake...

[92EcoDiesel Jetta]

Most mag pickup tachs I've seen triggers on one tooth (that 's one per rev) of the crankshaft. That tooth must stick out from the rest. If you try to trigger of the starter gear of a crankshaft with it's many teeth, you will get a very very high reading (if the tach can process it at all)!

Here's my latest tach. Click on the pic to play the video. It's a one piece self contained unit that needs just power and the mag pickup wired to it.! I hope I can fit it in place of the clock. If not I'll make it a stand alone unit.

[burn_your_money]

Thanks Maxfax. I've often wondered why that hole was there

[92EcoDiesel Jetta]

fixed the video link above, should work now.

[92EcoDiesel Jetta]

At idle the Mercedes tach says it was 1000 rpm, this one says 750. Not sure which is correct. I should be able to calibrate it with a 60hz 120 V AC motor. I think I have one that's 1725 rpm and one that's 3450 rpm that I can glue a tooth to the pulley on the motor and calibrate the tach with.