Designing a Better Tach Amp - Page 3 - PeachParts Mercedes-Benz Forum

techguy512 · #31 03-31-2009, 01:43 PM

Quote:

Originally Posted by marfa300td

My W123 tach does not work.

Can you describe symptoms?

1) Is it dead all the time?
2) Dead most of the time but springs to life every couple of months for a few seconds?

If #2, there's a 90% chance the amp is bad.

If #1, it could be more than just the amp. Crank sensor at the harmonic balancer, or bad wiring, or bad gauge, or even fuse.

It doesn't hurt to try. If it doesn't work, just send it back.....

mplafleur · #32 03-31-2009, 02:09 PM

I've no tach operation in my SD. I've never tried to diagnose this yet, however I have two spare tach amps I've pull at yards.

However, I applaud your effort to redesign this.

racebannon · #33 03-31-2009, 02:16 PM

If you can make one, I would buy one from you.
What steps do I need to take to make this happen?

My tach is in the #2 catagory - jumps to life now and again, but is all over the place. I would rather get a known working item than a used tach amp that might work for a while and quit.

I remember seeing these for sale on Ebay last year, but was skeptical. Then I found this thread, but was too late.

Please let me know if a purchase is possible.

Thanks

techguy512 · #34 03-31-2009, 02:18 PM

Quote:

Originally Posted by mplafleur

I've no tach operation in my SD. I've never tried to diagnose this yet....

If you ever want to diagnose the system, you can borrow the diagnostic tool. It gets loaned out periodically, but when it comes back I'd be happy to send it to you.....

tbomachines · #35 03-31-2009, 02:53 PM

Quote:

Originally Posted by techguy512

If you ever want to diagnose the system, you can borrow the diagnostic tool. It gets loaned out periodically, but when it comes back I'd be happy to send it to you.....

Can't you probe around with a voltmeter? Also I may be interested. If I can diagnose the problem (my tach has been completely non-functional) to the amp I'll PM you. Again great work and effort, much appreciated!

#36 03-31-2009, 03:01 PM

You can test the crank pickup by using a multimeter to read the millivolts produced while the engine is running. Set it to Ohms and test for wiring harness continuity from the amp plug to the tach plug.

H-townbenzoboy · #37 03-31-2009, 03:11 PM

If you still need someone to test a unit out on a W126 SD, send me a PM.

samdon · #38 03-31-2009, 04:11 PM

If your tach is dead; get one. Took all of 15 seconds to plug it in and it has functioned perfectly for about 6 months. The start up sweep is pretty cool, too.

techguy512 · #39 03-31-2009, 04:37 PM

Quote:

Originally Posted by tbomachines

Can't you probe around with a voltmeter?

Probing around with a voltmeter will give you only the crudest indication of functionality ("yep, it's there" is about all it can tell you).

The crank pickup is a switched reluctance sensor. The amp puts a constant current through the coil, and measures changes in the current as the pin swings through the magnetic field. Peak voltage of the signal is around 500 millivolts. Picking signal out of noise is the trick.

techguy512 · #40 03-31-2009, 04:38 PM

Quote:

Originally Posted by H-townbenzoboy

If you still need someone to test a unit out on a W126 SD, send me a PM.

Thanks. SunnyinTX helped me git it working last spring. It's all good.

mplafleur · #41 03-31-2009, 07:38 PM

If none of the units I have works, I can probe the pins with a scope to see if the signal is getting through. Does the amp load the signal or pull it up?

oldsinner111 · #42 03-31-2009, 07:55 PM

My SD when bought new,the owner had 3 amps put in.The Techs could not fix.Years later I did.What was happening is everytime you screw the amp on,it pushes the metal sockets and wires down.Mercedes should have allowed more flex in the harness.Solution is to wedge up the wires and metal sockets,so as you screw down the pins,the sockets don't move.

techguy512 · #43 03-31-2009, 08:53 PM

Quote:

Originally Posted by mplafleur

I can probe the pins with a scope to see if the signal is getting through. Does the amp load the signal or pull it up?

Using a constant current source, put 30 to 50mA through the pickup coil (pins 7 and 9 in the amp socket). With the scope on AC coupling, you should get a zero volt signal when measuring from pin 7 (gnd) to pin 9. When the engine is started, you should see a small 'blip' once per crank rev.

dcotejr · #44 03-31-2009, 08:56 PM

I bought one about a month ago after not having a tach for almost a decade.
Works perfectly; utterly simple installation. Great piece of work!

JackG · #45 03-31-2009, 09:37 PM

price????????