OM617A Tach Amp - Page 3 - PeachParts Mercedes-Benz Forum

pawoSD · #31 05-03-2008, 11:20 PM

Quote:

Originally Posted by pizzachef

Is it a 6 point? 1 1/4" is just shy of 32mm...careful, don't want to round THAT bolt.

Yes, when I said 1 and 1/4" I actually meant 1 and 1/16"

Wups.

It is a 6pt....works great.

pawoSD · #32 05-04-2008, 12:15 AM

Well, I got the pickup as close to the crank pin as I could, but it still reads the same....barely anything at idle, and 1/2ish of actual rpms when revved up.

Could it have anything to do with the fact that I have a gasser tach in the car? My original had a dead clock so I swapped it with one from a 280se....they have the same (large 2-pin) connector etc...the only difference is that the gasser one goes to 7k instead of 6k.....

Techguy? Anyone?

pawoSD · #33 05-04-2008, 01:36 AM

I think I have it figured out, the gasser tach is designed to read the pulses from the ignition system of the 6cyl gasser (280), which would happen more frequently than the crank pulse of a diesel, which is why it reads less than half of my actual RPM......sooo, I will be doing surgery on my cluster tomorrow and taking the working clock from my gasser tach unit, and inserting it into the original diesel tach with a dead clock that I took out of my car a long long time ago, then reinstalling that back into the car....hopefully it solves the problem!! I know the sender and amp work now, since the needle does move....

After my modifications tomorrow it should work again....

Johnt49 · #34 05-04-2008, 10:00 AM

FYI the meter movements are the same on all models. The only difference is the circuit board on the back & the face. It might be easier to change out the board.

techguy512 · #35 05-04-2008, 10:18 AM

Quote:

Originally Posted by pizzachef

Can you put me in line for borrowing the diagnostic tool?

PM your address to me. Should be ready to go Tuesday or Wednesday....

pawoSD · #36 05-04-2008, 10:29 AM

Quote:

Originally Posted by Johnt49

FYI the meter movements are the same on all models. The only difference is the circuit board on the back & the face. It might be easier to change out the board.

Yes, but the gasser ones measure the signal differently...so they are not interchangeable.......The part numbers are different also.

The gasser one says "21000 lmp" on it, while the diesel tach says "6000 lmp"

Gasser one also says 902/29/8 while the diesel one says 902/29/5

Interesting stuff....

pizzachef · #37 05-04-2008, 10:33 AM

Quote:

Originally Posted by pawoSD

Yes, when I said 1 and 1/4" I actually meant 1 and 1/16"

Wups.

Ah yes, sometimes I mix up my denominators too

1-1/16" should be dead on..

Quote:

Originally Posted by pawoSD

Well, I got the pickup as close to the crank pin as I could, but it still reads the same....barely anything at idle, and 1/2ish of actual rpms when revved up.

Could it have anything to do with the fact that I have a gasser tach in the car? My original had a dead clock so I swapped it with one from a 280se....they have the same (large 2-pin) connector etc...the only difference is that the gasser one goes to 7k instead of 6k.....
Techguy? Anyone?

I tried a gasser tach in mine and got no movement, so you're ahead of me...I guess you're finding that its hard to find a w123 diesel tach for less than $100? I lucked out on mine, it wasn't too expensive, but I'm finding that it may not work

Good luck.

pawoSD · #38 05-04-2008, 10:38 AM

It probably depends on the gasser tach, I got mine off a junk yard 1984 280SE euro.....the 8cyl ones are probably configured to read even more pulses per second equivalence....so on a diesel they don't register anything.

My car (that I am fixing this on) is actually a W126....our W123's tach works fine....annoyingly....

300SD81 · #39 05-23-2008, 03:02 PM

Can you publish the pinouts and signaling methods for the tach? Mine is working at the moment, but I don't think its very accurate, I'd like to be able to use my oscilloscope and signal generator to test it...

Also, if anyone has similar info on the cruise amp, I may be interested in making a similar product, since mine has been malfunctioning...

Cr from Texas · #40 05-23-2008, 03:18 PM

Quote:

Originally Posted by pawoSD

I think I have it figured out, the gasser tach is designed to read the pulses from the ignition system of the 6cyl gasser (280), which would happen more frequently than the crank pulse of a diesel, which is why it reads less than half of my actual RPM......sooo, I will be doing surgery on my cluster tomorrow and taking the working clock from my gasser tach unit, and inserting it into the original diesel tach with a dead clock that I took out of my car a long long time ago, then reinstalling that back into the car....hopefully it solves the problem!! I know the sender and amp work now, since the needle does move....

After my modifications tomorrow it should work again....

If you haven't already done the above, the easiest task is to replace the two capacitors in the clock. About $2 at radio shack. Do a search, there is a detailed thread on this repair. Did mine and the clock works fine, my tach is still not working but I'm sure it's the amplifier under the hood.

Charles

techguy512 · #41 05-23-2008, 05:13 PM

Quote:

Originally Posted by 300SD81

Can you publish the pinouts and signaling methods for the tach?

This is the best thread EVAR about the pinouts and signaling of the stock VDO amp.

OldPokey did a great job on this one......

http://www.peachparts.com/shopforum/showthread.php?t=206808

300SD81 · #42 05-23-2008, 05:36 PM

How did you measure the input pulses on the microcontroller? ADC with the sensor interface described, measuring for changes in voltage?

I'm going to have to build a circuit that can generate test pulses since my diy signal generator can only generate square, sine, or triangle waves...a 555 should do it

Bens lover · #43 05-23-2008, 07:52 PM

Im in for one also....Send me an email..
Dan
Thank you

techguy512 · #44 05-23-2008, 09:15 PM

Quote:

Originally Posted by 300SD81

I'm going to have to build a circuit that can generate test pulses

What exactly are you trying to do? Are you trying to debug your system, or are you doing this for fun and learning?

300SD81 · #45 05-26-2008, 05:32 AM

Quote:

Originally Posted by techguy512

What exactly are you trying to do? Are you trying to debug your system, or are you doing this for fun and learning?

Little bit of both, turns out my tach is a few hundred rpm off like i suspected, threw together a circuit that outputted 50 3ms pulses per second, and my tach was a bit off from the 3000rpm line... Not sure if its a constant difference or if it varies with speed, but it does explain my unusually low idle (on the tach)...