OM603 Ring Gear Tooth Count
 

I wanted to install a tach for testing purposes into my project, OM603 swapped into a 92 Sierra. I purchased a small/cheap unit that apparently can use a mag pick up, only requiring the correct amount of teeth on the ring gear to be input as a parameter. I read somewhere that the om603 had a 144 tooth ring gear, is that correct? During testing the tach seems correct at idle 650rpm ish but reads low at higher rpm.

Engine is out of an 87' 300TD

Thank you

Dan

This is not a result of putting in the wrong number of teeth. If you put in a lower tooth count then the rpm would register higher across the board. The opposite if you input a higher one.

Sorry I know that doesn’t really help you out. But, if the tach is just a frequency counter, getting it right at idle should make it right everywhere.

ETA - if I had to guess it’s 144 teeth. VDO put the full scale pulse output on the Back of the W123 tachs. 6000 for the old crank pickup systems and 14000 and for the ring gear systems. On your W124 tach I can decipher a sticker that says 12/14 400 Hz. My guess is that is German for 14,400 Hz full scale which would suggest a 144 tooth ring gear. Just a guess tho.

1) Quality may be suspect.
2) "reads low at higher rpm." What is reference that allows you to determine high or low RPM?

Idle seems high should be 550?

Admittedly only going on a gut feeling and information I have read on the net during the swap. For the Idle, I am not running the MB ecm so I adjusted the idle manually until it felt correct - transmission engages smoothly, it starts ok cold and idles nicely warm, I bought the tach to confirm these things (I was looking for ball park) but perhaps I should have spent more $$.

For high rpm I can tell from experience with all the other diesels in my life that it is not revving at the indicated 24-2600 rpm. It sounds great and is obviously revving much higher.

I have the stock gauge cluster with Tachometer but I assumed it required the stock ecm to for signal hence why I did not hook it up.

Thank you

It very well could be, I should have bought a digital rpm counter instead....The swap was definitely budget and time oriented and lacking some polish but it is functional and fitting to the vehicle it was swapped into. In Revelstoke we spend more money on our snowmobiles than trucks ;)

Here is a pic of the swap, sorry it took me a bit to figure out this Mac pos

I’m no authority on these cars but I do not think you need the ECM to run the tach. The tach requires 12v, Battery ground and signal. All the signal conditioning is done in the sensor. The gauge should be able to get it straight from the sensor on the bell housing. You’ll need to check yourself of course. I do not own a w124.

The ring gear on the 603 is same as the early 606 ring gear which is 144 tooth. I had to do rpm testing on it because my idle control was messed up.