OM617 Tach only works when blower is running - PeachParts Mercedes-Benz Forum

GhostRider8 · #1 08-02-2019, 10:50 AM

Tach seems to work decently (still hops around when first comes on but levels out) when AC blower is on high. When blower is turned off, it quits again. Anyone ran into this issue, with my limited experience on this subject it seems like possibly an electrical or grounding issue?

Rogviler · #2 08-02-2019, 06:33 PM

All of my tach problems were solved when I pulled the connector off of the little computer by the passenger door, had water pour out, and ended up bypassing all of that. But I would start with checking for the water thing (especially corrosion) and, assuming you want to leave all the EGR stuff intact, just clean it out really well.

-Rog

GhostRider8 · #3 08-03-2019, 12:30 AM

Quote:

Originally Posted by Rogviler

All of my tach problems were solved when I pulled the connector off of the little computer by the passenger door, had water pour out, and ended up bypassing all of that. But I would start with checking for the water thing (especially corrosion) and, assuming you want to leave all the EGR stuff intact, just clean it out really well.

-Rog

Forgive my ignorance, where exactly is this box by passenger door? Also how would removing the EGR effect the tach?

Rogviler · #4 08-03-2019, 12:52 PM

The EGR computer has the tach signals running through it for some reason (nefarious or practical, depending on who you ask). I've not actually explored if you can bypass the tach signal why keeping the EGR functional...

But anyway, if you look behind the panel in the passenger side footwell next to where the door hinges are you'll find a bunch of stuff that usually will fix your tach problems by fixing it. There's the fuse that can blow and you'll have no tach and there's the EGR computer with the connector that had water or at least some corrosion in it more often than not.

Here's a thread about bypassing it (very easy):

http://www.peachparts.com/shopforum/diesel-discussion/341652-quick-fix-1985-w123-tach-c-after-egr-failure.html

-Rog

GhostRider8 · #5 08-03-2019, 05:55 PM

Quote:

Originally Posted by Rogviler

The EGR computer has the tach signals running through it for some reason (nefarious or practical, depending on who you ask). I've not actually explored if you can bypass the tach signal why keeping the EGR functional...

But anyway, if you look behind the panel in the passenger side footwell next to where the door hinges are you'll find a bunch of stuff that usually will fix your tach problems by fixing it. There's the fuse that can blow and you'll have no tach and there's the EGR computer with the connector that had water or at least some corrosion in it more often than not.

Here's a thread about bypassing it (very easy):

http://www.peachparts.com/shopforum/diesel-discussion/341652-quick-fix-1985-w123-tach-c-after-egr-failure.html

-Rog

Very interesting! Thank you, I’m off tomorrow if I finish up my ball
Joint install I will start to tackle this one.

NZScott · #6 08-03-2019, 09:04 PM

Quote:

Originally Posted by Rogviler

The EGR computer has the tach signals running through it for some reason (nefarious or practical, depending on who you ask). I've not actually explored if you can bypass the tach signal why keeping the EGR functional...

But anyway, if you look behind the panel in the passenger side footwell next to where the door hinges are you'll find a bunch of stuff that usually will fix your tach problems by fixing it. There's the fuse that can blow and you'll have no tach and there's the EGR computer with the connector that had water or at least some corrosion in it more often than not.

Here's a thread about bypassing it (very easy):

http://www.peachparts.com/shopforum/diesel-discussion/341652-quick-fix-1985-w123-tach-c-after-egr-failure.html

-Rog

Different tach system on those 84-85 cars altogether

For OP, a good test of grounds and other connections on the instrument cluster would be my first try.

Rogviler · #7 08-04-2019, 01:04 AM

You're welcome!

-Rog