Quick Fix: 1985 w123 Tach & A/C after EGR Failure - PeachParts Mercedes-Benz Forum

andrewjtx · #1 07-24-2013, 11:16 AM

I've been pouring over all the threads on the 84 Cali and 85 Federal models with EGR and their propensity for water damage of the harness plug into the EGR control module. No one ever seems to offer anything other than repairing the harness and replacing the module.

While I did go and find a pristine EGR module, clean plug and large chunk of harness at the PnP yesterday, I wondered, "Why the hell am I even putting this module back in? All I need is my tach and A/C back. All of the EGR system has been removed!"

So I spliced the the white (RPM sensor signal wire) and grn/bk (to tach) wires from the harness together and guess what? SUCCESS!

I made sure to cover the power lead from the OVP so it doesn't short out on anything. But if you're running sans EGR, there doesn't appear to be any reason to run the EGR module.

YMMV

-Andrew

funola · #2 07-24-2013, 11:43 AM

Quote:

Originally Posted by andrewjtx

I've been pouring over all the threads on the 84 Cali and 85 Federal models with EGR and their propensity for water damage of the harness plug into the EGR control module. No one ever seems to offer anything other than repairing the harness and replacing the module.

While I did go and find a pristine EGR module, clean plug and large chunk of harness at the PnP yesterday, I wondered, "Why the hell am I even putting this module back in? All I need is my tach and A/C back. All of the EGR system has been removed!"

So I spliced the the white (RPM sensor signal wire) and grn/bk (to tach) wires from the harness together and guess what? SUCCESS!

I made sure to cover the power lead from the OVP so it doesn't short out on anything. But if you're running sans EGR, there doesn't appear to be any reason to run the EGR module.

YMMV

-Andrew

Nice! Do you have a pic and p/n of the egr module?

andrewjtx · #3 07-24-2013, 11:55 AM

No pic right now. Just imagine all the pins are shiny...

P/N: 002 545 6032

mach4 · #4 07-24-2013, 12:20 PM

Quote:

Originally Posted by funola

Nice! Do you have a pic and p/n of the egr module?

Found this one...

mach4 · #5 07-24-2013, 01:05 PM

Quote:

Originally Posted by andrewjtx

So I spliced the the white (RPM sensor signal wire) and grn/bk (to tach) wires from the harness together and guess what? SUCCESS!

Wow, that's pretty interesting. So what you are in essence saying is that the EGR controller is just serving as a "pass through" of the signal. And since the VR sensor is grounded outside the EGR controller only the one wire needs to be spliced. In thinking about that it makes sense in that the other major VR sensor on vehicles that have it is the speedometer, and that connects directly to the speedometer module in the cluster.

All along I assumed that the EGR controller did some kind of signal processing such that the Tach module in the cluster was just basically a voltmeter.

I'm not sure I want to rip out my module as it's working just fine, but I'd really like to be able to trace the circuit inside the EGR controller and verify that it really is just a pass through.

And the final speculative question would be, why would the German engineers route the tach through the EGR controller in the first place. I don't think it simplifies any wiring harness issues or solves any problem.

Anyway, bottom line, great find that can really simplify our cars and save big bucks on replacing modules that really don't need it.

pmckechnie · #6 07-24-2013, 01:30 PM

The signal goes through the EGR controller to keep people from removing the controller to disable the EGR. This was probably mandated by our Federal Government. They have been doing things like this for many years.

Paul

mach4 · #7 07-24-2013, 02:32 PM

Quote:

Originally Posted by pmckechnie

The signal goes through the EGR controller to keep people from removing the controller to disable the EGR. This was probably mandated by our Federal Government. They have been doing things like this for many years.

Crap, there I go again looking for a logical explanation when the explanation is a political one. I guess it's kind of like the regulation that says you can't vent that horrible, dangerous, environmentally harmful R134a refrigerant (it has to be "recovered") from an A/C system, but if you buy cans of computer duster, you can vent that same 134a all day long.

Thanks for bringing me back to reality

It also looks like you can remove the OVP relay, trimming plug, rack position sensor, EGR vacuum amplifier, coolant temperature sensor and recirc air safety valve to clean things up even more.

funola · #8 07-24-2013, 04:00 PM

Quote:

Originally Posted by andrewjtx

No pic right now. Just imagine all the pins are shiny...

P/N: 002 545 6032

I was wondering if it's the same p/n as the EGR computer in the 83 240D p/n 002 545 29 32, which it is not. I think they must do similar functions, which uses the tach signal to electrically control the EGR valve.

funola · #9 07-24-2013, 04:09 PM

Quote:

Originally Posted by pmckechnie

The signal goes through the EGR controller to keep people from removing the controller to disable the EGR. This was probably mandated by our Federal Government. They have been doing things like this for many years.

Paul

Don't think so. In 85 300D's the EGR controller needs the tach signal to control the (elec.) EGR valve, so the tach is driven off the EGR controller instead of a separate tach amp like in 84 and prior 300D's (which has a vacuum controlled EGR valve).

andrewjtx · #10 07-24-2013, 06:49 PM

Hmmm. the '85 still had a vacuum actuated EGR valve. Now, the vacuum is sent to it via the vac amplifier that is electrically controlled by the EGR module...

The EGR module is a consumer of the RPM sensor signal along with water temp and rack position. It combines this data to know when to active the EGR for best NOx reduction.

It doesn't amplify the signal. Passing the signal through without the module gives me accurate tach and the compressor cuts out when expected. My '84 did't have a tach amplifier either. I'm not sure the actual wiring differences that changed from 83-84, but the 85 just routed the same rear rpm sensor to through the EGR module that the 84 used.

mach4 · #11 07-24-2013, 06:49 PM

Quote:

Originally Posted by funola

Don't think so. In 85 300D's the EGR controller needs the tach signal to control the (elec.) EGR valve, so the tach is driven off the EGR controller instead of a separate tach amp like in 84 and prior 300D's (which has a vacuum controlled EGR valve).

I was actually wondering about that...which would make engineering sense, not just political sense.

Next time I'm out at the junk yard I'm going to pull an EGR controller and see if I can verify that there is an internal "Y" with the signal being shared by the EGR and the tach - the tach portion being just a pass through and no signal processing involved, as andrewjtx has apparently figured out!

andrewjtx · #12 07-25-2013, 10:19 AM

I've looked at the PCBs on my old module. If you can figure that out, all the power to you...

rmmagow · #13 08-02-2013, 01:10 PM

Full-blown dummy here.
Where are these wires, the white and br/gn? how is the splicing done? Am I just looking at the wires that are connected to the OVP?
I want to do this for my 85 and yep, it sounds easy and probably is I just don't know where to look. OVP is out, I know where the EGR thing is behind the little door.

sumarongi · #14 08-06-2013, 05:25 PM

Bwaa!!! I just did this and it worked. thank you guys!

Ok easy way. 5mins max. No cutting or splicing required.

Go to radio shack and pick up a "jumper wire kit" and some electrical tape.

Open 'secret' door, passenger side footwell, and remove EGR control unit. (flathead useful for pulling aside plastic retainer tab)

Look at the diagram below.

(80) Control unit

On the wiring harness (female) side, with your jumper wires, connect...

20 to 25 (RPM sensor-->to tach)

16 to 14 (RPM sensor-->ground)

Test it out.
Wrap up harness with electrical tape and chuck the EGR control unit!
No cutting, no splicing!

Working tach!
Good to go!!!

RavenTBK · #15 11-12-2013, 12:09 PM

Wanted to confirm the above as working.

In my case, my AC and such was all functional, just could never get a response from the tach. I finally got around to doing this. Popped my door and popped the plug from the EGR computer, expecting to find nastiness like others have reported. My pins and harness are pristine, like new. I figured I'd disect the computer just to look since I will not be replacing it. Again, like new.

As such, I have no idea why my EGR system and everything else worked fine, but the tach wouldnt.

I grabbed some scrap 22ga speaker wire and shorted pins 20 over to 25 and 16 over to 14. Instant signal. Its reading low... 500rpm idle, 3100rpm no load governed. Not sure if the reading discrepancy lies within the jump "around" the EGR computer, or in the junkyard replacement sensor I swapped in a month ago to try and fix the problem.

Either way, here's some pics for those of you that are diagram challenged.

And my final product once I trimmed down the excess. I tucked it into the opening around the edge. Theres no reason for the wires to come loose from their pins let alone fall out with the middles being jammed into the gap.

I would guess, ideally, you'd put a zip tie or something around the plug to be sure nothing moves. Whatever. I fixed my tach for $0. I slid the EGR computer back into its "port" and laid the plug on top. Once I replaced the kick panel, the plug cant move anywhere.