1985 300 D Tach Problem - PeachParts Mercedes-Benz Forum

neuse pirate · #1 12-01-2005, 09:23 PM

Thanks for the earlier suggestions on tach difficulties. I have completed the last round of repairs to no avail ( replaced tach sensor, replaced instrument cluster and replaced power relay with fuse) There are no obvious electrical problems. The tach does jump to proper RPM at start up but quickly dies. Does anyone have any other suggestions?

boneheaddoctor · #2 12-01-2005, 09:29 PM

Have you tried running an addtitional ground to the chassis from the cluster....just to eliminate that possibility? I know its probibly not it...Unfortunately I don't know the '85 well enough to know where the tack amp is if it has one....(its the same as the W126 but not the '84 and earlier W123 and W116)

JimmyL · #3 12-01-2005, 11:22 PM

Is the tach sensor on the front of the motor or in the back near tranny area? Seems like I remember someone saying it is near the balancer. Mine doesn't ever move, never has. I have replaced the computer module. The pins were green and corroded, so I just knew that was gonna be it, but, uh....NO. Took me awhile to find where that computer module was too.

JimmyL · #4 01-15-2006, 06:00 PM

Still could use some help with this one. I can't find anything on the 123 Service cd relating to the tach, pickup, etc..... Then I throw in the uniqueness of the '85 model and searching wasn't conclusive.
My tach is inop. Always has been. I have replaced the computer module, as stated above. I finally sprayed off the front and rear of engine, and found the tach sensor up front, and some type of sensor, under oil filter housing on drivers side of bell housing.
I was going to remove them, clean and inspect them, take readings, etc..... But during my searching, somebody raised the possibility of one of those being utilized for the cruise control, which actually does work. Perfectly in fact.
So, I want to trouble shoot my tach without rendering my cruise inop. Make sense?
Fuse for overload protection relay is not blown.
#1 - is the sensor up front where the rpm signal originates?
#2 - what does the sensor on the bell housing do?
#3 - does anyone have this circuit diagram that could scan it and post, or cut and paste if it is electronic?
#4 - what senses the vehicle speed for the cruise system.

Thank yall in advance.

Eskimo · #5 01-15-2006, 06:57 PM

Quote:

Originally Posted by JimmyL

#1 - is the sensor up front where the rpm signal originates?

#2 - what does the sensor on the bell housing do?
#3 - does anyone have this circuit diagram that could scan it and post, or cut and paste if it is electronic?
#4 - what senses the vehicle speed for the cruise system.

Jimmy,

I can't solve your problem, but I think I can answer a couple of your questions.

The sensor up front is not, to the best of my knowledge, where the tach for an '85 gets its signal. That sensor goes to a socket on the driver's side fender, and is available for diagnostic purposes (such as reading the engine speed during service).

I think the sensor on the bellhousing is the one that's actually used for the tach in that car.

my123ca · #6 01-16-2006, 11:14 AM

Quote:

Originally Posted by JimmyL

Still could use some help with this one. I can't find anything on the 123 Service cd relating to the tach, pickup, etc..... Then I throw in the uniqueness of the '85 model and searching wasn't conclusive.
My tach is inop. Always has been. I have replaced the computer module, as stated above. I finally sprayed off the front and rear of engine, and found the tach sensor up front, and some type of sensor, under oil filter housing on drivers side of bell housing.
I was going to remove them, clean and inspect them, take readings, etc..... But during my searching, somebody raised the possibility of one of those being utilized for the cruise control, which actually does work. Perfectly in fact.
So, I want to trouble shoot my tach without rendering my cruise inop. Make sense?
Fuse for overload protection relay is not blown.
#1 - is the sensor up front where the rpm signal originates?
#2 - what does the sensor on the bell housing do?
#3 - does anyone have this circuit diagram that could scan it and post, or cut and paste if it is electronic?
#4 - what senses the vehicle speed for the cruise system.

Thank yall in advance.

Does your car have an EGR valve? If so, your tach runs via the EGR contol unit. The sensor in the bell housing is the engine speed sensor and the vehicle speed sensor is the small black box behind the speedometer.

Wasuchi · #7 01-16-2006, 11:28 AM

The sensor on the front of the engine I've always believed was the tach sensor..... it does indeed run to the diagnostic port in the left wheel well area (round black thing with a cap that screws on).... but there is also a wire or possibly wires leaving that port that join a large bundle going into the cabin. Seems funny to have more than one sensor for the same thing but I suppose it's entirely possible. The only thing I could see in the area described on the bell housing looked like a braided ground cable but I didn't have a flashlight so it was hard to see.

If you run out of options for looking for the tach sensor let me know and I'll unplug the sensor on the front of my engine and see if my tach works or not then.... I would do it right now but it doesn't look like it'll be too easy to do (plus it might quit working altogether if it gets messed with).

my123ca · #8 01-16-2006, 09:37 PM

Quote:

Originally Posted by Wasuchi

The sensor on the front of the engine I've always believed was the tach sensor..... it does indeed run to the diagnostic port in the left wheel well area (round black thing with a cap that screws on).... but there is also a wire or possibly wires leaving that port that join a large bundle going into the cabin. Seems funny to have more than one sensor for the same thing but I suppose it's entirely possible. The only thing I could see in the area described on the bell housing looked like a braided ground cable but I didn't have a flashlight so it was hard to see.

If you run out of options for looking for the tach sensor let me know and I'll unplug the sensor on the front of my engine and see if my tach works or not then.... I would do it right now but it doesn't look like it'll be too easy to do (plus it might quit working altogether if it gets messed with).

I've pulled that sensor in front of the engine. It didnt do anything to the tach. I think its for diagnostics only.

MarkM · #9 01-16-2006, 10:55 PM

On the 85, the tach pickup is located on the transmission bell housing, just behind and below the oil filter housing.....I forgot where the other end plugs into, but that is easy to follow.

On the 85 you do not have a separate tach amp....on this model, the tack amp is part of the egr control module, located behind the kick panel in the front passenger foot well.

Lastly, you need to check the overcurrent protection relay, located inside the dash on the passenger side....I think you can access it by removing either the foot well panel above your knees, or removing the glove box. You will see a silver (aluminum) rectangular box with a 10 amp spade fuse...check that fuse....on my car, the tach hadnt worked for years (for te previous owner....when I checked around, I found the 10 amp fuse blown..when I replaced it, the tach has worked fine since.

good luck,

Mark

JimmyL · #10 01-16-2006, 10:57 PM

So far that is good info, and at least helps in starting to understanding this weird one year only circuit. When and if I actually get home this week before dark I am going to pull the one on the bell housing. There is still some crud on that one. Does it just "pull" off (doubtful), or does it have some sort of securing bolt or bracket?
OK, the vehicle speed sensor behind the speedo..... is that tied in to the cruise operation? I also saw an electrical wire/plug on the back of the tranny. Is that tied into said vehicle speed sensor?
I've swapped EGR control unit. Old one was heavily oxidized, green pins and all. Didn't fix it to my suprise and dismay. They hide that thing well, don't they? As stated in post #4, Overload Protection Relay fuse is good. Took relay apart, and all seemed well.
I would love a working tach, and I'll get there some day.......

**edit**
I was typing same time as post above this one, and didn't see it until after I typed the above.....

Wasuchi · #11 01-17-2006, 10:47 AM

Yes, the speed sensor on the back of the speedo is for the cruise (I tied into it for my aftermarket Audiovox cruise control). I think it just gets it's signal from the speedo so I don't know about the wires on the transmission, maybe neutral safety or something like that?

bkeese · #12 06-06-2008, 09:06 AM

It's been awhile, but was your 85 tach problem ever solved? I am have a similar situation with my 85 300D.

No response 2 years ago, but... does anyone have a wiring diagram for the tach circuit on an 85 123 that they'd be willing to post? Or know where I could go find one?

Many Thanks,

Brian
1985 300D - just passed 200K miles - need the tach back so my AC will start working again

Jeremy5848 · #13 06-06-2008, 12:29 PM

The 1985 300D (W123, OM 617) has a unique tach circuit. The tach is driven from the EGR control module, which is located behind the right front passenger kick panel. There's a "secret door" in the kick panel that you can see if you stick your head down in the footwell and look to the right. Opening the door will reveal the "control unit" and the "trimming plug" (an idle speed adjuster like the later 603 engine has in the W124 diesels).

I will post a diagram as soon as I get my scanner fixed (later today). The diagram is in the "Introduction into service" manual for the 1985 model year. Much of the unique 1985 300D stuff is in that manual.

Jeremy

JimmyL · #14 06-06-2008, 12:50 PM

It is funny to go back and see your posts from years ago when you knew absolutely nothing. Ha. {actually, still know almost nothing}

Tach still doesn't work. Never has. I've thrown OVP relays and measured voltage from tranny pickup, which apprears to work.
I'm starting to think that with the corrosion on the pins to my original EGR computer that maybe a pin broke off the connector from the car to the EGR computer.
Can anybody count the pins for me on there '85 connector and tell me how many pins I should have. I would love to actually see what a working tach looks like....

AdvisorGuy · #15 06-06-2008, 03:10 PM

Take the tach amplifier off, fold a very small piece of paper into a small square, put it in the tach amplifier and tighten it back down...