Hi, I started my 1985 300CD yesterday and the tach was not functioning. It has worked flawlessly up to now (although the car is new to me as of last October). It has about 253,000 miles on it. Any ideas? Could it be the gauge itself? How would I diagnose this?

Thank you all in advance!

The tach on the 1985 is controlled through the EGR mini-computer, which in turn runs through an overvoltage protection relay (aka OVP), which in turn has a spade fuse (10A I believe, not certain).

Chances are either the fuse has blown or the relay is faulty.

Had you done any electrical work on the car in the last few days? (I blew mine when we installed a new alternator.)

The OVP is on the passenger side kick panel area. You will need to remove the large trim panel under the glove box to get to it. It's about 2.5x the height of one of the generic 'ice cube' relays and the fuse is on top.

__________________


Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Thank you Zacarias, I'll check that a little later today.

The fuse checks out. Maybe it's the relay? I'll see if I can run one down unless there is a way to test it.

Thanks

You might try unplugging the connector to the EGR computer. Water poured out of mine and it was quickly obvious that the problem was the common one of corroded contacts. Since I have no EGR stuff I just bypassed the connector so the tach goes straight through. Problem solved forever.

-Rog

On my 85 300 sd the problem was the ovp relay which is located in fuse box engine bay drivers side. Even though the fuse on top was good relay was bad. Not sure how to check relay one way would be to plug it in at a known spot that has current and see if it clicks or not. Some people shake and rattle them and not sure if you hear it rattle that means if it is good or bad. I guess compare it to a known good one.

If you don't have a known good one or access to another car to test your OVP in you can try bypassing it so long as you know the alternator/battery are operating correctly and are not experiencing anything odd.

You'll want to pumper pins 30 and 87. That will apply power to whatever the relay was powering.

In later cars there were more 87's I think than just a single pin but this should be a quick enough test I would think.

If bypassing the OVP does nothing than either the problem is in the EGR unit or the tachometer.

I've done a good amount of worth with these tachometers. I'd be happy to test/rework a unit if so desired.

__________________
Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

I'm such a dummy sometimes. I have two other 123's sitting right there at my house. I can pull the OVP relay out of one of them and try it in the coupe. I'm out of town until Sunday morning but as soon as I get back I'll swap one in and see if the tach comes back. Thank you all so much!

Will

Just an FYI, I'm fairly certain that the OVP was only installed in 84 Cali, 85 Federal cars.

__________________
Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

Thanks James. All of the 123's I have are Federals and my wagon is an '85 like my coupe.

__________________
Will



72 280SEL
85 300CD 288k
00 E320 Wagon 4-matic 100k
11 E350 Wagon 4-matic 45k

73 VW Westfalia Camper
67 VW Westfalia Camper

66 Porsche 912

Just wanted to let you all know that the cause of my tach suddenly not working was indeed the OVP relay. I swapped one from my 85 wagon into the coupe and the tach worked. Thank you Zacharias, and the rest of you, for your kind assistance.

Best,

__________________
Will



72 280SEL
85 300CD 288k
00 E320 Wagon 4-matic 100k
11 E350 Wagon 4-matic 45k

73 VW Westfalia Camper
67 VW Westfalia Camper

66 Porsche 912

The pick up signal comes off the FW. There is a sensor that fits into a hole on the Adaptor plate,
the black cable then goes to the EGR Computer (Black Box) behind the pass side kick panel.

This is on the 84 and 85 model W123`s.

Good you got it worked out.


Charlie

__________________
there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

What is the FW?

Thanks

__________________
Will



72 280SEL
85 300CD 288k
00 E320 Wagon 4-matic 100k
11 E350 Wagon 4-matic 45k

73 VW Westfalia Camper
67 VW Westfalia Camper

66 Porsche 912

FW = Fly Wheel

Look down on the drivers side below the oil filter housing, and you can see where the Sensor fits into the adaptor plate.
Has a small bolt that holds it into place.
Picks up the revolutions from the FW Teeth I assume.

The Tach will work with a manual FW also. I swapped in a 4-spd into the 85 and using
the manual FW and the Sensor, everything works the same.

Charlie

__________________
there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works