1980 300sd, honk my horn - killed my tach - PeachParts Mercedes-Benz Forum

240diowa · #1 10-31-2012, 08:19 AM

Just bought a 1980 300Sd that has been sitting for 5 years...Nice car, have most things fixed except an electrical issue. My check seat belt light comes on if I do any on the following. turn on or off the heat, use a turn signal, honk the horn, turn/off the radio....basically anything.

Today I honked the horn and the tach stopped working, after I shut the car off and started again the tach was back. I suspect I have a grounding issue. Have found some grounding wires by the radiator drivers side, one on the drivers side inner fender. One for the battery and one from the motor to the chassis. Have cleaned all of those, have installed a new alternator as well.

Question: does anybody know where all the grounding wire to frame locations are?

Don't think I have found them all yet.......

vstech · #2 10-31-2012, 09:12 AM

I would pull the instrument cluster and look for ground connections in there.
someone with a 116 should chime in soon.

paging Doktor Bert!!!

240diowa · #3 10-31-2012, 09:19 AM

I did remove the driver side interior panels to look for ground connections, all I saw was a large bundle of brown wires (I assume those are ground) but did not see any connections. I once pulled an instrument cluster out of a 123 and forgot to reconnected the oil pressure line......what a mess after i started the car......... "black gold...texas tea"

vstech · #4 10-31-2012, 09:22 AM

yes, brown is ground on the MB. you did not see any bolts attaching those wires?

240diowa · #5 10-31-2012, 09:26 AM

didn't see any bolts or connection points etc. , the wires were all wrapped together running toward the firewall, I'm guessing 12-15 wires. I looked and looked and didn't see any ground locations under the dash.

bbboomer · #6 10-31-2012, 04:19 PM

I had a very similar thing just happen to my 1984 300SD.
I checked all the grounds and found no problem.
A local indy said he thought is was in the fuses. He suggested I rotate all the fuses while still in the fuse holders. Just spin them back and forth. A couple were kind of stuck in place at first but finally turned.
Fixed my problem!

240diowa · #7 10-31-2012, 04:49 PM

I have rotated the fuses already and also checked them in place with an ohm meter and found no problems. Think I'll take them our and rub the ends with Emory cloth just to make sure. Still wish I know where all the ground connections are located.

bbboomer · #8 10-31-2012, 08:23 PM

Sorry,
I meant to add this to my other post.
The grounds for the dash are behind the instrument cluster.
There are 3 bolts into a metal bracket with several wires connected to each bolt.
At least that is where mine are.

240diowa · #9 11-01-2012, 08:29 AM

I did remove all my fuses last night and made sure they had good contact. Didn't change my electrical issue.

I'll check behind the instrument cluster to see if the grounding bolts are there. did you have to pull the cluster out?

240diowa · #10 11-01-2012, 12:45 PM

The PO just gave me a CD of all the shop manuals etc for my car, there is a ground connection behind the instrument cluster according to the wiring diagram.

w123love · #11 11-01-2012, 05:33 PM

I have had this EXACT problem before. It happened when I put in a new head unit (radio).

What I did wrong was I used the wrong power/constant/neg/ground wires for the new unit. I had previously installed new head units in other W123’s like mine, but for some reason I tried to do it differently this time.

What I suggest, (given the stereo has been changed/ messed with) is that you unplug it and see what symptoms you still have.

I still can’t get my instrument cluster lights to work after my debacle.

240diowa · #12 11-06-2012, 10:11 AM

The car still has the original radio so there should not be any issues with the grounding on that. I took out the instrument cluster, located the grounding connections. They are behind and below (really hard to see) the instrument cluster. Cleaned the connections etc and I still have the issue but it is less frequent now. My check seat belt light comes on if I operate anything electrical...sometimes.....not all the time like before....when I honk the horn the tach needle moves a little and doesn't kill the tach like before. don't know what other grounding connections to test.

240diowa · #13 11-19-2012, 09:42 AM

Found all the grounds except one and cleaned them etc.....According to the shop manual wiring diagram there should be a ground with 11 wires behind the RH headlight assembly. I took out the battery etc and could find any ground (except the battery neg ground)