1982 300SD speedometer out - PeachParts Mercedes-Benz Forum

vadkit · #1 09-19-2012, 01:42 AM

Hi guys.
I recently bought this 82 300SD with a lot of electrical problems. I would like to start with the speedo first. Well, it's not responding at all. I tried to swap it with donor one that was tested on 83 380SEC and seen to be OK. I'm not sure were donor came from but in 380 SEC it was working fine, the top speed is 80MPH on it, but in 300SD no response at all. So my question is did i use the right unit and what can be done next? By the way it is a electronic speedo. My experience with diesel is 0, and all the help will be very appreciated.
Thank You.

vstech · #2 09-19-2012, 08:03 AM

First off, WELCOME to the forum!
we're a helpful bunch.

top speed is 80? ahh, I bet there is another line after the 80, that would be the 81/82 US mandated 85mph speedo... could be a ground issue on the cluster. do the other gauges work? is there any problem with the ignition key? there is a connector under the dash beside the throttle pedal behind the side panel, if it's loose you will get no speedo, but the key can also fail to work... check all fuses in the panel, if they are aluminum, remove them clean the contacts with a pencil eraser, and install proper copper fuses.

also, the SEC speedo will be geared for a 2.47 rear diff, your speedo will be geared for a 3.07 diff. so when you fix the issue with the signal, your speedo will read faster than you are actually traveling.

JamesDean · #3 09-19-2012, 12:26 PM

There is a connector to the right of the accelerator pedal. The speedo signal passes through this connector. Remove the center console carpeting and check the connector. On a 560SEL I worked on it was in pieces, fixed the speedo a some light issues at the tail.

vadkit · #4 09-19-2012, 02:47 PM

Thank You guys for reply's. I will check the connector first. the other gauges are working except temp, and the dimmer switch falls apart in my hands, so i put a temporary wire over two contacts, but still no gauge lights yet. Got some work ahead.

oldsinner111 · #5 09-19-2012, 02:57 PM

I added extra large ground wire to instruments

vadkit · #6 09-20-2012, 07:11 AM

Quote:

Originally Posted by oldsinner111

I added extra large ground wire to instruments

Wich one is ground wire?

vstech · #7 09-20-2012, 08:31 AM

ground connections are on the steering column, under all the brown wires.
the cluster has two copper tabs on the sides of the speedo section, lift one of the screws holding that down, and install the ground wire there.

vadkit · #8 09-20-2012, 11:59 PM

Quote:

Originally Posted by vstech

ground connections are on the steering column, under all the brown wires.
the cluster has two copper tabs on the sides of the speedo section, lift one of the screws holding that down, and install the ground wire there.

Thanks for the details! Hope I can put my hands on it tomorrow.

vadkit · #9 09-24-2012, 03:10 AM

I added a ground wire and check the connector, it looks OK to me. When i took it for test drive the speedo came on for split second, then went out again. few minutes later I went over a pot hole and most of the instruments went nuts, the fuel gauge dropped to zero and oil pressure. The temp gauge wasn't working and I just replace sensor, the needle won't go above 40, and the only thing that's working is the odo. So looks to me that I need a whole different instrument cluster at this moment. Now i hope to find one at pickandpull.

vstech · #10 09-24-2012, 06:19 AM

with the car running, what is the voltage at the battery?

JamesDean · #11 09-24-2012, 08:15 AM

Quote:

Originally Posted by vstech

with the car running, what is the voltage at the battery?

Here here, you could have a low voltage issue. Not providing the cluster with appropriate power would result in erratic behavior.

Aside from a power or wiring harness issue, the only thing I can think of it aged capacitors might cause some things not to work. I know there are two on the clock, one or two on the tach, three or four on the speedo...Not sure about the fuel, oil, temp units.

SD Blue · #12 09-24-2012, 10:00 AM

Don't forget to check your main battery-chassis and especially the chassis-engine grounds. These can also give you low voltages to instruments, as well as starting issues.

vadkit · #13 09-24-2012, 05:52 PM

When car running the voltage goes to almost 13v, so alternator must be alive. I'm not sure how can I check the voltage on the cluster itself. Any ideas?

vstech · #14 09-24-2012, 09:02 PM

... almost 13v...
do this test,
open hood, and measure battery voltage. make a note:
have someone turn on the glow plugs, report voltage:
have them start the car, and measure voltage during cranking:
after car is started, where does the voltage sit?
after 1 minute where does the voltage sit?
exact numbers are important.

vadkit · #15 09-29-2012, 10:02 PM

Quote:

Originally Posted by vstech

... almost 13v...
do this test,
open hood, and measure battery voltage. make a note:
have someone turn on the glow plugs, report voltage:
have them start the car, and measure voltage during cranking:
after car is started, where does the voltage sit?
after 1 minute where does the voltage sit?
exact numbers are important.

Here comes the numbers.
Engine off 12.16V, glow plugs on 11.5V, cranking 10.0V and minute after 12.6V. Thanks!!!