Speedometer help

[Ncbenz]

1985 300sd
I'm having a problem with getting my speedometer working. I checked for signal, power and ground. They all checked out. So I replaced the speedometer head and the new one worked for a few weeks but the needle kept getting stuck and after trying to figure out what was making it get stuck it stopped working. So I replaced the unit with a speedometer that looked good. Now I'm getting nothing, and I re confirmed power, ground and signal. Anyway does anyone have any ideas of what else I should check? I'm not sure if this matters but I by passed the tach signal through the egr control computer recently. Any tips would be greatly appreciated

[SD Blue]

Try disconnecting the Cruise Control Amplifier. It's located below the instrument cluster and can be easily accessed by removing the knee panel.
Maybe it is loading the signal down due to an internal problem.

[Ncbenz]

Originally Posted by SD Blue

Try disconnecting the Cruise Control Amplifier. It's located below the instrument cluster and can be easily accessed by removing the knee panel.
Maybe it is loading the signal down due to an internal problem.

Hmm that's a great idea actually. I'll try that tomorrow morning. Thanks

[optimusprime]

Speedo cable faulty /. These are cable driven .

[Ncbenz]

Originally Posted by optimusprime

Speedo cable faulty /. These are cable driven .

On the 85 300sd the speedometer is all electronic

[Ncbenz]

Originally Posted by SD Blue

Try disconnecting the Cruise Control Amplifier. It's located below the instrument cluster and can be easily accessed by removing the knee panel.
Maybe it is loading the signal down due to an internal problem.

Alright I disconnected the cruise control amp and the speedometer is still inoperative.

[Rooster300SD]

Also check the cable end where it inserts into the tranny.
Mine was acting up and it turned out that the little bolt that
attaches the cable to the tranny housing was missing and the cable end
had worked its way out of its hole on the drivers side of the tranny.

It's tight up in there, but once I got a new bolt in it fixed the speedometer.

Jeff

[Diseasel300]

It is possible you may have bad capacitors in the speedo unit. There are some electrolytic caps in it and while you don't read much about the speedo going out, you do read a lot about the tach and clock dying from caps failing.

[ROLLGUY]

Originally Posted by Rooster300SD

Also check the cable end where it inserts into the tranny.
Mine was acting up and it turned out that the little bolt that
attaches the cable to the tranny housing was missing and the cable end
had worked its way out of its hole on the drivers side of the tranny.

It's tight up in there, but once I got a new bolt in it fixed the speedometer.

Jeff

Please read the entire thread.

[Ncbenz]

Originally Posted by Diseasel300

It is possible you may have bad capacitors in the speedo unit. There are some electrolytic caps in it and while you don't read much about the speedo going out, you do read a lot about the tach and clock dying from caps failing.

That's kind of what I was leaning towards. I just wanted to rule everything else out. this is the 3rd speedometer I've been through including the original and I just find it hard to believe that many are bad. Do you know of way to test the individual capacitors?

[ROLLGUY]

Does the odometer portion work? I am not sure exactly how the odometer and speedometer interact, but for sure the odo is driven buy gears, and they do go bad. Does the same motor, servo or whatever it is called operate both?

[Diseasel300]

The speedo drive and Odo drive are separate. The 126 is all electronic. There's a stepper motor to drive the odometer, and a torsional motor to drive the speedometer pointer.

[ROLLGUY]

Originally Posted by Diseasel300

The speedo drive and Odo drive are separate. The 126 is all electronic. There's a stepper motor to drive the odometer, and a torsional motor to drive the speedometer pointer.

You learn something new every day, thanks!

[ROLLGUY]

Originally Posted by Diseasel300

The speedo drive and Odo drive are separate. The 126 is all electronic. There's a stepper motor to drive the odometer, and a torsional motor to drive the speedometer pointer.

But do they both get their signal from the same sensor?

[dude99]

Have you checked the OVP relay? Does the tach still work?