1979 300sd

This morning I was driving on the freeway and noticed a very faint hum at 70mph. The hum became more pronounced as I accelerated.

About 30 minutes in to my drive, the hum began to get louder and louder.
I then accelerated to 75 mph and the hum became a high pitch sound.

I couldn't stand how loud it got and it was then that I noticed the temp gauge needle bouncing around.

I got off the freeway and hoped that the sound would die down. It did.
Came to a complete stop and then accelerated again to gain some speed.
No hum, no high pitch noise. Noise gone for the rest of the day.

I was thinking that maybe the speedometer cable was guilty of the hum and the high pitch noise.

Everything seems to be working fine on the cluster.

Please advise

It may not be the speedometer cable but I would pull clean and lubricate it. It is the strongest contender.

The input shaft in the speedometer could be causing the noise and vibration. That grease is almost 40 years old and is probably dry. The speedometer can be disassembled, cleaned and re-greased if you have a steady hand. A light grease would be required.

Don't leave it too long before fixing (or having it fixed) as it is going to wear the shaft and bushing at an accelerated rate without the proper lubrication.

It's most likely dried/dirty grease on the speedo gears. I've had this occur on a couple of W116 speedos and the solution has been to clean and re-apply some clean lithium grease.

__________________
Joe

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1980 300SD - 495k miles - 'The Ambassador'
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Former Family Members
95 C280
73 280SEL
90 300D
87 300SDL (X2)
86 560SEL
84 300D
80 300SD

As everyone has said it is the speedometer cable. Freaked me out the first time I herd it, has a higher chance of happening the colder it is.

To solve:
Remove the speedometer cable from the back of the instrument cluster and pour some transmission fluid down the cable at about half a tablespoon. Valvoline Maxlife tranmission fluid meets MB ATF spec (always good to keep a quart of the correct fluid around)

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Nosce Te Ipsum "Know thyself"