Author Topic: Speedometer Cable keeps snapping  (Read 324 times)

Jjutzy

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Speedometer Cable keeps snapping
« on: 21 February 2020, 10:09 AM »
OK. 1979 Mercedes 300 SD. I’m on my third speedo cable. It keeps snapping about 18” down from the gauge. I’m binding somewhere! Am I not routing the cable correctly or is this a problem with something binding in my gauge?

When a new cable is installed, Speedo and odo work great, for about 80 miles. Then both quit working. Is it time to pull the gauge or do I need to be meticulous with cable routing?

Thanks,
John
1979 300 SD 348,000+ Miles

rumb

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Re: Speedometer Cable keeps snapping
« Reply #1 on: 21 February 2020, 11:33 AM »
Pretty hard to bend a speedo cable too tight, I would look at speedo rebuild/replace.
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ptashek

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Re: Speedometer Cable keeps snapping
« Reply #2 on: 21 February 2020, 12:01 PM »
The speedo works as a magnetic pick-up. Internally the shaft attaches to a spinning magnet, and the faster that turns, the higher the indicated speed. So it can't really be binding here if the needle moves without jerking.

The other connection is to the geared shaft that drives the odometer. This could be binding, but I think the cable would enough force to break that first, before snapping.
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Jjutzy

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Re: Speedometer Cable keeps snapping
« Reply #3 on: 21 February 2020, 12:12 PM »
I would think that one end or the other has to stop spinning in order to snap this cable. The only thing that makes sense is something binding on the gauge side, unless the cable is snapping due to friction from not being routed properly. I’m admittedly shooting from the hip with this assumption.

Has anyone else had experience with something similar? I may just pop the speedo out and make sure it’s lubed and spinning freely... I’ve got 283,000 on the clock. May just be time to send the entire cluster off to be gone through...
1979 300 SD 348,000+ Miles

ptashek

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Re: Speedometer Cable keeps snapping
« Reply #4 on: 21 February 2020, 01:11 PM »
I was recently refurbishing a 112k spare cluster from my stash, and while I was at it I've taken the odo apart (see below).
The only parts that had any grease on them is the plastic shaft from the speedo cable pick-up to where it interfaces with the brass gear of the odo. The odo hears had no grease on them.

« Last Edit: 21 February 2020, 01:19 PM by ptashek »
1993 "Pearl Blue" W124 280TE
1979 "Icon Gold" W116 450SE (history, resto)

Diesel 617

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Re: Speedometer Cable keeps snapping
« Reply #5 on: 21 February 2020, 01:32 PM »
Is there any visible damage to the cable? Could you post pictures?
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UTn_boy

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Re: Speedometer Cable keeps snapping
« Reply #6 on: 21 February 2020, 01:34 PM »
Lucas, did you disassemble and lubricate the rear of the speedometer head where the bearing is?  That often seizes up from the grease becoming too stiff. 
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ptashek

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Re: Speedometer Cable keeps snapping
« Reply #7 on: 21 February 2020, 02:17 PM »
Lucas, did you disassemble and lubricate the rear of the speedometer head where the bearing is?  That often seizes up from the grease becoming too stiff.

I did check it, and it was moving freely. For peace of mind I gave it a shot of Normfest's High-Press Ultra sprayable grease. Initially it's liquid like WD40 and gets into the smallest crevices, then takes on a grease-like consistency and sticks to things like there's no tomorrow. That should cover it for years to come.
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mrkozzy

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Re: Speedometer Cable keeps snapping
« Reply #8 on: 22 February 2020, 03:55 PM »
Last year, I bought a used speedo cable and it snapped in 2 weeks, just like the original.

I heard the "twang" as it finally let go.

No money back on a used cable of course.

I sourced another (used) cable and had it installed 3 months ago.

The mechanic said, this time he was going to take the inner cable out and lubricate it, before installing it.

Whether he did or not, I can't say, however its still there running smoothly.
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raueda1

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Re: Speedometer Cable keeps snapping
« Reply #9 on: 23 February 2020, 12:33 PM »
Isn't there some kind of fitting along the speedo cable that feeds a signal to the criuse control?  I recall it being well hidden under the dash.  If that's binding up somehow it might explain it.
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

raueda1

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Re: Speedometer Cable keeps snapping
« Reply #10 on: 23 February 2020, 03:06 PM »
Isn't there some kind of fitting along the speedo cable that feeds a signal to the criuse control?  I recall it being well hidden under the dash.  If that's binding up somehow it might explain it.
This is the thing I'm talking about.  Behind the brake pedal.  Seems worth checking.
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Jjutzy

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Re: Speedometer Cable keeps snapping
« Reply #11 on: 24 February 2020, 08:20 PM »
My cable goes directly from my speedo, through the floor, to the tail cone.
1979 300 SD 348,000+ Miles

mrkozzy

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Re: Speedometer Cable keeps snapping
« Reply #12 on: 25 February 2020, 02:30 AM »
Same here.
MrKozzy