1940TORPEDO

calibrating speedometer to correct speed

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I have a 1940 Pontiac and the speedometer stays at 80 MPH while sitting still. When the car accelerates the needle moves and appears to move on scale with each speed increase (90 MPH = 10 MPH / 100 MPH= 20 MPH). 

 

How do I get the needle to read 0 MPH when sitting still? I have looked in my shop manual but there does not seem to be a section that addresses speedometers other than the two gears in the transmission (drive and driven).

 

Thanks for looking,

Allen

speedometerponitac.PNG

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Is the cable broken?

 

There will be a mechanical problem. They are factory calibrated with the drive gears in the rear of the gearbox for tyre size.

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Allen,

You will have to pull the speedo and take off the back "can".

Inside you will most likely find a set of "fins" that rotate around a drum which is magnetic.

The fins are attached to the cable drive and are probably being "return to zero" limited by dirt or grease from the cable.

Not a big deal to clean this out.

HOWEVER, do not spray this area with brake cleaner, as it will take the numbers off the odometer drums.

Ask me how I know.

 

This opinion comes from me doing this fix on my '40 Buick.

Mine was grease that was pumped up the cable right into the speedo.

Too much grease in the cable assembly by the previous owner.

 

Mike in Colorado

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18 minutes ago, FLYER15015 said:

Allen,

You will have to pull the speedo and take off the back "can".

Inside you will most likely find a set of "fins" that rotate around a drum which is magnetic.

The fins are attached to the cable drive and are probably being "return to zero" limited by dirt or grease from the cable.

Not a big deal to clean this out.

HOWEVER, do not spray this area with brake cleaner, as it will take the numbers off the odometer drums.

Ask me how I know.

 

This opinion comes from me doing this fix on my '40 Buick.

Mine was grease that was pumped up the cable right into the speedo.

Too much grease in the cable assembly by the previous owner.

 

Mike in Colorado

I'm sure your '40 buick would be the same technology. I will check out the head unit.

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