Mechanical speedo cable transducer

To correct the speedometer reading on my '87 Suburban, I need to change the transducer that is now on the speedo cable in the vicinity of the catalytic converter. These transducers have stamped on them a number indicating the gear ratio inside. But exactly what does the number mean?

To increase the speedometer reading by 130%, do I need a transducer with a number higher (130%) than the one there now, or do I need a transducer with a number lower (77%, which is = 1 / 130%)?

TIA,

Una

Reply to
Una
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Got my hands on one, so can now answer this. The number is the ratio of output to input. To increase the speedometer reading you need a transducer with a number greater than 1.

The transducer has a square pin (like a wood matchstick) that goes to the transmission, so that is the input side. The output side has a square hole.

Una

Reply to
Una

Since it seems this business of getting correct mechanical speedometer readings is highly obscure, here are some more details for the record.

This '87 Chevy Suburban has the 454 engine and TH400 (turbo hydromatic) transmission. My mechanic says that for this transmission there are only 3 speedo drive gears available, with 38, 37, or 36 teeth.

A difference of 2 teeth allows for a correction of at most

38/36 or 5.6%. My speedo is off about 30%; to correct this large an error a transducer may be installed on the cable from transmission to speedo. As it happens, there already is a transducer installed; obviously it does not have the necessary ratio.

When actual speed is 65 mph, indicated speed is 50 mph, so

50/65 = 0.769. The goal is to increase this ratio to 1.0. This ratio is occuring with 38 teeth and transducer ratio 0.833. Changing from 38 to 36 teeth helps slightly: 38/36 = 1.056. So with 36 instead of 38 teeth, the error ratio will be 0.769*1.056 = 0.812. It is very close to the 0.833 ratio of the transducer now on the cable. If I remove the transducer I'll get an error ratio of 0.812/0.833 = 0.975, which is just 2.5% slow. A transducer with ratio 1.026 would be exactly right, but if none is available, having no transducer at all may be good enough.

How accurate does the speedo need to be?

Una

Reply to
Una

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