GTD Speedo

Paul Williams

Supporter
HI guys my digital speedo on my 1991 GTD keeps jumping thousands of miles on the readout which is obviously not much good for insurance purposes especially when you only have 2000 miles a year . Not sure if it would be the speedo or the sensor any ideas

Cheers Paul
 
Hi Paul,
I presume that you have something similar to the attached, which is a Smiths branded gauge made by Caebont. They have a good technical help desk and should be able to point you in the right direction.
Caerbont Automotive Instruments | Homepage

Rgds,
Andy
 

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Mike Pass

Supporter
The one I have is from ETB instruments. They have two different types of sensors which need wiring up differently. I have attached the two different wiring diagram types. If your speedo is an ETB give them a ring. I found them very helpful when I got the wiring mixed up when I put a new fusebox in.
ETB Instruments link
https://www.etbinstruments.com

Speedo link
100mm Programmable Speedometer 200mph GT40

Cheers
Mike
 

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Ian Anderson

Lifetime Supporter
I believe the unit also has a battery in it that charges when in use but with long lay ups goes flat and causes this type of problem.

But I do not believe the battery is user swappable.

Ian
 

Ian Clark

Supporter
Hi Paul, If the gauge is otherwise working properly it's worth checking the integrity of the ground connections in the harness, ignition system and engine block, would be a shame if you bought a new instrument and it did the same thing.

Spurious noise from the harness can effect the gauges... tach, volts, alt input wires to gauges could be suspect.

Something else all GT40 replicas should have is a master battery disconnect switch that shunts to ground on shut off. Can also save your ignition box or other onboard electronics.

Are there any markings on the back of the gauge to identify the manufacturer? Could be a VDO. Customer support at Caerbont and VDO/Siemens is excellent, you'll get it sorted:)
 
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