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Old 11th January 2017, 10:13 AM   #1 (permalink)
paulexige
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GTD Speedo

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
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Old 11th January 2017, 10:56 AM   #2 (permalink)
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saxoncross
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Re: GTD Speedo

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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Old 11th January 2017, 11:35 AM   #3 (permalink)
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Re: GTD Speedo

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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Old 11th January 2017, 03:01 PM   #4 (permalink)
paulexige
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Re: GTD Speedo

Thanks guys this is the speedoClick image for larger version

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Old 11th January 2017, 06:13 PM   #5 (permalink)
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Re: GTD Speedo

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
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Old 11th January 2017, 07:59 PM   #6 (permalink)
paulexige
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Re: GTD Speedo

Maybe i should just get a new speedo
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Old 13th January 2017, 01:51 PM   #7 (permalink)
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Re: GTD Speedo

Have a look at Speedhut gages. I put a complete set in my SLC.

Here is a example of a GT40 looking GPS speedo.

4" GPS Speedometer 200mph. Auto Clockwise Without Turn Signals From Speedhut.

Note: they come in any color combination you would like. Play with the customizer.
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Old 15th January 2017, 12:02 AM   #8 (permalink)
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Re: GTD Speedo

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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Old 15th January 2017, 06:33 AM   #9 (permalink)
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Re: GTD Speedo

Agree with Howard,

I've had a Speedhut for many years now. Trouble free :-)
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Old 15th January 2017, 11:51 AM   #10 (permalink)
paulexige
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Re: GTD Speedo

Thanks for your advise guys
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Old 15th January 2017, 05:51 PM   #11 (permalink)
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Re: GTD Speedo

Hi Paul, Micks thread about his speedo has also brought out good tips. I've seen a few cars gain mileage on the electronic odometer so an external signal generator such as the VDO GPS Sensor may provide a clean filtered signal to your speedo. Hunting down wiring voodoo is no fun...

Cheers

PS here's the instruction sheet: https://www.vdo-gauges.com/media/ins...0Sender%20.pdf
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