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aerobod
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The pulsed signal voltage at the speedo is not in the correct range for it to register the pulses. Do you have the 1000 ohm pullup resistor installed between the 12V and signal wires on the speedo (Pin 1 to Pin 7), if it is not confirmed to be internal to the speedo?
 

James

John Vine
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Re #22:

Good call. The required 1K ohm resistor is shown in #10. See also #2 and #3.

Your second speedo (black dial, P/N 71255) certainly looks like one that can be programmed via a pulse code, so I'm puzzled that you're unable to display the existing code.  You could ask Caerbont to confirm.

Should you still need a silver-dial MPH speedo, CC have this one with 20% off.  They describe it as KPH but the pic shows an MPH one.

JV

OrangeMax
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So, try placing a 1 ohm, 0.25w resistor inline on the red/blue signal wire? Ok, I'll try that!. Thank you for the info on the silver MPH Tach. I will contact Caterham on Monday Morning to confirm its a MPH version. Thanks!

OM

*driving*Max

aerobod
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Re #24, the resistor should be placed across the 12V and signal wire, not in-line in the signal wire. What you can do to temporarily test this without cutting any wires is to insert the resistor into the back of the pins in the plug, one end into pin 1, the other end into pin 7.

James

John Vine
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It's 1K ohm, not 1 ohm.  Assuming the resistor is simply missing, there should be a mating connector for it in the loom already.

JV
 

simon_h
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The second speedo, if you take the grommet out it might have the rows of dip switches to change the cal. Mine looks similar from memory and isnt programmable by the trip button but has the switches behind the grommet

John Vine
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Re #27:

Good idea. 

Looking again at your second speedo, the odometer screen is black rather than green, and the date on the label is 30/04.  AFAIK, the later version (green screen, pulse code) was introduced in 2005.  So yours could well be the type programmed via dipswitches, as simon_h suggests.

JV

OrangeMax
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So, the 12v green wire from Pin #1 supplies the added voltage required for the speed sensor (reduced by the resistor)?

Yes, the black tach is a dip switch version. Thank you for pointing that out. Is there a document covering the dip switch settings?

OM 

*driving*Max

John Vine
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Re #29:

Some info on pull-up resistor here.

Is there a document covering the dip switch settings?

Dipswitch table

Speedo info

JV

OrangeMax
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Three cheers! I have a working speedometer, thanks to the great advice from the folks who frequent the tech section of the Lotus 7 Forum.

Just in the rare case that others may find themselves with a similar challenging speedometer I'll recap the things that were required to get it working.

1. The sensor distance to the trigger wheel was not correct, as it was not lighting on every tooth.

2. Ground modification was not done. (Sometimes this is not required, but it can't hurt to do it)

3. Sensor wire was going to the wrong pin on the speedometer. I changed it's location from pin #6 to Pin#7.

4. The wrong code was set in the speedometer. Thanks to the folks here I was able to set in the correct code.

5. There was not a 1K ohm .25w resistor between pin#1 and pin#7 wires. This was added.

Thank you all again.

OM

*driving*Max