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2001 Sporster LX tach and fuel gauge not working

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I have a newly acquired 2001 Sporster LX.

First the tach. I bought it with the tach not working. I posted earlier about wiring diagrams and someone provided a link. I did the continuity check and made sure the gray wire was in the correct position on the MDC. I did find the gray wire was not connected behind the dash. Spliced them together and the tach started working. It functioned for about 30 mins and stopped. Checked wires again and took it out this past weekend and the tach worked again for 30 mins and stopped. I took the spice apart and measured voltage on both legs while engine was running. From the engine, I was seeing -3VDC. From the controller I was getting 12.5 to 13 VDC. Why would the controller be putting out 12VDC+ for an input wire? Confused

Other issue is the fuel gauge. It reads 1/8 to 1/4 full even with a full tank. Checked resistance at the MDC and measured 200 ohms which squares with what the gauge is showing. I grounded the pink wire and the gauge read full. Is it common for the sending unit to go bad on these? Does anyone know the part # for the sending unit? It has a 38 gallon tank.

It appears that this boat may have had perfect pass installed previously as there are several plugs which are not used along with several single female spade wires going nowhere. Wondering if this may be some of my issues.

Thanks in advance for your help.

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This is fuel sending unit I had in mine: https://www.amazon.com/Moeller-Marine-Products-035722-10-Electric/dp/B000MTI15K/ref=sr_1_1?keywords=Moeller-Marine-Products-035722-10-Electric&qid=1558360010&s=gateway&sr=8-1

I believe the tank is 8 inches deep but someone else may be able to confirm - Moeller has a couple different options depending on the depth of the tank.

What happened to mine is the float became fuel-logged and no longer floated on top of the gas. The sending unit itself wasn't bad.

Regarding the tach I would bet it's the tach itself that is intermittent. It's normally how these gauges start to fail.

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I'd replace with something other than a float arm.  Good video.  Also, you can try swapping one of the KM plugs from say the tach to the speedo to see if that fixes it, which means - as suggested - that the gauge is on its way out.  Bakes sells replacements for a crazy amount.  Like https://www.bakesonline.com/malibu-boats-tachometer-gauge-5-black-white.html


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Thanks for the responses guys. Yes I have swapped plugs and nothing changes for either the tach or fuel gauge. I am suspicious of the float soak too. Just  need to let the fuel tank empty more before I attempt to pull it out and check it. LOL

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