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Old 06-02-2020, 04:45   #1
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Rio 14,20 prior 1979 fuel gauge/sensor issue

Hi all,
"I have a friend" - no, genuinely! - that has the motor cruiser above and the fuel gauges only read "onto reserve tank" irrespective of fuel level. The thought is that they may still be working but the sensor may be covered in crud. It seems very difficult to remove to check as the float pipe is a) behind the saloon seating but then b) the top Allen key bolts are difficult to access but then c) even if we removed the top, we probably couldn't remove the float as there is no room and d) if we removed the other hoses coming off the float pipe, we may not get them back on again or disrupt the seal at the bottom which is equally inaccessible.

Not really my bag so all positive suggestions welcome. (Currently the thought is "Leave it alone and use a dip stick!")
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Old 06-02-2020, 06:43   #2
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Re: Rio 14,20 prior 1979 fuel gauge/sensor issue

Greetings and belated welcome aboard the CF, bgler.

Basic Engine Gauge Theory and Testing
https://www.cruisersforum.com/forums...sting-645.html
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Old 06-02-2020, 07:18   #3
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Re: Rio 14,20 prior 1979 fuel gauge/sensor issue

The senders are variable resisters. That gives you a clue as to how to test whether the rest of the circuit is OK and it's indeed a sender problem. Having eliminated the rest of the circuit as the problem, yes, it's get them out for repair or replacement or use a dip stick. If there is a ray of hope in here, it's that both circuits are doing the same thing. That leaves the possibility of an over-arching failure on the circuit side rather than the sender side.

That's a nice theory of boat construction. Start with a hull, install tanks, slap the salon on top of the tanks, wirth no regard for how to get to the tanks. Easy to build it that way, and it saves interior space. There was a story going around in the 1950s (apocryphal) that Ferdinand Porsche had designed the Renault (the one that looked like his beetle) so that it could not be serviced. It was supposedly his pot-war spite at the French for holding him in prison as a potential war criminal.
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Old 06-02-2020, 08:00   #4
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Re: Rio 14,20 prior 1979 fuel gauge/sensor issue

Thanks all - my mate is a Sparky so this should be helpful so I will see what he makes of it.
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Old 06-02-2020, 09:08   #5
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Re: Rio 14,20 prior 1979 fuel gauge/sensor issue

Basic Engine Gauge Theory and Testing
https://www.cruisersforum.com/forums...sting-645.html

The standard Voltmeter type Gauge receives it’s reference voltage from a Sender unit, mounted on the Engine. The Sender is a device, whose resistance varies with changes in Level, Pressure, or Temperature. As the sender resistance increases, the reference voltage transmitted to the Gauge decreases.

All Gauges:

1. Remove Sender Wire from “S” Terminal on Gauge, and Turn ON Instruments Circuit.

2. Test Voltage between “I” Terminal (External Power Supply) and Ground Terminal “G”.
Tested voltage must match nominal voltage of the Gauge (ie: 12V Gauge must test between 10 - 16VDC), and the Battery Voltage.

If no voltage - check Ignition or Instrument Switch(s) and/or Fuses.
If tests less than battery voltage, repair or replace (Violet) wire from Ignition or Instrument Switch.

If a Sender fails “Open” (Infinite Resistance), one or both (if Dual Station) Gauges will read Zero Scale.
If a Sender Fails “Closed” (shorted - Zero Resistance), one or both Gauges will read Full Scale.

Other confusing Ohmmeter results, such as readings lower than expected, could indicate a poor Ground Connection at the Sender Body (Sender to Engine Block). Check for pipe dope or tape on Sender Threads.

Fuel Gauges:

1. Disconnect Pink Sender Wire from Gauge, and turn power ON.
The Fuel Gauge should read ZERO Scale (< Empty)

2. Short the Gauge Sender Terminal to Ground.
The Gauge should read FULL Scale (> Full).

3. Measure resistance of Pink Sender Wire to Ground.
American Floats: 240 Ohms (Empty), 103 Ohms (˝ Full), & 33 Ohms (Full)
European Floats: 10 Ohms (Empty), 95 Ohms (Half), & 180 Ohms (Full)
Automotive & other Float Switches may use other resistances such as 0 - 90 Ohm, 73 - 10 Ohm, & etc.

4. Disconnect Pink Wire from Sender on Engine Block.

5. Test continuity of Pink Wire.
Zero Ohms from Block End to Gauge End - If not, repair or replace Wire.

6. Measure Resistance from Sender Terminal (on Block) to Ground on Block.
Should read as per (3) above - If not, replace Sender.
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