livetohunt54

No power to my fuel sending unit,,any ideas on what to check to find out what the problem is. Thank you

Lemme

Must be supplied by a fuse at some point. What year?

AGENT 86

There is no power to sender, it measures resistance to ground. 90 ohms full > 0 ohms empty.

AGENT 86

Read this > http://forums.corvetteforum.com/c4-t...h-article.html

Lemme

There has to be a circuit with a power source that flows current through the sending units resistor. A resistor on its own will not achieve a measurement.

pcolt94

I'm not specifically familiar with the 84 vintage as I have a 94 type vintage. But I would be surprised if there was as fuse associated with the sending unit and the read out assembly.

However there has to be some voltage to the sender on the top side of the wiper. Don’t know what the sender connects back to on the 84 but if it comes from a unit like a CCM, there has to be either a 5 or 12 volts to the sender. Whatever makes the measurement of the fuel senders position (resistance), a current has to be measured and you need a voltage to produce a current.

If someone actually has a schematic of the circuit, that would take a lot of the guess work out of it.

Lemme

My 87s service manual shows a pink wire to the digital cluster from the sending unit. The cluster must provide the power source which may be only as high as 5V DC as it is a solid state transistor circuit.

Try measuring voltage at the fuel tank. If no voltage then disconnect sending unit pink wire (unplug 3 wire connector) and measure resistance of sending unit to make sure it is not open cct. Do not measure resistance with wire connected to cluster as this could damage the solid state circuitry.

leesvet

There is NO power to the sender....Its as simple as simple can be. One wire to the coil contact, grounds are thru the frame. The whole assy only has 3 wires, 2 of which go the the fuel pump.
Please do not try to "jump" the wire to the sender....in theory the tank cannot explode but another theory that I heard says it WILL when you turn a small coil into a glow-plug submerged in gasoline...there IS a ground...so don;t supply a pos source.

Its just one wire...believe me, I've been in there 20 times.

leesvet

Thats because this is not just a resistor. There are resistors involved but the intank unit is more of a potentiameter....rheostat in reverse. Float arm moves a contact across a coil. The position of which changes the resistor (variable resistor) value that becomes the signal that you see on the dash gauge.

pcolt94

Potentiometer or rheostat can be the same physical device but only differ in configuration or circuit connections. That is not an issue here.

Three wires;
1 for the common ground for both fuel pump and sender unit.
1 wire for fuel pump power…and
1 wire for sender unit connected in a rheostat configuration

This is not magic. This is an DC electrical circuit and electronics work on voltage. And with that is the associated basic ohms law of voltage, current and resistance. You need to have a potential across the rheostat and that will produce a current. There's no getting away from it.

But if you made the measurement at a empty or near empty tank condition, then the voltage would be minimal because the top end of the rheostat is at ground potential therefore no (or minimal) voltage might be measured. (Got a good ground for the meter).


I have measured on my 94 and with the sender un-plugged it measures 11 volts. And with the sender connected there is approx 2.4 -2.5 volts across sender with a basically full tank.

AGENT 86

It's obvious I'm no electrical guru, my comment was based on reading the FSM's panel connector pinout info. I see now that the info is with panel removed, so that's why they list 0-90 ohms and no voltage.
When I think about it, my meter uses it's battery current to test resistance, so it only makes sense for the same to apply here.

ddahlgren

There is a voltage but it is part of a resistance bridge circuit. It is not directly connected to 12 volts but across a 4 resistor bridge. If it was directly connected at 1 ohm it would draw 12 amps.. E=IR....