Has anyone found a solution for the linear resistance designed into the Spectra Premium fuel sending unit?
This 2006 post discusses it but does not have a solution.
The problem is this. The Spectra Premium sending unit increases resistance at a steady rate (the resistance is "linear") as the fuel level drops, whereas the factory unit increases resistance very slowly at first, then more rapidly as the tank approaches empty. With the Spectra unit installed, the gauge reads full when the tank is full, and reads empty when the tank is empty, but reads very low when the tank is partly full. In other words, as you drive, the gauge quickly drops down toward E, but then hangs out just above E, creeping almost imperceptibly closer to E as you continue driving.
As SlantSixDan suggested in the 2006 post, I called Spectra and explained the problem. Here's a paraphrase of the relevant part of the conversation:
Quote:
SPECTRA REP: What's the part number?
ME: FG69A.
SPECTRA REP: And what's the problem?
ME: It doesn't make the gauge read correctly. The resistance is not correct at all fuel tank levels.
SPECTRA REP: It works to the factory specs: 10 ohms at full and 80 at empty.
ME: It reads correctly at both extremes, but too low in between.
SPECTRA REP: We have been doing this for many years and we haven't had any reports of this problem.
ME: Here's your report, I'm reporting it now.
SPECTRA REP: What's the application?
ME: 1974 Dodge Dart.
SPECTRA REP: OK, I'll post a report to the quality team.
I followed up the phone call with an email saying:
Quote:
Part no. FG69A should have its resistance increase *nonlinearly* from 10 ohms (tank full) to 73 ohms (tank empty). To work properly with the factory gauge, the resistance should increase slowly at first, then more rapidly as the tank approaches empty. As designed, the resistance increases linearly. That causes the gauge the read half when the tank is 3/4 full, and near empty when the tank is half full.
In the photos below, I've opened up the box containing the sliding contact and the resistive wire. The variable resistance comes from the total length of wire between the sliding contact (that moves with the float) and the case, which is electrical ground. Each time the contact touches a new turn of the wire, an additional length of wire is added to the circuit and so the resistance increases. You can see that all turns of the resistive wire are of equal length, so as the sliding contact moves along, the resistance between the contact and ground increases by the same amount each time a new turn is contacted. Also, the turns are equally spaced, so every inch of contact movement adds the same number of turns to the resistor.
It occurred to me that I could adjust the response by stretching some of the turns apart while pushing others together, but the wire is wrapped pretty tight and I don't think I can do that without breaking it. The wire may actually be glued or clamped in place.
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