I recently refurbished a 70 Eliminator and noticed that it was missing a resistor wire that bridges two contacts on the connector going to the low fuel relay. Hmmm…the wiring otherwise is the same as earlier years that do have that resistor wire. I checked the MPC and the same relay is used throughout the Cougar years. So…either the resistor wire was done away with or possibly the low fuel thermister changed at the same time, but i doubt that.
Now then, that resistor wire is incredibly fragile and once it breaks, it is nearly impossible to re-crimp it again. Did FLM do away with the resistor wire completely and not change anything else? If so, that really helps the 67/68 crowd whose resistor wire has broken, which shouldn’t affect functionality at all. It does help those of us who are trying to repair it: there’s no sense in doing so, if my hypothesis outline above is correct.
You can use a 5 watt 50 ohm resistor to replace the resistance wire. Unpin the connections and solder the resistor across the two connections. The resistor is in parallel with the coil in the relay dividing power.
My November-1969 built 1970 Cougar does not have the resistor either. Same with another 1970 XR7 harness I had.
This was probably a running change but I did not find anything about it in the L/M TSBs.
I believe the purpose of the resistor is to increase the current (2x) through the thermistor in the gas tank - probably to increase the sensitivity in colder weather. It seems like the thermistor value would need change with the addition of the resistor.
In the '80s, I found the OEM source (KCC) for these thermistors and obtained a small supply. I put one in my '68 XR7 and my '70 convertible - they both worked correctly until few years ago. Unfortunately, I no longer have any left and KCC will not manufacture them anymore.
Interesting.
I have a colleague that analyzed the circuitry and claims the resistor is absolutely needed for the circuit to work correctly. To the best of our knowledge, neither the relay nor the thermister changed during this time, only the size of the gas tank. What I would like to know is whether the early version (with the resistor) will work with the resistor removed from the circuit. Surely, someone has tried this combination…
Another reason resistors are placed across relay coils is to discharge the energy stored in the coils magnetic field when current through it is interrupted. Without it, the relay coil (which is like a tiny version of the ignition coil) could produce a spark in the fuel tank if the thermistor suddenly opened up. If they later removed this resistor wire, I wonder if it was because the resistor was moved internal to the low fuel relay. Or sometimes they will use a diode across a relay coil for this purpose. I can’t imagine not having this protection at all.