So I did use search and even google so bare with me. My mom has a 98' Navigator that died on her on the way from work one day. I was deployed at the time and there wasn't much I could do. I'm in town on leave so I would like to get as much done before I leave for Italy. Anyway,the truck will start and run and then overheat the 20amp fuel pump fuse and it will blow. I tested power to the inertia switch and it show's 6.8 volts. I know it should be 12 volts at least.

When I test at the relay it show's 12.25 volts from the fuse (pin 86),and 12.25 volts from the control power side or key (pin 30). ECU to from the relay is 0 as it should be. However,on the relay to intertia switch/ecu pin 40 side I'm seeing 6.85 volts from pin 87. I'm pretty sure it should be 0 volts but I'm not 100% sure. I'm inclined to think there is a short but I don't understand where it could be coming from. The diagram doesn't show anything between the relay and the inertia switch other then the ECU input for pin 40.

I did notice some smoking from the cruise control deactivation switch. I don't think it's part of the issue though but I could be wrong. Any insight would be great !

Thanks,
Bob

 

1) IMMEDIATELY, go out and unplug that switch's connecter from the master cylinder. Then, have Mom schedule an appointment with a dealer for recall 05S28 if it hasn't been done.

2) If the fuse (F19) is blowing, there's too much current being pulled through the circuit. That low voltage at both the inertia switch and the relay output suggests the fuel pump relay or the fuse box is probably already damaged from the heat.

Fuel pump relay pin 30 is the ALWAYS HOT feed from fuse F19 and should always measure battery voltage. Pin 87 is the switched output of the relay and should measure the same whenever the relay is energized. Your relay (or its socket, depending on where you took your measurements) is bad from the excessive current draw.

If I had to take a stab at it, I'd say you probably have a bad fuel pump (excessive current draw) which has damaged the relay. Replacing both ought to fix it.

Hey, thanks so much for your service! Good luck!

 

short circuits wont cause low voltage, they will cause no voltage

open circuit, or partially open can cause excessive current draw and thus low voltage readings

 

My analysis is based on the assumption that the presented readings were made with the fuel pump running. If the pump was not running at the time, the voltages measured were something else and my analysis doesn't apply.

No, neither a short nor an open circuit is or has been present, it's an in between condition. My analysis is that the pump has been pulling excessive current and the relay contacts are now exhibiting excessive resistance (contact deterioration). It's possible that it's a wiring harness with a partial short-to-ground, but it would require additional diagnostics which are certainly recommended to either confirm or rule out that idea.

An open circuit or excessive resistance will cause either NO or LESSER current, not more.

FYI, I've been an electronics tech since 1976. I have a pretty good understanding of this stuff.