Electrical Mystery

[LX5liter]

I'm working on a friends car and am a little stumped. The car(1988 Mustang 4 cyl.) runs fine when it is warm outside, but as soon as it gets below zero, (and it does get cold A LOT up here) the fuel pump will not run. Does anybody have a decent schematic that says where the wire from the inertia switch goes? I know one goes to the relay, but I can't tell where the other one goes. I figure there must be a broken wire in the harness somewhere, and since copper shrinks when it gets cold, it is cutting out the power to the inertia switch, since there is no power at the inertia switch when it is cold out. What do you guys think?

[moosejaw]

The circuit goes from the main bus -> relay -> inertia switch -> the pump -> ground.

The relay is triggered by the computer. The ground leg of the shutting coil on the relay goes to the computer. The computer then grounds this leg to fire the relay and turn on the pump. I have seen faulty computers that refused to ground that pin. Test all aspects of the relay and relay connector to verify them. The cold could be affecting the ECU or the relay.
Fuel pump relay should be under the driver seat on an '88. If not it is under the hood by the AC relay.


Mike

[LX5liter]

I originally thought relay as well, so I replaced it to no avail. I had found a schematic from an 86 Mustang that said the power went through the inertia switch first...did they change this in 87? I know power is getting to one leg of the relay, but it is not being triggered to send it back to the I/S or the pump. Do you know what pin the relay signal comes from?

[moosejaw]

On the A9l series it is pin 22. '94 - '95 is also pin 22. I'll go out on a limb and say it is probably the same on '86. Some things they haven't changed over the years.

Also, when the key is on there should be +12V to the shutting coil. So there should be two pins in the relay plug with power. See if you can jumper the grounded side(ecu side) of the shut coil and see if the pump runs.

On the car that had the pump circuit fail on the ecu, we wired a manual switch (with the owners permission).

Mike

[moosejaw]

Just remembered....

Check for a broken but not blown fusible link. Check all the wiring at the starter relay distribution. Another car (a taurus) had a fatigued and broken fuse link that would go intermitent at the worst times! Pull lightly on the wiring to see if a break is present. The sleeve over the link will stretch if broken. It should not stretch at all.

If the inertia switch is under the dash and the relay under the seat then it would make sense that the inertia switch comes first in the circuit.

Mike

[LX5liter]

I'll check for that fusible link. The inertia switch is in the hatch, so it would make sense it's inline between the relay and pump. I have to wait until it gets cold again now before I can work on it again. It runs fine now that the weather is warmer.