7v sounds way too low but if there is no gas in the tank u wont hear it either.
dave

 

Gara, What year donor harness are you using? I have a '93 and my inertia switch is wired just before my fuel pump. I also have a special ground for the fuel pump, which is in fact a diode, that originally was wired near the front of the car, but I moved it to the rear during my wire diet. 7 volts is definitely too low to operate your pump correctly. Your fuel pump relay is operating for sure? Which side of the fuel pump relay is the inertia switch?
Doug

 

Doug,

I have an 89 harness from and Automatic trans car. I checked and double checked the relay, the voltage going into the relay is definitely 12volts R/B and O/LtB. The output going to the fuel pump is Pk/Bk is definitely only putting out 7V. I did some playing around and tapped into the Down stream 12V lead R/B on the inertia switch, I also located a new ground. connected both these new lines to the fuel pump and whoosh it works fine. The way it is now, when I turn on the ignition the pump comes on. When I turn off the ignition the pump shuts off. If I trip the interia switch the pump shuts off. I am not sure if I can do this long term, if I can, I will leave well enough alone. I am still curious whats up with the voltage drop across the relay. Could this simply be a bad relay?

Thanks

Gara

 

Gara, let me try to explain this the best I can. The red/black going into the fuel pump relay is the battery supply to operate the f/p relay. the voltage that gets cut thru to the fuel pump on the pink/black actually originates from the black/orange that also goes into the fuel pump relay. You mentioned a O/lt blue wire. Does this go into the f/p relay maybe instead of the bk/org? If you still have the WOT relay, it is interchangeable with the f/p relay. You could test the relay by exchanging it with the WOT. Also, the f/p relay operates from a ground from the computer on pin 22. Should turn on for 1 sec., then go off until you turn the ignition to the start position. You are staying EFI with the computer I take it? Your fuel pump diode is wired a little differently then my '93, but it is in there to act as sort of a surge protector, to keep any voltage spikes from getting back to the computer. In my opinion, you are better off keeping the wiring as close to stock as you can. I can't say that the way you have it wired now won't work as well.
Doug

 

Gara, you were right. I was looking at the wrong drawing. the o/lt bl is the battery supply for the fuel pump. And your f/p diode is probably the same as mine. The ground for your fuel pump should go throught this diode. It looks like a little silver squarish can about 1 inch by 3/4 inch and is bolted to ground. The terminal coming off this little can goes to the ground side of your fuel pump. Hope this hasn't confused you too much.
Doug

 

Gara, if you have 12V into the relay on the O/LB wire and only 7V out on the PK/BK wire you have high resistance between the two wires. Unplug the relay and inspect the pins on the relay and the pins in the connector for corrosion. If they look O.K., replace the relay. The contacts in the relay are probably burnt, resulting in high resistance.

I did some playing around and tapped into the Down stream 12V lead R/B on the inertia switch, I also located a new ground. connected both these new lines to the fuel pump and whoosh it works fine. The way it is now, when I turn on the ignition the pump comes on. When I turn off the ignition the pump shuts off. If I trip the interia switch the pump shuts off. I am not sure if I can do this long term, if I can, I will leave well enough alone.

I wouldn't advise leaving it wired this way. Normally the fuel pump is powered by a dedicated 18 gauge fusible link. You now have it powered by a 20 gauge fusible link which is also needed to supply power to a lot of other things. The 20 gauge link won't like all that electrical load for long.

 

OK. Appearantly everything is working except 12volts at the pump from the PK/BK wire. I have an 89 harness on the bench for a wire diet and I can say that the PK/BK out of the FP relay splits into two directions. First it goes directly to the pump but also splits and tells the computer whether the engine is running. For 3 secs. you should have full voltage out of the relay followed by a voltage drop to 6/7 volts.

If this is not the case try another relay, the convertable top relays have the same #s as the FP, because they typically work or not at all. Check the continuity of PK/BK between the FP relay and the pump with the key off. If you have continuity the next thing I would try is another computer.

Roger
949 275 5702 9-3PM PT

 

Roger,

From what you are saying, the 7v I am reading is normal. I have been taking longer than 3seconds to take a voltage reading when I turn the key on. I am still confused why the pump would not turn on during the 3 seconds of 12v. I will check again tonight. Maybe I have a loose ground somewhere. At least I know that the pump works when I bypass. Oh by the way, I started the car for the first time yesterday, what a rush...

Back to work, and then the fuel pump tonight.

 

Sounds like you need to determine if the relay is functional, then you can determine why things aren't functioning properly when you turn the key. You say the R/BK wire has 12V. With the relay plugged into the harness, jumper the T/LG wire at the relay to ground. This should cause the relay to close (and it will remain closed as long as the key is in run). With the relay closed power should flow from the O/LB wire, across the relay contacts to the PK/BK wire.