Fuel pump rewire on a 90 AWD

Rewiring the fuel pump with a larger gauge wire can help the stock pump (or any aftermarket pump) maintain higher fuel pressure/volume throughout its' entire operating range, as the stock wire is pretty small. The small wire limits the voltage to the pump, and under high boost, the lower voltage can severely affect fuel output. Some people have measured 10 volts or less at the pump under high boost with the stock wiring. Adding a larger wire and relay to feed the pump can restore the voltage to that of the battery (usually 13-14V), maximizing pump output.

So, how do you do it? It's pretty simple - just add a larger gauge wire (anywhere from 12 to 8 gauge will help, the larger the better, though 10ga seems to work fine for most people) from the battery, and run it to a relay mounted near the pump. Then cut the stock 12V wire leading to the pump, and splice the relay inline.

The following is a short primer on how to do it on a 90AWD, I will add pics for other DSMs as I receive them from people. Once I actually rewire my car, I will add in more detailed pics of the install.

    This is a schematic of your basic relay used ( SPDT (Single Pole Double Throw) Bosch relay, others may be laid out differently, but the numbers of the terminals should match - you can buy these at any decent car stereo shop, NAPA, or even Kmart, sold as a foglight relay. Supposedly Walmart sells amplifier install kits in their car stereo section, if you find 2 kits, buy the larger one).
    For reference, the terminals are:

    • 30 - feed wire
    • 87A - Normally Closed (NC) terminal
    • 87 - Normally Open (NO) terminal - the wire that the relay switches to when the relay coil is energized
    • 86 - relay coil
    • 85 - relay coil
    Note that these relays can be used in many different ways, and the terminals can be used for input or output depending on use, but I am only going to cover the way I would use them for the rewire.
    My usage for the pump rewire:
    • 30 - feed wire from battery
    • 87A - Not used
    • 87 - when cutting stock 12V pump wire, this is to the pump
    • 86 - when cutting stock 12V pump wire, this is to the car
    • 85 - ground

    Note that the relay coil terminals can be switched, 86 can be ground, 85 can be power from the car.

    Note that a very quick and easy anti-theft additive is to add a switch to the 85 wire (or 86 if that is what you use for ground) - with it turned off, the pump will not be powered with the ignition on, so a thief can't start the car until he finds the switch.


    Here's the 90 AWD pump wiring unplugged. Note the circled section - that is the end of the plug from the car side of the harness. While you can rewire with heavier wire all the way to the pump, including running through the pump assembly top plate, it is usually unnecessary. Splicing the relay into the wiring near the circled section should do fine, as you are only trying to remove the voltage drop caused by the tiny stock wire from the front of the car to the rear of the car. A foot or less of stock wire shouldn't drastically lower voltage once you are feeding it with the heavier wire and relay.


    Here's a closeup of the 1G AWD pump assembly top plate. The circled black wire with a white stripe is the 12V supply wire to the pump. Follow this back through the harness to the plug shown in the pic above, and then match that up with the male part of the plug attached to the trunk floor to figure out which wire you will be splicing into.


    Here's a closeup of the 1G AWD pump assembly top plate again. The circled junction is the one for the black wire with a white stripe, the 12V supply wire to the pump. If you feel the need to totally rewire all the way to the pump, you will need to carve that black crud off (it is tough going with a knife), and you will find...


    that the junction isn't going to be that easy to handle. It is a riveted fitting, that runs through a plastic grommet to the underside of the pump assembly plate. You would have to drill this out and run a wire directly through the grommet, or find a brass nut/bolt to replace it with, and run new wire to the top of the bolt, sealing it with RTV or JB Weld, and installing a new in-tank wire with ring terminals on both ends.


    Here is the underside of that grommet/riveted junction. If you drill it out and install a new brass or copper bolt through it, you could install a larger wire all the way to the pump. Again, I don't know if it is worth the trouble to do so.


    Here's the pump assembly. The 12V feed wire is on the left. If you wish to rewire it all the way to the pump, this is the one you will have to deal with. Not worth the trouble in my opinion, but if you are an overkill kind of person, go for it.


    OK, so now you have the wire located on the car side of the fuel pump harness plug. Now what to do? Simple - cut it in half. I suggest removing the rear plastic side panel, and cutting it back behind there, so you can mount the relay behind that panel. So you now have a cut wire, what to do. Well, you wire the relay in as I said above:

    • 30 - feed wire from battery (Large RED wire)
    • 87A - Not used
    • 87 - when cutting stock 12V pump wire, this is to the pump (yellow wire in my picture, not stock wire color)
    • 86 - when cutting stock 12V pump wire, this is to the car (long green wire in my picture, not stock wire color)
    • 85 - ground (black wire in my picture, not stock wire color)
    So get the wire you cut in half, and attach the end that runs towards the pump to terminal 87, and attach the other end to 86. This causes the stock wire that normally feeds the pump to now turn on the relay. The relay gets power directly from the battery, and feeds it into the stock wire that leads to the pump. This lowers the voltage drop caused by the small gauge stock wire that runs from the front to the rear of the car, since now all that stock wire has to do is turn on a relay. Note the 2 short green wires in the photo - these are an emergency bypass. If the fuse on the feed wire blows and you have no spare, or the relay dies, reconnecting these two bypasses the relay. These are simply a jumper from 86 to 87.

Oh, this normally doesn't have to be said, but I'll say it anyways...
MAKE SURE TO FUSE YOUR LARGE GAUGE FEED WIRE AT THE BATTERY. I'd suggest using a fuse the same rating as the stock fuel pump fuse (if you can even find such a rating), or slightly higher (but use a slightly higher one at your own risk). Personally, I use a 20A fuse and have had no problems.

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Last modified: Mar 5, 2001
Copyright 1998-2001, Tom Stangl
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