fuel tank question

[fluffyfreak]

I gotta 86 C10 I want to convert over to tbi. Will a 87 sending unit/fuel pump work with my tank or do need to get 87 tanks. Also id like to keep the dual tank setup, would I need to change wiring or can I keep it?

[Zane M]

I believe you can use your existing tanks, but you will have to be a little more mindful of fuel slosh at lower fuel levels since the 86 tank doesn't have the baffle.

The tank selector wiring can stay, but you are going to have to modify or replace the wiring to the fuel tanks to accommodate powering the fuel pumps-one in each tank.

[hatzie]

The sender wiring is different and the polarity reversing cab switch may need to be changed from Momentary in both positions to ON-ON. Not sure why GM used the momentary switch at all. The valve motor shuts off at the end of its' travel regardless.

The 87-91 R/V senders have 3 wires. A 2 position Weatherpak connector for pump power & the gauge sender wire and a ring terminal to ground the pump and sender.

Pump power is from the polarity reversing switch in the cab that reverses fuel valve motor. When the switch is set to LH then the LH fuel pump has Power and Ground and the RH pump has two Grounds. Flipping the cab switch reverses the polarity so the RH tank has power and the LH tank has two grounds.

If you can get a largely unmolested 87-91 dual tank harness it's the easiest way to do this. There's a disconnect in the single Pink sender wire along the frame between the bulkhead plug and the valve plug. There's a 2 position switch disconnect on the engine compartment side of the firewall.

If you intend to roll your own it can be done. I'd get a Weatherpak extractor and crimp tool along with some male and female 2 position Weatherpak connectors, seals and terminals.
This kit will cover the shells and terminals... http://www.amazon.com/Position-Weather-Pack-3-pack...
If you're only doing a few this crimp & extractor kit will work... http://www.amazon.com/Delphi-Packard-Weatherpack-C...

Grab the GM 1989 ST350 wiring manual from my service manual post and jump to PDF page 42 for a good wiring diagram of what You want to end up with.

You can clip the sender leads close to the tanks and crimp on the appropriate Weatherpak terminals to mate with the new sender assemblies fuel pump and sender connector.
Write down the wire colors for circuit 920 & 921 on your vehicle harness.
Extract the circuit 920 & 921 terminals from the valve plug with the terminal extractor and modify from there.
Soldering on the 35 year old wire is probably going to be dicey.
I'd put the 920 & 921 terminals into a 2 position Weatherpak tower. 920 in A and 921 in B.
Cut a 2 6" wires and 1 7' and 1 3'.
Peel a 1" section of insulation from the center of the 6" wires then... Solder the 7' and 4' wires to the center of the 6" wires and cover with ahdesive lined heatshrink.
On the 6" wires... Crimp female terminals to insert into the valve connector and Male terminals to plug into the original harness.
The 7' should reach the LH tank plug and the 4' should reach the RH tank. Tie these wires to the sender wires and clip to the same length as the sender wires. (These are your fuel pump power leads)
The 7' wire is your new circuit 921 and the 4' wire is your circuit 920. Insert in the connector cavities accordingly.
Crimp on the appropriate Weatherpak terminals to mate with the new sender assemblies fuel pump and sender connector.
Insert the sender and fuel pump wire in the shell that mates with the new sender connectors mating with their respective components. Sender to sender and pump to pump...

You may want to insert the sender and fuel pump power wires in expanded mesh sleeving or split poly loom. You definitely want to tie the wires up to the frame to protect them.

[Zane M]

Well spoken and detailed explanation Hatzie.
It is this kind of information that makes this forum such a valuable resource for GM trucks in general, and square bodies specifically.

[dieseldawg142]

when i changed my 85 k30 to efi, kept the same tanks, sending units, (dual tanks), all i did was use an Aeromotive external fuel pump about a foot from the switching valve & plumb in a new line (old line became return).
tank selector & everything else stayed the same.

[tex_waco_usmc]

I have recently done the same swap. 86 to 87 drivetrain. I used one brand new tank for an 87 as one had a rust hole in it. The other was from the 86, when it gets to below half a tank it will just die for no reason running down the road. I am about to put a brand new tank in on the passenger side to correct the issue.

I used the 86 tank, put the 87 sender, fuel pump and complete wiring harness. It functions but it will piss you off when it shuts off on you going down the road. It always fires right back up once you bring it to a complete stop but as soon as the slosh starts happening it dies again!

[mattcrp1]

I am doing an ls swap right now on my 76. I had to change the tank and sender to get the correct set up for fuel injection. The sump in the 87 tank should make a difference in the driveability. The new sender is a lot different then the old carbureted one.

[hatzie]

Originally Posted by tex_waco_usmc

I have recently done the same swap. 86 to 87 drivetrain. I used one brand new tank for an 87 as one had a rust hole in it. The other was from the 86, when it gets to below half a tank it will just die for no reason running down the road. I am about to put a brand new tank in on the passenger side to correct the issue.

I used the 86 tank, put the 87 sender, fuel pump and complete wiring harness. It functions but it will piss you off when it shuts off on you going down the road. It always fires right back up once you bring it to a complete stop but as soon as the slosh starts happening it dies again!

First comment I've heard about any unpleasant effects that the 73-86 no baffle tanks have on the TBI in-tank pump operation...
It's not a cruddy sock because this is a new sender.

If it's cutting out just running flat on a stretch of road it's probably not the missing tank baffles... I'd try something cheaper that will probably fix the problem before spending your dough on a new tank...

Change out your dash switch with a new $14 AC Delco D7089C.

This sounds odd until you think about how the fuel pump and tank switching circuit works.
The fuel pumps' operating ground is the short pigtail to the frame shared with the gauge sender.
The dash switch is a polarity reversing DC switch. It's only hot to one fuel pump via one half of the switch. The other half of the switch is feeding a second ground to the resting fuel pump.
When you flip the switch the wire that was ground becomes hot and and the wire that was hot becomes ground. This reverses the DC motor in the valve moving the shuttle to the opposite tank. This also feeds hot to the previously resting fuel pump completing the circuit thereby turning it on and double grounds the un-selected pump breaking its' circuit thereby turning it off.

Whichever pump is at rest has two grounds so that half of the switch is not carrying any current to the resting tank. When the good tank is selected the weak half of the switch is not carrying any current.

Then you switch over and kick in the weak tank. But it's full. So you chug merrily along and the fuel pump is fat dumb and happy...
As the fuel level in the tank, on the weak side of the switch, drops... the fuel pump has to do more work and the current draw across those weak contacts rises. When you reach around 1/2 tank the current draw reaches the point that the weak switch contacts will no longer carry it and they intermittently arc dropping the pump current below its' cutout threshold. Dead truck.

If it's only when you're on curves or changing speed then it points more toward the missing baffles... But. The fact that it doesn't recover till you bring it to a complete halt with around 1/2 tank on that side is odd at best. It should buck and stumble as the pickup sock is covered and uncovered. It should not cut all the way out without recovering.