Location of oil sending unit on 88' V30? Keep blowing ECM B fuse

[Chevy3+3]

Originally Posted by JW76

Check the diamond shaped plastic block on the firewall (to the left of the brake booster) that that power wires connect to. There is a metal tie bar on the back side. On my truck the back of the block cracked (couldn't see it from the front) and when the cab flexed it would contact the firewall blowing the ECM B fuse.

Ok I will check that out also. Thanks for the tip.

To everyone else trying to help me I apologize that I have no updates. I had to watch my daughter most of the day. I'm working night shift this week so I only have the mornings to look at it. I hope to get some time in on the truck in the morning.

[Chevy3+3]

Alright guys. I'm with my truck now and been disconnecting and checking continuity between wires. Here's is how I have the truck sitting at the moment.

1. Both fuel pumps are disconnected
2. The fuel tank selector valve pigtail is disconnected
3. The ECM B fuse is not plugged in

I will start at the tank side. The pigtail (harness side) for the pump shows some resistance between the 2 wires. If this is a 12v & ground then I doubt it should be this way

I believe I traced the correct wires back to the fuse box along the drivers side frame rail. The wires are colored orange/reddish and a tan wire. When I ohms between the 2 wires under the engine bay I almost peg my analog ohms meter. Remember the fuse is not installed.

Thoughts?

[Chevy3+3]

I will also add that I cannot get 12V at either wire on the passenger side pump pigtail. When switched the passenger side tank I only have power going to the tank selector valve on the red/pink wire.

I apparently have 2 different demons to sort out. I believe my passenger side pump/tank not working had nothing to do with my fuse blowing. That tank didn't work when I got the truck but I ran on the drivers side tank for a couple of years. But if I can't get power to the pigtail then I would assume that the switch inside the selector valve is faulty and not switching.

[hatzie]

Originally Posted by Chevy3+3

I will also add that I cannot get 12V at either wire on the passenger side pump pigtail. When switched the passenger side tank I only have power going to the tank selector valve on the red/pink wire.

I apparently have 2 different demons to sort out. I believe my passenger side pump/tank not working had nothing to do with my fuse blowing. That tank didn't work when I got the truck but I ran on the drivers side tank for a couple of years. But if I can't get power to the pigtail then I would assume that the switch inside the selector valve is faulty and not switching.

That would be an incorrect assumption.

Download the 1988 wiring diagram PDF file. Look in the 1973-1991 link in my signature.
TBI option NL2 (dual tanks) wiring diagram is on PDF page 279 of the 1988 GM light truck wiring diagrams.
Power to the NL2 wiring is Circuit 120 from the Fuel Pump Relay and the Fuel Pump Oil Pressure Switch. These are on PDF Pages 286 & 287.

RH & LH means as the driver is seated. SO... North American Passenger side is RH. Driver side is LH.

First...
You will not get fuel pump power with the engine stopped. Ignition on or off makes no difference. The oil pressure fuel pump safety switch contacts are open so you will not get juice with the ignition on the ON position. The ECM is also not commanding the fuel pump relay to provide juice to the fuel pump system because your ignition switch is not in the CRANK position.
NOTE: There is a fuel pump prime wire in the wiring diagrams attached to the fuel pump relay socket. Applying power to this wire should apply power to the dash switch and the valve wiring for testing.

The Dash switch routes power and ground to the fuel pumps' Hot leads... this is not done by the valve. The dash switch output wires are wyed to the motor terminals D & E on the valve as well as the LH & RH fuel pumps.

There is a Double Pole Double Throw switch inside the Valve but it does not route power to the fuel pumps.

1. Switch half one is... The overtravel switch. Terminal E is the exposed Common terminal with the valve motor wired to both switch terminals through properly biased diodes. The valve motor only runs till it reaches the internal overtravel switch point. When the overtravel switch trips... it routes the E terminal to an opposite bias diode blocking current to the motor till the dash switch is flipped to the opposite polarity. NOTE Terminal D routes DIRECTLY to the valve motor.
2. Switch half two is... The gauge selection switch. All three terminals are exposed to the outside world. B is Common to the gauge, A is wired to the RH sender, and C is wired to the LH sender. Since this is the second half of a DPDT switch it trips at the same time as the overtravel switch.

This is the state diagram from my prior post showing the switch and valve in the RH position.
With the switch in the RH position the Dark Green circuit from the switch is HOT and the Light Green Circuit is Ground.
This gives the RH fuel pump power and a Frame Ground turning it ON. The LH fuel pump is OFF because it has TWO grounds. One on the Light Green Circuit and one from the Frame.



Flip the polarity reversing dash switch to LH.
The Dark Green circuit is now Ground and the Light Green Circuit is now HOT.
This gives the LH fuel pump power on the Light Green Circuit and a Frame Ground turning it ON. The RH fuel pump is OFF because it has TWO grounds. One on the Dark Green Circuit and one from the Frame.


I believe the 1988 fuel pump and valve motor power wires from the dash switch are TAN and GRAY.

[Chevy3+3]

Ok well I thought maybe it would be calling for a fuel pump when I turned the ignition on. That's my bad. I wasn't thinking I guess. Back to the drawing board. I'm not really sure where to go from here. I guess I will wire up some leads and test each pump by hooking directly to them with an in line fuse wired in line. That way i can determine if my pumps are bad by bypassing the rest of the system.

[Chevy3+3]

Ok I looked at the '88 wiring diagram. Seeing it on a bigger computer screen makes it a lot easier to read and understand, lol. I am beginning to understand the workings of it more now and from the diagrams it appears my truck is still wired this way.

One thing I can't understand is if you said I shouldn't be calling for a pump without cranking the engine then why is my pink/white wire have 12V (drivers side tank selected). My tan wire also faintly illuminates the test light. When my switch gets flipped to the other side my pink/white wire still has power but the tan wire does not illuminate any longer. Maybe I chasing something I should be???

Also does each fuel pump have their own individual relay?

[345HP87SSAC]

............following

[345HP87SSAC]

............following

[hatzie]

Originally Posted by Chevy3+3

Ok I looked at the '88 wiring diagram. Seeing it on a bigger computer screen makes it a lot easier to read and understand, lol. I am beginning to understand the workings of it more now and from the diagrams it appears my truck is still wired this way.

One thing I can't understand is if you said I shouldn't be calling for a pump without cranking the engine then why is my pink/white wire have 12V (drivers side tank selected).

In 1988. The three pink wires are the sender circuit. One stripe wire will always get some volts through the gauge.

• Pink/Black is the RH sender to the valve plug A terminal.
• Pink is the valve plug terminal B to the Fuel Gauge.
• Pink/White is the LH sender to valve plug terminal C.

Originally Posted by Chevy3+3

My tan wire also faintly illuminates the test light.

Use a digital or analog multimeter. Test lamps can smoke I/O channels on ECMs and they only give vague indications of voltages and no indication of polarity.
Tan and Gray are your valve & fuel pump power wires from the polarity reversing dash switch. They are the Green circuits in my state diagrams.
In '88 Tan should be hot with the LH tank selected and Gray will be ground.
In '88 Gray should be hot with the RH tank selected and Tan will be ground.

Originally Posted by Chevy3+3

When my switch gets flipped to the other side my pink/white wire still has power but the tan wire does not illuminate any longer.

The valve isn't moving.
You need to meter terminals D&E on the valve plug. In 1988 D is Tan. E is Gray.
D should be hot and E ground with LH selected.
D should be ground and E hot with RH selected.

Originally Posted by Chevy3+3

Also does each fuel pump have their own individual relay.

Just one relay and oil pressure switch that provide power to the dash switch while cranking and running.

[Chevy3+3]

I need to check a couple more things but I'm almost positive my PS pump is bad. I wired it directly to a 12v power source and it did not run. I didn't the same to the DS pump and it cycled.

I did check for voltage at the selector valve pigtail connector. I did have 12v to both the tan & gray wires when either the left or right tank was selected. I will add that just for fun I checked the tan & gray wires for voltage when they were assuming the ground responsibility. Both wires carried 5.5 volts to ground (frame of truck). I wasn't expecting voltage of any kind if those wires were switched to the ground position when reversing polarity at the switch.

[Chevy3+3]

Just getting back here for anyone following. It took me a while to fix it since I was out of town for a while and then an unscheduled surgery slowed me down.

I replaced both fuel pumps and was back to running without blowing the fuse. My passenger side tank would still not work. I found out the selector valve was not working properly so I replaced it. Now both tanks work and I am not blowing the fuse any longer. So the culprit was a bad fuel pump causing the ECM B fuse to blow.

[hatzie]

Thanks for closing the thread with a solution for other folks that may have the same issue.
Glad it's fixed.