Electrical Question about Fuel Sender Circuit

[ehjorten]

So...many of you have probably seen my slow moving LS Conversion in these two threads: http://forum.73-87chevytrucks.com/smforum/index.php?topic=19098.0 and http://forum.73-87chevytrucks.com/smforum/index.php?topic=34079.0.

Last thing I did was to power up my truck to check-out that all of the forward harness work I did was working properly.  Check headlamps, turn signals, horns, aux. electric fan, etc.  I ran the #2 circuit from the firewall junction block to the + battery and also looped in the #2 circuit to the alternator stud into the + battery.

Everything worked great!  A few minor hiccups like the RH front fender marker was not lighting-up properly (a little work with a pair of needle-nose pliers and some emery cloth fixed that!), and the Aux. fan wasn't spinning (a little hand rotation and now the fan works great!).

One thing I noticed was that the fuel pump, that was selected on the switch was slowly rotating.  You could hear it, but it wasn't rotating at speed.  I had previously tested the pumps, but on this go-around the fuel pump feed wasn't powered!  The LH gauge correctly read 1/2 tank as I had put 10 gallons in both tanks.  I then switched the tanks to the RH side out of curiosity and immediately I noticed that the gauge pegged full to the right!  LH pump quit rotating and then the RH pump started rotating.

After a bit of fiddling around with going back and forth between tanks and listening...checking for proper grounds to frame ect.  The pumps quit rotating all together and did not make any more noise, and now the guage doesn't seem to be responding properly.  I am pretty sure when I powered the pumps and checked the fuel system that the gauges were reading properly.

Could this be some weird electrical issue?  Is it backfeeding the pumps until they pressurized to some point and then there wasn't enough current to turn them anymore?  I have been looking at the circuit diagrams for fuel pump and aux fuel tank and selector valve and I am confusing myself.  Power is coming off of circuit 120 and going through the fuel selector valve and then out to the pump and also the fuel selector valve.  That is where I get confused by the diagrams.

Anyone have some insight?

My next step I guess is to power the pumps and look to see if the guage responds normally.

[ehjorten]

I think through talking to one of the electrical engineers at work that I figured it out.  The circuit for the fuel tank selector valve basically flips polarity between power and ground.  Also, both pumps are tied to each other through three grounding circuits: One through the ground to frame, one through the ground path through the selector valve, and one through the ground path through the selector valve itself.

I think I must be back-feeding some power through the firewall connector as circuit 120 to the pumps is tied to circuit 120 at location F1 on the firewall connector.  I don't have a relay connected up to power the fuel pumps.

[VileZambonie]

First thing to do is verify the battery voltage an polarity.

[ehjorten]

Powered up the pumps last night and my gauge read accurately on both tanks.  It just must have something to do with how the circuits are connected that is affecting it.