Apparently, the wire for the sending unit must have got pinched or stretched when pulling the old motor. The shielding was intact but the wire strands were broken.

I spliced in a new wire and the gauge works fine.

 

1990 5.7 TBI full size blazer 4x4

I changed the oil pressure sending unit, which fixed the pegged gauge. The gauge now works properly and is indicating at 45 (psi???). I also changed the fuel pump relay. TRUCK IS STILL POPPING ECMB FUSE IN REVERSE!!! Its kinda weird and becoming progressively worse. It used to pop like a short, real sudden and with no other indications (only in reverse). Now, when I drive in reverse and purposely give it more gas, the engine starts to cut out, then you can feel the engine stall and then the fuse pops and it pops loud!!! The only things in that ECMB circut are sending unit, fuel pump relay, and fuel pump. I am starting to wonder if it is a gravity issue in reverse and the fuel pump is failing or shorting out for some reason in reverse. I was thinking oil pump failing and the circut is protecting the fuel pump as it should, but when I gassed it in reverse and forward, the gauge stays steady and it appears there is no loss of oil pressure. It just feels now the truck is losing gas pressure to the engine and then pop goes the fuse!! I have traced the wires to the major areas and they appear fine. I am beyond confused now and wondering if its not a grounded out circuit but instead a bad fuel pump. HEEEEEEEEEEEEELP!!!!! I do not want to take this to the shop!!!!

 

Check the wiring were it goes down from the firewall to the frame and then on back to the fuel tank. I have seen them short out at this location. It is more prone to this if the body is sagging on the mounts a bit more than it should.

 

Yeah, i checked those wires from the previous post I had and ya recommended that..there is a point where wire bundle crosses between the frame and body next to a body mount and it is kinda a tight space. But, the wires move freely between and when I shake or put pressure on that point, the motor does not cut out and fuse does not pop. From the firewall/frame transition the wires are straight shot down the frame and are not obstructed. I really am a loss at this point....how safe is it to change out the fuel pump with a full tank of gas?? The P/O was nice enough to cut out a square in the body for an access point to the fuel pump on top of the tank so no need to drop the tank.......

 

*Threads Merged* - Please do not DOUBLE POST.

You have a short in that circuit and you have to trace the wires until you find it.

 

Do you think if I attempted to do a short check on the wires by disconnecting the battery, ECMB fuse, oil sending unit, fuel relay, and fuel pump, then start at the fuel pump wire harness and check with a multimeter for ohms reading on the wires that I could possibly pick up a short or a dead short? Would I have to disconnect the ground to the fuel pump relay? Infinite reading would mean circut is open and not shorted?? a zero or high reading means there is a short????? same test with a self-powered light meter????

 

I would do a continuity test to ground on the main power wires then move the harness around while you listen to the tone from your multimeter.

 

How do you do a continuity test to ground from the main power wires?

 

Most multimeters have a continuity test. With the fuse out and each item on the circuit disconnected. Set your multimeter to the continuity setting and touch the two terminals together to verify that it is set properly. Your multimeter should emit a sound. Then connect one lead to a good ground and probe the power wires. While you are probing the wires, have someone move the harness around.

 

My problem was that the wire going to the sending unit was broken inside the shielding. The wire looked fine but really wasn't. I tested this by disconnecting the battery, then I disconnected the wire from the OP switch and disconnected the two harness connectors at the ECM. I looked for the same color wire coming into the harness connectors (I believe it was orange w/black stripe) and checked continuity from the harness connector to the OP switch wire.