My car is a t a local shop and an interesting problem showed up. Fuel pump went out and they replaced with new one... With the key on and motor off, the fuel pump does not run. Put power to it and the car fires and runs fine. As soon as you pull the outside power source, the car dies... He is having trouble locating the relay. Anyone have some pictures or an exact location of the relay or any advice? (1996 LT4). We were also told to replace oil pressure switch but, that doesn't explain why the pump does not run or prime with key on and motor off...

Thanks for the help!
Marco@cuabsi.com
303-905-0238

 

Look at the 10 amp fuel pump fuse in the fuse block above the passenger's right knee (the panel you access with the passenger door open).

Since both the fuel pump relay and the oil pressure sensing sending switch fail to power the fuel pump I would start with that fuse since it is "down stream" from both.

I suspect you will find damaged wiring that creates a short to ground after that fuse. But only you two on scene can make that call.

ref: FSM 6E2-A-36 chart A

 

Start with power source... fuses, then move to any fusable links under the battery. You should get the 2 second prime at key ON. The Oil press switch is highly unlikely to cause the problem. Test the plug for power. It can be jumped.

Relay is somewhere on or around the brake booster, or wheel skirt/well. test for power there. Bad pumps cause fuses to blow. Easy enough. I hope they are using a FSM as a guide to trouble shoot this.

 

Thanks for the reply... The fuse is good... Mechanic is checking wiring. Bizarre... The car was running fine when it got there and the first day it was there. It got low on fuel, they put some in it and bam! No warning or anything. They replaced the fuel pump right away and the problems began from there...

 

I would then put a test light on that fuse and watch it when the key is switched to "run" from "off". You should see the light come on for two seconds.

 

Thank you "Leesvet". The mechanic at the shop is watching this thread.

 

I really appreciate the help guys! Hopefully this get us to the bottom of this...

 

Its often very simple...its challenging during the "process".

Satrt with the basics of what this fuel system does and why...

Key ON...2 second prime.

Engine cranking.....pump OFF until engine fires and runs on its own. Signal comes from dist to ECM.

Power source for pump/relay/switches...fusable links as well as fused circuit.

The oil pressure SWITCH is the smaller one with 2 wires. The oil sender is the bulb or larger of the 2. The switch and sender are next to each other on a "T" backside of left cyl head. Under the wiper motor where you cant get to it...

IF there is no power to/from fuel relay, longer cranking will generate 4 psi oil press to operate the oil press switch...that will complete the circuit to the pump. SO, IF there is power at the switch plug, then there is (should be)power to the pump. Checking the pump plug-in with test lite as eng is cranked may show the 2 second pump run.

Install the fuel test gauge on the rails.
Crank engine.; IF the gauge shows pressure, 40 psi, that means the pump has run. Assuming there IS pressure, during cranking you should see the injectors pulse by watching the needle on the gauge flicker with each inj pulse. ONLY when the pump runs with the OK of the ECM ( from the dist) will that 40 psi be maintained and the engine will run. No PSI...no engine. It takes 20psi to make it burp.

ok, if there is NO pressure generated (no pump at all) at anytime, then you need to start thinking about some damaged wiring in the harness. THAT is more likely than VATS issues. The harness in C4s rots too soon and this causes all sort of elec issues. The insulation rots off, cross signaling starts, grounding or broken paths...$10,000 cars get sold for $500 over $7 worth of 16 gauge wire and some solder. The VATS issue is highly UNLIKELY since you have demonstrated that the system works by jumping power to the pump. If it were VATS related..the injectors would not cycle at all. VATS cuts the inj out, not the pump.

Use a NOID lite to see if the inj are operating during cranking or intermittently. That will show the pulse. The INJ are GROUND path completed to operate..NOT power complete. The grounding is done in the ECM. Everything under the hood is operated and managed by GROUND PATH completion. Everything is always HOT with power to everything all the time...some only at key on, others even key off. Its the grounds that matter.

Check & clean harness grounds on the block over the oil filter. 5 wires on one bolt. More important than the neg battery cable...

Jumping the ALCL in the cabin is forcing the ECM to take all the grounds so everything wants to run. Fans...pump..etc. As if you were reading the flash codes.
Get a copy of the FSM and get in the 2nd book thats 100% electrical. The system is very easy to understand the how and why when you have those drawings. I won't even look at a C4 repair without the FSM to consult. The car is too complicated. Even though it is 25 yrs old it still baffles most mechanics that have not worked on one before. Its complicated. Unique.

 

The fuel pump relay is located in dash under the passenger side knee bolster. I had checked mine once, using diagrams my brother found on his GM computer at work. Unfortunately, I have since lost the diagram so I couldn't tell you for sure which one is the relay.

Now, the purpose of the oil pressure switch, is to complete the circut to the fuel pump at 4 psi oil pressure. This is to operate the fuel pump if the relay goes bad. It also turns the pump off if oil pressure drops below that if I recall correctly.

Would seem a long shot that both the relay and the oil pressure switch go out at the same time. Probably neither one. If it runs fine hooked up to outside power source, then lack of power is your dilemma. My first guess is the harness at the pump connections?

 

[1] Yes.
[2] No. (Common misconception)

 

That wives's tale refuses to die!!!

 

Thank you again for all the help! You guys are great! They found it. Broken wire in the back of the car... All is good. He said it broke where it made a bend... The car is 16 years old so, I guess some of that stuff may happen...

Thank you again!!!

 

You're too easy, I would ask them why they replaced the pump when it was a wiring problem.

 

You probably bought a pump you didn't need unless they broke the wire during the swap!