Space387

So I am having a no start issue with my 1990 L98.

It started and ran fine with out any lights about 4 days ago. This morning I went to start it up and she would turn over but not fire. After a few tries I got it to fire off but it ran a little rough and died out after about 30 seconds. While it was running the SES light was on. At this point I am late for work so I used my Firebird instead. The car sat for about 12 hours before I came back to it and it fired right up no mess no fuss no lights. Shut down and go to restart to make sure I am set and this was just a fluke, now it wont even turn over. The dash lights turn out like they are supposed to leaving the security light on.

One oddity not sure if it is related is that when the car started this evening and was running fine the indicator for the AC control selector was flashing. At this time I am thinking the no start is either vats or the clutch release switch. For good measure I tried both sets of keys I have for the car and both had the same result.

65Z01

Insert a paper clip into ALDL A&B, ignition on and read the code(s) from number of SES flashes.
It will flash "12" three times, each code number three times, followed by "12" three times at end of all codes.

Space387

looking head on with the vertical tab up the A pin is top left or top right?

Space387

Got DTC 46, Vats error, and 56, low fuel pressure voltage.

So here is where we stand. The Vats system was just being a pain and needed to sit for a little bit. Now the engine cranks and sputtered once or twice so fuel seems to be a likely cause. Being I have recently replace the fuel pump and it is unlikely ( but not impossible to be the issue) Where is the fuel pump replay located? I am going to try to test the relay and power to the pump to see if that is the issue before buying a new one.

65Z01

For my '88 the FP relay is on the firewall to the right of the brake master cylinder.
Mine has 2 red, 1orange, 1dk grn/white, and 1 black wires.

Hot Rod Roy

Does your '90 have a FP fuse in the fuse panel? If so, are you seeing 12 v. at both of the small test tabs in the top of the FP fuse for about two sec. when you first turn on the ignition? That would say your FP relay and the fuse are good.

Space387

At this time I have tested Voltage at the input plug, 12-11.8V during prime and 10.8V during crank. New and old pump spin up properly when ran directly from the battery and from through the pump assembly driven from the battery. So as far as I can tell I have voltage to the pump and a working pump that wont turn on when powered from the back of the car. Checked the fuse panel and the 10A fuse is intact. At this point I'm at a loss. It sounds like and amperage issue but I have never known a relay to deliver too low an amperage

Hot Rod Roy

Are you saying that you have 12 v. at the fuel pump input terminal at the top of the fuel tank?
What is the voltage on the other (ground) terminal at the time you have 12 v. on the input terminal?

If a relay contact is closed, and is conducting voltage, the only reason you won't have current flow (amperage) is because of something missing at the load (light, motor, or heater), or a bad ground.

cardo0

A better test would be of fuel pressure. A good fuel pressure gauge is really a must for FI car. And Harbor Frieght does not sell a Good gauge. Connects to little Schrader valve on the fuel rail. You can observe how long it takes to bleed off fuel pressure after shutdown. A good pump and fuel system will hold pressure all night long.

Space387

when testing for voltage I am using the power in and the ground at the plug on top of the tank. When I tested the pump in the tank by bypassing the entire switch system I had ran both 12V+ and Grnd to the plug, Attached the 12V+ first and the pump turns on with the built in ground and with the external ground connected.

I am at the point of trying a new relay for the sake of it but for the life of me can not find an image of or description aside from its under the dash to find the relay. Under the passenger side where I have read many time the 90's have it there are no visible relays.

I have a fuel pressure gauge on the fuel rail. That's how I determined the pump worked when I bypassed the entire electrical system but not when using the key.

Hot Rod Roy

This response does not answer my question. It sounds to me like you have a bad connection at the tank. Either the 12 v. power wire, or the ground wire. If you do my test, and you have 12 v. on the ground wire, the connection of the ground wire at the frame is defective.

Don't just throw money at the problem without knowing why you have the problem!

Space387

Just because I am not seeing the err in my ways at this point. You are asking to test for voltage between the black(ground) and chassis/ battery ground with the pump plugged in during the prime cycle.

If this is the case forgive me for not understanding earlier. I have always been under the impression if I can measure voltage across the 12v in and ground out from a plug it works. I went as far earlier to pull the plug off the pump ( inthe tank) and leave the input plug connected to see 12v at the pump specifically.

I will go test ground and see what results we get

Results:
Across the plug no pump - 12v during prime
12v to chassis ground - 12v during prime
plug ground to chassis ground - 0v during prime, 46ohm resistance
across the plug with pump - .9v during prime
12v to ground with pump - .9v during prime
plug ground to chassis ground - .1v during prime

Both pumps show 0ohm resistance when testing if that matters and both will pump if ran strait from the battery

Hot Rod Roy

There's your problem! The pump can't run with only 0.9 v! What's the voltage at the FP fuse during prime? You've got a loose/corroded/burned connection someplace!

Space387

1v fuse to ground during prime
remove the fuse and probe pole to pole the fuse slot is 12v during prime

C5 Diag

Hello my friend !!
Possible bad fuel pump huh ??...as far as checking voltage it means absolutely NOTHING...ALWAYS ALWAYS do a voltage drop check...don't know if you know how to do this (MANY don't) but here is a video...a video is worth a thousand words !!

Space387

Looking at the schematics and being an amature at best in electrical engineering, I am confused as to how exactly the oil safety switch will prevent the car from powering the fuel pump. I see there are 2 oil pressure switches off the back of the engine and I know I had to replace the single wire as it was bleeding oil through it. I dont remember its purpose to be honest. I know the second one shows it serves 2 different circuits. If it has gone bad can I try "jumping" the C and D pins to bypass it for the purpose of a start and pressure test?

If it matters with the entire system together as it should be if I jump the pump test probe to the battery the pump turns on and I get 45PSI on the rail

I feel like we are back at the point I was this morning determining I need to probe the 2 items left in the circuit, the relay and the oil pressure switch. I have a spare relay I just have no earthly idea where in the bloody hell it is supposed to be. I have read it should be in a block of relays under the passenger side dash but I look and see nothing but wires and a fuse block of 4 fuses.

Space387

I pulled all 3 relays under the passenger side dash and bench tested them to find they all work and carry current. I don't hear it engaging under the dash so now to see why its not going that.

Hot Rod Roy

So, rwobs, we're going to bump heads again? Your video is good, but it's the hard way to solve Space387's problem. Right at 31 sec. in your video, the guy says: "The engine (oops, typo: "pump") must be running for this test". We don't have that condition, here. Please quit throwing rocks at me.

The 12 v. at the input side of the missing fuse during prime proves nothing. We need to have the circuit complete, with the fuse in place, so current is flowing thru the FP, even though the FP isn't running.

Earlier you said:This is a confusing statement. Later, you said you have only 0.9 v at the pump during prime. Which is true? Please explain.

In post #5, 65Z01 asked whether your FP relay might be like his car. Did you look?

Space387

The test for the fuse was done first with the fuse in place than with the multi-meter AS the fuse. Second when I gave 12V and 0.9V under prime the difference as noted was if I had the pup in the circuit. For the relay location no it is not in the engine bay, it is indeed under the passenger side of the dash as I stated in my last post. Turns out the FP relay has been changed once before and after having to guess which relay in teh block of 3 was my pump relay I got it fixed.

For anyone who finds this in the future or sees this question in the future the fuel pump relay on a 1990 corvette is behind the passenger side kick panel. You only drop the lower plastic piece and reach up over what should be your left leg. There is a block of three relays about 1-2 inched up from the remaining ledge, the relay you seek is to the passenger side of the car.

Jumping to a pump cost me about $75 but now I have a spare for my corvette and my firebird. Thanks for your help guys

C5 Diag

Roy,Roy Roy...Now are you one of those "MANY don't" that I had mentioned in my post ???...LOL !!...now if you've ever done a voltage drop check on a fuel pump circuit you KNOW the engine does NOT have to be running to do the test...are you just testing me ???...in the video the gentleman said "pump running"...not engine...you better go check your ears for potatoes ...LOL !!...I also believe 387 said he has a 2 second prime. Have a great weekend !!!