covzr2

help, no feul pressure in 1990 L98. I changed out the pump and filter first, big mistake! i'm not getting any juice to the pump or the fuse in the panel. I cannot find the feul pump relay to check it out. Its not on the firewall or under the dash on either side. I'm tempted to c hange out the ecm. Does anyone know where the relay is?

0Paul Ruggeri

Should be under passengerside of dash. 1st one on the left in group of 3.

JOHN89

FYI...Power to fuel pump/fuse goes through oil pressure switch on back of intake manifold on drivers side. Power is cut off if no oil pressure is seen within seconds. Just making sure you are aware of this little feature while troubleshooting.
John

M.J.L.

jfb

The fuel pump fuse gets power from two places, the oil pressure switch which closes at 4 psi and the fuel pump relay contacts. The fuel pump relay coil is energized by the ECM for two seconds when the ign sw is in run or past that to crank. The relay coil will also get power if the ECM see pulses from the distributor. The relay is located on the firewall to the right of the brake booster on a bracket with two relays and it is the rear relay.

66IISS

Sorry to resurect, but I've got a similar issue on my '96 LT4. There's no juice to any of the relays or the pump. I also don't hear the pump click on when I turn the ignition, nor do I get 12 volts to the pump when I turn the ignition.

Anyone know how to test if it's the oil pressure switch? The switch is independent of the gauge, correct?

floridamale

Here is a link to a thread from other day which shows wire diagram
Good luck
http://forums.corvetteforum.com/c4-t...questions.html

94vettelover2

According to diagram,put power(12v) to point A & try cranking engine to see if it starts.If it does not start keep 12v at point A & check for power at fuse.If power is at fuse check for power to pump.If there is power through fuse & no power to pump,there is a bad connection or broken wire between pump feed & fuse.This test is assuming fuel pump relay & relay signals are working properly.Also make sure fuse is good(fuel pump)& switch with another fuse even if fuse "appears" ok.

LD85

This forum has a lot of great guys with tons of knowledge!

Karp

Hi guys, after reading multiple forums and multiple troubleshooting times on the primary fuel pump circuit still not receiving power. I accidentally noticed that one side of my fuse was only 3.4 volts. Upon further inspection I found the fuse holder prongs were actually wider on one side like something was jammed in the prongs at one point. I can only imagine a multimeter lead? That widening was enough to prevent a good connection, preventing the primary two second start circuit. Hopefully this may help someone in the furure.

Cliff Harris

If you want to test the fuel pump circuit, you can force the pump to run by unplugging the oil pressure switch and jumpering the contacts in the connector together.

raypdq

Did you ever find the problem for no voltage at the fuse. I am having the exact same problem on a 1990 base model. Checked fuse with meter, good, replace relay under glove compartment with new one. Even swapped out all of the relays with new one just in case I didn't have the right one. Still no voltage at fuse. Red a thread about circuit for fuse running oil pressure switch. My sws only has 1 pin and plug only has one wire, but there is a sws with three pins and I am sure that is the model that the circuit to the fuse runs through, I am changing the sws any way to to eliminate any possibility. First thing I did was replace the fuel pump even though it was only a 1 yr old because i ran ran some injector cleaner threw it about a month ago and I thought I trashed it. The reason I thought that is because it was not starting cold as quickly as it had been when I first replaced it. I am trying to find the wiring diagram on line to make sure there isnt some where in the system that might of failed. If you have the solution for the problem I would really appreciate the info. Thanks Ray

raypdq

This is the first time I have used this system, please check your original message for my inquiry. Thanks Ray

Karp

Aloha Ray,

My issue was someone had widened the two prongs that holds the fuel pump relay fuse. When I hit a bump or something. The connection was lost at the fuse. It looked like someone jammed a probe between the fuse holder clamps and it widened up enough to make me check the entire fuel circuit wiring from battery to pump. It was a bear to find. Hope you get your vette on the road soon. My is down for a T56 upgrade 🤗
Good luck with your issue and let us know what it was

Karp

Aloha Ray,

My issue was someone had widened the two prongs that holds the fuel pump relay fuse. When I hit a bump or something. The connection was lost at the fuse. It looked like someone jammed a probe between the fuse holder clamps and it widened up enough to make me check the entire fuel circuit wiring from battery to pump. It was a bear to find. Hope you get your vette on the road soon. My is down for a T56 upgrade 🤗
Good luck with your issue and let us know what it was
V/r,
Karp

playsdixie

On my 85, if you apply 12v to pin G on the aldl...it directly turns the fuel pump on...if the fuse is ok....I too had a problems with the pins in the fuel pump panel...too wide for a good connection.....also the bulkhead connector by the fuel pump in the rear can be a problem....

raypdq

Problem is when driving car shut down like I turned the key off not sputtering or anything else just stopped. When attempted restart engine turned over fine but no attempt to fire. Could here that fuel pump when turned key on Towed home
John I have a 1990 base coup I have replaced fuse, and relay under passenger side dash, I have replaced fuel pump I have 2 second initial pulse, confirmed with meter and heard when key is turned on. My oil sending switch only has 1 pin and plug only has 1 pin, replaced it any way. To the side I also a have another, what appears to be a sending unit screw into the same channel very hard to eye ball but it has mutable wires going into plug, looks like 3 wires, this is very hard to get to to remove the plug and the unit. If the plug has a clip it is on the bottom side and can't see, I will have to get all full creative to release the clip and eremove the plug to jump it to test. I just want to confirm that that is the right switch you are referring to and if have any magic instructions on how to remove plug and sensor . FYI when I went to parts store they showed another sensor with 3pins for my car which was about an inch longer and I think that is probably is the one I am looking now. Any clarification or validation of this would be greatly appreciated. Thank you for your attention to my delemma. Raypdq

raypdq

Hey thank for your reply, corvette forum site problems and computer hick ups won't go into detail. Where I am at is I have replaced, fuel pump, relay, fuse, oil pressure sending unit(single wire to plug). I have the initial 3 second pulse to pump when key is turn on (verified with meter and by hearing) but when cranking no signal. There seems to be conflicting information about the circuit for the fuel pump relay going through a oil pressure switch which does not close until the oil pressure is at a safe level when cranking, which only has seconds to achieve. Some people say this is a urban meth others subscribe to the fact. I replaced the oil pressure switch even though it had only one wire going to it, logically how is that going to make a sws or complete a circuit. Under and to the side installed in the same channel as the pressure switch is another similar but longer pressure switch with three wires going to the sws plug. You have no Idea how difficult and how bad it was to see let alone to get to. Buried under vacuum lines, soft and hard, wiring and plugs you name it. One of the post I read, cant find it again, mentioned removing the plug and jumping the pins to complete the circuit to the relay, just got the plug off last night, clip for plug was on the back side had to improvise a tool to get at it. It would nice to know for sure which 2 pins in the plug to jump but I think eventually I will will safely find out. I am hoping that this is the oil pressure switch that they are referring to and it is not a urban meth. This is about all I can do with out a wiring schematic, by the way you don't know where I can find on do you. FYI my car died like turning the key off no coughing spiting or anything just stop running, had to get towed home. If you have any additional info or know where I can find any please advise. Mean while I am going to jump that plug and I 'll let you know if its a urban meth! Raypdq

playsdixie

I had the exact same problem...lots of searching...easy fix once I found it......car would just shut off....

playsdixie

I don't think your problem is with the oil sending unit....your car should pressurize to 36# for 2 sec when you turn the key on...you say you have verified this by meter and "hearing"...are you saying you hear the pump turn on at the gas tank? or the "click" of the relay?..
I don't know all you've done so far, but I think you should go back to basics....get a fsm 1st of all, it'll have all the diagrams you need. Secondly, for issues with the fuel pump, you should have a gauge taped on your windshield from the schrader valve...
Next, my question would be, did it work before you changed the fuel pump if that's what you did 1st....I'd recheck that wiring at the tank...I had a similar problem and there is a recent thread about it and these guys posted tons of wiring diagrams involving your problem....mine turned out to be dual....bad connection at the tank and not a good connection at the fuse panel....If I was good at navigation, I'd find that thread for you....good luck...