Low Voltage Output

Hello. I have a 1996 Nissan Altima with a Manual Transmission. I have been trying to figure out why the fuel pressure is lower than normal. I have checked all of the stuff you can think of (fuel filter, clogged lines, injector pulse, dirty fuel, fuel pump (mechanic put in a new one), etc.). There are no CEL codes stored in computer. So, I investigated the fuel pump circuit. When turning the ignition switch to the on position, the fuel pump gets primed and normal pressure is present. But when I measured the voltage at the fuel pump it shows 10.89V and battery voltage is 12.40V. So, I'm losing 1.51V somewhere between the battery and the fuel pump. I measure battery voltage at the ignition switch and then at the connection of the switch to the fuse panel. But the voltage coming out of the fuse panel to the fuel pump relay is 11.92V. So, just there as soon as it touches the fuse panel, travels through the fuse, it loses around 0.5V. Then when it reaches the fuel pump it is 10.89V losing roughly another volt. I'm baffled how I can lose 0.5V at the fuse panel. Finally, I tested the voltage drops on both the power and ground circuit. The voltage drop from the positive battery post to the positive lead connected to the fuel pump is 1.36V. The negative side had 0.17V. I have taken it to 3 "mechanics". All they want to do is change parts or rewire. I was hoping to get some suggestions here before I look for a 4th. I know my car is old, but everything works and has a good engine and transmission. Thanks for any help you can provide.

is there peformance issues? hesitations? what about the fuel pressure regulator? does the issues disappear if you run a jumper straight from the battery to the pump do the issues disapear if there are any. have you tried a new fuel pump relay as that can be a place to have a voltage drop

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Thank you for the reply. No performance issues that I have noticed. I stopped driving it around 3 or 4 months ago. I would drive somewhere, the CEL, keeps blinking as it is driven. Then after finishing my business at a store or restaurant, I would get in the car and it would crank but wouldn't start. The fuel pressure was low at 20 psi. So, I finally realized that the pump wasn't receiving the sufficient amount of voltage while troubleshooting at home. New fuel pressure regulator, pump, and relay. The voltage at the relay with the key ON, is 11.92V. So, no voltage drop from the fuse panel to the relay. But was mentioned in the original post, battery voltage (12.42V) is present at the connector from the ignition switch to the fuse panel, but only 11.92V present at the fuel pump fuse.

I haven't tried a jumper wire from the battery to the positive lead of the fuel pump. Would that not fry something? Would that be a permanent solution if it worked?

By the way, I do have the factory service manual. The ground circuit checked fine as did the signal (ECM) circuit. But it is the power circuit that is not checking out. Thanks for any suggestions.

For our info, how low is your fuel pressure compared to spec? I'm not familiar with the fuel system on this Altima... Does this have a return regulator?

Your voltage readings definitely aren't ideal. But, I'm not sure they'd necessarily account for low fuel pressure. Not that it's not worth checking! Just that I'm sometimes amazed how well some electronics work with poor connections. :lol:

Running a jumper like Cheryl suggested would be totally acceptable, as long as you're careful and double check your connections. The danger would be in jumping 12V to the control wire at the fuel pump relay while the relay driver is on. Stick to the load side of the fuel pump circuit, and there's no chance of frying something.

A good place to start would be jumping from the fuel pump relay output, black/red at the relay, to the power wire at the pump. If the voltage drop improves significantly, then you know there's an issue in that wire. If you're concerned about a fuel pressure issue, then this would be the time to watch the gauge and see if the pressure goes into spec with the jumper applied.

Thank you. The fuel pressure has gone as low as 18psi. According to the service manual, it should be around 32psi at idle. When you put the key in the ignition switch and turn it to ON, it primes the fuel rail and reaches the normal pressure it should have. Once you start the car and let it idle, the CEL begins to flash randomly, and flashes a lot more if you accelerate. I then look at the gauge and it is around 30psi. The more you drive it, the lower it gets. I will then park it and turn it off at the grocery store. A few minutes later I come back and it has hard time starting. The pressure is not there. When I use my Power Probe to power the pump, it works just fine. It is a new pump, the resistance is within spec for the pump too, and the ground checks out fine. So, I can install a 12V jumper wire from the battery to the power output of the relay (Black and Red Wire). IF you can see the diagram provided from Cheryl, the blue with black (L/ wires, there are 3. The main one comes from the fuse panel and automatically loses 0.5V as it leaves the fuse panel to D1 connector (I think the problem is at this connector). Then it splices and goes to the relay. The voltage there is still the same (11.90V). So, the problem is not at this Joint Connector-4. The power comes out at the Connector 5 (black and red wire) still at (11.90V). Then it goes back to the D2 connector where it loses again some voltage down to (10.89V) when it reaches the fuel pump. So, at the D1 and D2 connectors something is happening. But there are about 100 wires connected to these 2 connectors. I wouldn't know how to repair just one or more. Both connectors are secured with a 6mm bolt in the middle. I already tightened both bolts to torque specs. The connections are very old (almost 22 years old, in 2 months).

So, what if I connect a jumper wire from the battery to the power wire on the fuel pump bypassing any of the intermediate connectors? Would that fry anything or cause the ECM to act funny? I'm including some diagrams. Thanks again.

Love the diagrams. A jumped hooked up like you drew in the second shot would be just fine. The only difference would be that the pump will run any time the key is on, instead of doing it's prime and only turning the pump on when the PCM gets an RPM signal. The PCM has no way to diagnose a fuel pump circuit issue on this design, so the computer will never know the difference.

...A few minutes later I come back and it has hard time starting. The pressure is not there. When I use my Power Probe to power the pump, it works just fine...

So, you can hit the pump with your Power Probe, and get the pressure up to spec? :huh: If that's true, then you've already done the above jumper test. You could try making the jumper semi-permanent just to confirm your theory.

Thanks. That's what I was thinking. But no damage will come to the PCM by doing this jumper wire? Should the jumper wire be the same gauge as the power wire going to the fuel pump? Some months back I was trying to see if any codes were stored on the PCM. It has a screw that you turn and then read the number of flashes of the CEL, etc. Well, on the last try of that doing I turned the screw clockwise to reset the PCM after reading the codes and the screw kept going. It didn't stop. I don't remember if the problems began since that day or not. But in any case, could the PCM be damaged already? Thanks again for the comments.

I'm not familiar with the switch you mentioned, but I tend to doubt you caused any issues. Or at least this issue, anyway.

The jumper wire should ideally be as big or bigger than the existing fuel pump wire. If you're still concerned about damaging the PCM, you can remove the relay or fuse while the jumper is in place. That'll isolate the PCM from whatever you're doing with the fuel pump.

I'm not sure if the switch you are referring to is the screw (knob) on the PCM that allows the user to retrieve OBDII Codes. That's what I was referring to. You turn it counterclockwise until it stops. Then you can see if any codes are stored in there by how many times the CEL lights up. 5 Quick Blinks followed by 5 Long Blinks means code 55 which stands for no codes, etc. I have used it many times until I finally purchased a Scanner that allows me to see Live Data. The last time I used it, I turned it clockwise to reset the PCM and it didn't stop like what happens when you strip the threads. But it wasn't that, there are no threads, and I didn't apply any force. I know how to use it and I wouldn't damage it on purpose due to a mishap. Anyway, I was asking if that could have caused the damage since the PCM is connected to the fuel pump circuit. I don't know, just shooting in the dark I guess.

IN any case Tyler, wouldn't the jumper wire from the battery be HOT all the time, and thus, the fuel pump would operate even when the car is turned off? Isn't removing the FUSE or RELAY from the PCM make operation of the vehicle impossible? Doesn't the PCM control the fuel pump, injectors, etc. Thanks for the comments.

Not to worry. Wire that feeds fuse #3 (the fuel pump fuse) is only hot in run or start. Therefore, if you wire it like you have drawn, then the pump will only run with the key on.

Also, fuse #3 only feeds the fuel pump relay, so you won't interrupt power to anything else by removing the fuse or relay. Check out this power distribution diagram for more info:

I'm sorry Tyler. I understand your point...kind of..lol. That Fuel Pump circuit is only hot on RUN or ON when the wiring is the way it is right now. But if I "jump" from the battery that is HOT all the time, wouldn't that make that wire (Black/Red) HOT all the time too. That is what I was worried about. I'm not an expert with circuits, but I do understand most concepts. I'm sorry if I don't understand. From the Fuel Pump circuit schematic the PCM is also powered by the Fuel Pump Relay at connector 8. But I would think that is only there for the priming perhaps, or to simply open the circuit. I plan to install a "JUMPER" wire that is fused perhaps with the same fuse value as the circuit "10A" to start with and hope it works. I believe the problem is either at the "fuse panel, which is totally mind-blowing" or at any of the junction connections (M7, E103, M6, and B1) that are very difficult to check. So, you stated that I could remove the fuel pump fuse? I thought you mentioned to remove the PCM relay or fuse and run it like that! My fault if I misunderstood. You never suggested that, right, lol? Thanks again Tyler.

nissan956 wrote: I'm sorry Tyler. I understand your point...kind of..lol.

Sorry man, I'm really not trying to confuse you! :blush:

That Fuel Pump circuit is only hot on RUN or ON when the wiring is the way it is right now. But if I "jump" from the battery that is HOT all the time, wouldn't that make that wire (Black/Red) HOT all the time too. That is what I was worried about.

You're right. If you jump straight from the battery then you'll definitely be hot all the time. I kept going back to that diagram you had drawn up:



Wired this way, getting power from the wire that comes off the ignition switch to the fuse box, then it won't be hot all the time. This lets the ignition switch decide when the pump runs, instead of the fuel pump relay.

From the Fuel Pump circuit schematic the PCM is also powered by the Fuel Pump Relay at connector 8. But I would think that is only there for the priming perhaps, or to simply open the circuit.

Negative, that pin #8 at the PCM isn't a power, just the PCM's control of the fuel pump relay. The PCM will ground that pin for the prime, and once the engine starts turning. That wire is grounded any time the engine is running. This is why I suggested removing the relay as an option - the PCM has no way of determining if the relay is there at all. Or the fuse, for that matter. Some systems do have diagnostics for the fuel pump relay circuit, but this isn't one of them.

So, you stated that I could remove the fuel pump fuse? I thought you mentioned to remove the PCM relay or fuse and run it like that! My fault if I misunderstood. You never suggested that, right, lol? Thanks again Tyler.

If you have a jumper from B+ to the fuel pump power wire, then you can totally run without that fuse or the relay. You can leave them in, too. I only suggested removing those so you could take the PCM out of the circuit completely.

Thanks! That makes a lot more sense to me now. I have been really pondering what to do to fix this once and for all. I have fixed everything on this car since I purchased it January 2000. I have had it for almost 18 years, but it is actually gonna be 22 years old in 2 months. The wiring is old and perhaps not working properly. I am seriously considering installing another fuel pump relay in the trunk close to the pump to power it. I would use the positive lead from the circuit <from original relay> (black and red, after I cut it) as the signal wire to "OPEN" the new relay, send a 10 gauge fuse wire to the battery + from the relay, send power from the relay to the harness side of the wire I cut (black and red), and ground the black wire. What do you think? That would in my mind would always provide battery voltage to the pump.

I finally received my fuel pump relay kit. I am planning to hardwire my fuel pump to get battery voltage since I have determined that something is not allowing the fuel pump to receive battery voltage. I suspect the wiring since the car is 22 years old. Anyhow, I was looking at the kit and the power wire that sends voltage from the relay to the pump is a 12 gauge wire (purple in the diagram). That wire needs to be connected to the (black/red) wire that is on the harness. But that wire is much smaller than the 12 gauge wire sending power. The (black/red) is at most 16 gauge in my opinion. My question here is whether it is safe to connect the 12 gauge wire coming from the relay to the (factory, black/red) 16 gauge wire? I would have to solder it because they are different gauges too. I don't know how I would crimp them together. I would appreciate any comments. Thanks.