Not the fuel pump! But...??? Renix 89

Had a return of the no fuel issue on the 89, but once again it was, is, not the fuel pump.

The fuel pump currently is not priming, not getting power during the prime cycle, but does pump in the start mode and then keeps pumping in the run mode once it starts, and then it keeps running.

2 years ago, cleaning the e-connector at the fuel pump got it working again, and that was probably 15,000 miles ago.

This one has me baffled!!!!!!!

The harness (wires) for the fuel pump go through numerous connectors and actually the wires change colors once and back again. I had a connector in front of the left rear wheel well that was black and crispy.
The connector seemed to be acting like a resistor, was generating heat, but only cut back the available voltage to the pump a little (measured witha volt meter). It did cut down on the amps noticeably though.

I got just about the same results once with a bad ballast resistor. It wasn't completely bad but was pretty much melted into a lump on the inside, the back side of the ballast resistor was burnt black. Top speed was 60 MPH after that it ran out of fuel.

IMO, volts aren't everything, the fuel pump seems to do just fine with low voltage (9 volts or so) to a point, but needs sufficient amps to work properly. A larger supply wire IMO could help avoid many problems, the one they have in there with all the connectors (possible trouble spots) is under engineered.

I'd likely start troubleshooting by swapping the fuel pump relay out. I've taken a few apart and the contacts were pretty crispy. The relay block can get pretty corroded on the back side, it is open to the elements in the back and years worth of trash and gumbo die electric grease can make an interesting mess on the back/bottom side of the relay block.

Just some ideas.

I pretty much did all that, and this time, like last time (2 years ago) it had NO effect. I did a pretty good cleaning job last time on the contacts, and the relay is new. I do hear a relay switch in the prime cycle, but not the pump.

I am leaning towards a ground issue, that might get moved, and grounded by vibration-movement of cranking the engine, but the part that bothers me is the fuel pump has far more voltage at the prime stage (bypassing the resistor IIRC?) than it does during the cranking stage when the battery voltage drops to say about 10.x volts, and yet it starts and runs with no prime cycle?

I did not think this was even possible with the Renix fuel system!!!! It also has has less voltage while running with the resistor in line, though I do not know what the normal running voltage after the resistor is, it might 12.7 volts, same as the prime cycle, or a lot lower?

I would think Renix would be wired to by pass the resistor while cranking????

There is a time cycle on the prime, and the ECU it would seem controls that, so perhaps the ECU signal to time-prime is not getting to the relay!!!!!

I still have C-101, so I guess I need to trace the ECU prime signal to the fuel pump relay???? Question is, is that the same signal as the run signal form the ECU, or different wire????

One thing for sure, this is going to be an interesting thread!!!!

I have had fuel pumps that are so quiet that you didn't hear them but they worked anyway. Are you sure there is no fuel pressure? Clamp the return hose, you could have a failed fuel pressure regulator. If the fuel pressure regulator is wide open there is little load on the pump and it will make less noise.

old_man said:

I have had fuel pumps that are so quiet that you didn't hear them but they worked anyway. Are you sure there is no fuel pressure? Clamp the return hose, you could have a failed fuel pressure regulator. If the fuel pressure regulator is wide open there is little load on the pump and it will make less noise.

Click to expand...

I am pretty sure. It is a rather noisy pump to start with (possible the OEM Renix). Always has been noisy, since I bought it 5 years ago with 114,xxx miles on it. Less noisy with a full tank. The FPR is only about 5 months old.

It was making the usual noise one minute, and then the jeep died, no pump noise, would not restart, then the next day I worked on it, and for some reason I tried cranking it anyway and surprise it started, and ran. :clap:
Now that I think about it, I was using a fuel pressure gauge when I started troubleshooting it. That may be why I tried to start it with no prime noise???? I forget.

Now I am trying to figure out why it runs, LOL!!!

I will put the fuel pressure gauge back on it the next time I play with it and run tests.

During cranking, the power for the fuel pump bypasses the fuel pump relay circuit. My thinking was after the power bypasses the fuel pimp relay circuit and spins up the pump during cranking, there may be enough voltage/amperage for the fuel pump to run, but not enough voltage/amps to initially spin up the pump during the prime cycle. Just a thought. Pumps usually take more amps tp start, than they do to run.

You are probably right, the signal circuit from the ECU to prime the system is weak or open. And that may be causing a lazy relay or something similar. You've likely had the ECU our before for circuit testing, the pins do back out of the plug on occasion.

Maybe check the prime signal circuit for resistance, next maybe unplug the pump, jump the fuel pump relay and start checking for line loss back to front or front to back. After my episode of the crispy connector in the fuel pump harness (with no wiring diagram) I always have that in tha back of my mind.

The relay sockets are pretty much junk IMO, but they seem to last, that is the ultimate test.

Mike, 10.2 volts at the output side of the resistor. How many volts at your fuel pump connector?

8Mud said:

During cranking, the power for the fuel pump bypasses the fuel pump relay circuit. My thinking was after the power bypasses the fuel pimp relay circuit and spins up the pump during cranking, there may be enough voltage/amperage for the fuel pump to run, but not enough voltage/amps to initially spin up the pump during the prime cycle. Just a thought. Pumps usually take more amps to start, than they do to run.

You are probably right, the signal circuit from the ECU to prime the system is weak or open. And that may be causing a lazy relay or something similar. You've likely had the ECU our before for circuit testing, the pins do back out of the plug on occasion.

Maybe check the prime signal circuit for resistance, next maybe unplug the pump, jump the fuel pump relay and start checking for line loss back to front or front to back. After my episode of the crispy connector in the fuel pump harness (with no wiring diagram) I always have that in that back of my mind.

The relay sockets are pretty much junk IMO, but they seem to last, that is the ultimate test.

Click to expand...

Interesting, and very ODD, that they would install a fuel pump relay, then by pass it, but that all makes sense the way you listed the order. The part that I found oddest at first was that the run operation has to get through the resistor and the Prime - start circuit does not. Or does the prime also go through the resistor?

I mentioned the C-101 in a post below, but it is wrong, this is the 89, and it has no C-101.

I need to dig out some wiring diagrams next, and see the different paths.

Let me see if I have this right:

Prime goes through the relay only and is timed by the ECU?

Start by passes the relay and the resistor, but has lower voltage due to the starter load?

Run goes through the relay and the resistor, but has about two more volts input from the alternator overvoltage?

Ecomike said:

Interesting, and very ODD, that they would install a fuel pump relay, then by pass it, but that all makes sense the way you listed the order. The part that I found oddest at first was that the run operation has to get through the resistor and the Prime - start circuit does not. Or does the prime also go through the resistor?

I mentioned the C-101 in a post below, but it is wrong, this is the 89, and it has no C-101.

I need to dig out some wiring diagrams next, and see the different paths.

Let me see if I have this right:

Prime goes through the relay only and is timed by the ECU?

Start by passes the relay and the resistor, but has lower voltage due to the starter load?

Run goes through the relay and the resistor, but has about two more volts input from the alternator overvoltage?

Click to expand...

I'm not going to tell you I actually know the path of the prime circuit, I never really had the need to figure that circuit out. I noticed there was a wire spliced into the fuel pump circuit, I never really did figure out it's function. I always guessed it might be where the prime circuit spliced into the fuel circuit. But I don't know for sure.

I honestly don't know if the prime goes through the ballast resistor or not. It doesn't seem logical that it would, but they are going to save wire when they can.

I always assumed the prime was controlled internally in the ECU and just closed the fuel pump relay for a few seconds when the key hit the run position on the way to the start position. I may be wrong, logic says I should be wrong. You'll have to educate me on this one when your done and have the sub system figured out. I think the French engineered the electrical system for the Renix .

I do know the power for the fuel pump when cranking comes from the starter relay and splices into the fuel pump supply circuit after the ballast resistor. My guess was always, that the power for the pump when the motor was cranking. came from a source as close to the battery as possible and bypassed the fuel pump relay and ballast resistor, to help offset the voltage drop during cranking.

The ballast resister cuts back on the voltage/amps to the pump when the motor is running (after the fuel pump relay closes), they say to reduce pump noise. The ballast resistor increases resistance when it gets hot and lowers resistance when it get colder. in effect it is a primitive voltage regulator.

Still not sure what the real story is here. Finally got rid of some back ground noises, and had someone else sit close to the fuel pump as I flipped the ignition switch from off to run. And sure enough the pump IS priming after all.

It's just too damn quite now to hear from the drivers seat!!!

WTF????? A quite Renix fuel pump on prime????

????

Still not sure what the real problem was back when I started this gremlin hunt, so far it is working fine now, has been the last 2-3 weeks of test runs. It as been parked getting front end damage repairs for awhile, and gets started and run twice a week.....