OK- HELP !!! What should voltage to Fuel Pump Be ?

Yes, I did a search.... 'fuel pump voltage' could not find this answer. My '93 had the in-tank fuel pump replaced June-2002 (by me). Now the pump does not run. The part in there now is a NAPA Balkamp (Carter) and it has run flawlessly for nearly 4 years. All of a sudden, with key on there is no tell-tale sound of the pump running. Obviously, it will not crank or run. I checked the voltage on the three wire input and it has (with center pin positive and two outer pins negative) +6.6 v at the two pairs outside-center and outside-center. (Hope I am making this clear) Is this the right voltage? Is it split between two pairs? I do not have a wiring diagram or voltage list. Have never had to deal with this before..... Please help if you know...... TIA Bill-93XJ

DANG !! Is everybody mad at me or something? 35 minutes and no reads or replies..... well in the meantime I checked out some other posts and a couple mentioned the ballast resistor. Mine appears to be good.... 1.3 OHMS is that a good number....?? Is anybody out there...??? LOL.... TIA Bill-93XJ

WOW this is weird..... ZERO reads in 45 minutes....??? Too tough a question?

Thought it was 12?

Rev

Yeah, Rev me too. That is why I posted..... Two 6 volt splits equals 12..? Do they add to get to 12 ? I am stumped......... Bill-93XJ

Some guys on here have been through at least 3 pumps. im on my 2nd

with the pump disconnected you should get a battery reading(12v)
spray starter fluid in you TB, its probably done.

I don't know off the top of my head how much voltage the pump receives but I assume its less than 12 because the ballast resistor cuts it back some to avoid excess fuel pump noise(jeep had complaints from customers concerning a loud humming noise so they added the ballast resistor)

of course I've been wrong before.....

Hey, that is the mystery.... Pump disconnected. Key on. I read 6.6V from each outside wire to the inside wire. The polarity looks like the center is positive and the two outer wires are negative. I do not see 12 at that point. How can even a good pump run on the 6.6 volts?

i could be wrong too. bet its a shot pump though. dig up some fuel pump threads, heard of them going 10 years to 1 month.

If the fuel pump is bad it should still crank. You might want to turn your attention to the NSS. If it cranks and does not start then it may be the fuel pump, but a bad pump will not stop the engine from cranking.

The starter cranks the engine. It turns over. The fuel pump does NOT run. The voltage reads 6.6 volts from each outer wire to the center wire. Where is the 12V ..?? Do they add together somehow?

If you cannot read 12vdc on your DMM, then you ain't got 12v.

You could try a jumper from the fuel pump power wires to the battery, see if it runs.

Rev

If the 93 is like most other XJ's when you first turn the key on it primes the system (runs the pump) for a few seconds, the pump relay closes and after a few seconds opens again. Then when the key is turned to start, the ballast resistor is bypassed and gives whatever battery voltage (current) that is available, with the starter motor turning (it will suck the battery down some).
The ballast resistor works almost like regulator, but actually cuts back a percentage of input voltage (actually watts, volts X amperes = watts). Typically ballast resistors are rated in percentage. A 15% ballast resistor with a 12 volt input. will have a 1.8 volt drop and be +/- 10.2 output (but like I mentioned voltage isn't a true indicator). Most times when with the motor running, the resistor has around a 9-10 volt output, it is variable. The ballast resistor heats up and cools some and this it what controls how much current it will pass. Primitive but effective and fairly reliable, but not real exact.
You almost have to have the motor running to do a voltage test at the pump. Or jumper the relay. What you are measuring could be standing voltage from a poor ground someplace and not the true pump input voltage.
I may be off base here, but likely close to the mark.
Unless you have jumped the relay, the motor is running or are turning the starter over, you are likely not reading true input voltage at the pump.

Bill-93XJ said:

The starter cranks the engine. It turns over. The fuel pump does NOT run. The voltage reads 6.6 volts from each outer wire to the center wire. Where is the 12V ..?? Do they add together somehow?

Click to expand...

No, one lead is likely the gage current and the other pump current.
Jump the ballast resistor and/or run a hot wire over to the ballast resistor. In effect run battery voltage to the pump circuit and then check the pump plug for juice, to chassis ground and then to the black ground wire from the pump (don't trust the ground wire).
I had one 87 that cooked a pump harness connector where it runs behind the drivers side rear cargo panel. It was a head scratcher, trying to figure out where the voltage went. I once tore the pump ground wire out by the roots, with a tree branch.
It could be the pump, it may be something in the wiring.

I've seen these XJ pumps ground/short out to the bracket due to poor install. The new pumps do not fit the old style brackets well at all. I've had to modify the last couple that I've done to keep the pump sitting in the top bracket properly.

it'll take two people to check, but if you bypass the resistor and have someone crank the starter, you should be receiving 12v to the pump input wire. (I think the resistor is bypassed in crank mode, but since I'm not sure, I'd remove it from the circuit just-in-case)

also check that the wiring from the resistor is sound by measuring voltage on the output side of the resistor and looking for the same at the pump in run mode.
--Shorty

P.S. measure voltages from source wire to a known ground-- not to another wire that 'I think is a ground'- it'll remove one more variable and give a better reading of what is actually happening-- then investigate if the 'ground' is really a ground.

Check it out....this happened to me just not to long ago and I was about to pull my hair out. The fuel pump harness carries about a 6 volt or less on one of the wires to the pump but all that wire is for is your fuel gauge, that wire will probably be red but it is the orange wire that carries the 12 volts that runs the pump. You might check the fusable link that comes off your starter relay. This is what fixed mine. I could not tell the link was burned in two because it did not burn the insulation of the wire. Now on mine (which is an 88 so I'm just telling you my experience) the starter relay is a silver square box that is on the passenger side and is right by the 4 square relays that line up right beside each other. Counting from the left the third relay is the fuel pump relay, that could be bad but not likely. The fusable link is possibly the culprit. It will be a red wire that comes right off the starter relay center post and hits a fusable link and then turns into an orange wire and that orange wire then runs into a harness and loops around and into the bottom of the fuel pump relay. Now from the fuel pump relay it runs to the fuel pump via the unibody. Take a test light and test that wire right next to the starter relay for continuity with key on and if the light is on then keep checking about every 1/4" to see if you can find where it is burned in two. When I found where mine was burnt I just clipped that part out and put a fusable link back in and presto the problem solved. Keep in mind that there are about 4 fusable links that run off the starter relay and it wouldn't hurt to check them all before and after the fusable link for power.

8Mud said:

No, one lead is likely the gage current and the other pump current.
Jump the ballast resistor and/or run a hot wire over to the ballast resistor. In effect run battery voltage to the pump circuit and then check the pump plug for juice, to chassis ground and then to the black ground wire from the pump (don't trust the ground wire).
I had one 87 that cooked a pump harness connector where it runs behind the drivers side rear cargo panel. It was a head scratcher, trying to figure out where the voltage went. I once tore the pump ground wire out by the roots, with a tree branch.
It could be the pump, it may be something in the wiring.

Click to expand...

8Mud,

Sounds like you have slayed the gremlin I'm chasing, or at least his cousin. I've got a 96 that started with intermittent fuel gauge dropping out. I ignored that at the time and just used the trip odometer. I offer that for background.

Now I'm having trouble with the fuel pump fuse (30A mini under the hood). It started as an intermittent problem. The fuse would blow, I'd pop in another, and it would run again. Now it's worse. It blows immediately and obviously the rig won't run.

I disconnected the fuel pump harness under the truck, and with the key on the fuse blows. I chased the wire into the panel behind the spare tire. I disconnected every plug I could reasonably get to (all but the ones wrapped in the foam bundle back in the corner). Turn the key on, and the fuse still blows. So, the problem is somewhere between the connectors I got disconnected and the ignition switch.

My question is: Which bundle of wires is the fuel pump and gauge? Where to do they route to get to the front of the truck?


Thanks in advance.

Never mind. It starts. Now won't idle.

Chased wires from under back seat to harness under dash (I think stuff behind spare was other stuff, but some does go overhead and forward, some comes from forward.) Decided I wanted to do more research before digging too much under dash.

The 30A fuse that was blowing protects lots of stuff, not just fuel pump.

Swapped relays around (put start relay, known good since it cranked, in fuel relay spot and ASD spot). Still blew fuse with key in run position. Put relays back.

Disconnected both oxy sensors. Still blew fuse with key in run position. Reconnected both oxy sensors (connections might have been iffy before?) Still blew fuse with key in run position.

Turned key to run and shifted to neutral, installed new fuse. Now it starts but won't idle, seems to run OK over 1500 RPM or so. Obviously does not blow fuse. As it warms up it goes slower before dyeing, but still will not come close to idling on it's own. Every time it dies, I restart it without much trouble. Now it will start in park too, but still won't idle.

I hate sparks.

Any ideas? I have every access cover from the driver's rear taillight all the way up the interior driver's side floorboard and the lower part of the dash removed, so I can get to most stuff if somebody can tell me what to look for.

Did I mention that I hate sparks?