VW wiring problem


#1

This is a continuation of “Jetta Fuel Pump”



My pump has been blowing fuses, and I find it will do so even when the connector to the pump is disconnected, making a short circuit highly suspect. I have made the following measurements on the pump connector pins:



brown to red/yellow 18.6 ohms

brown brown/gray 18.2

brown lilac 397

red/yell brown/gray 0.9 (both ground wires, no surprise)

red/yell lilac 379

lilac brown/gray 378



brown and red/yellow are the pump power wires

brown/gray and lilac are the fuel level wires



Any suggestions as to where the problem is?










#2

I took a look at the wiring schematic on the AutoZone website and as per the usual; it’s a joke. Obviously a poor scan from what is apparently an illegible page out of a Chiltons manual. It’s unreadable even with glasses and a magnifying glass. Diagram 9?)
http://www.autozone.com/shopping/repairGuide.htm?pageId=0900c152800b054a

Anyhoo, you might consider checking the wiring and wire ends on the backside of the panel where the pump relay plugs in. If a wire end is getting burnt this can cause a poor connection. A poor connection will then cause high resistance in the circuit and this could blow the fuse.

Since you state the brown and red/yellow are the pump power wires have you used the VOM to test those wires individually to ground rather than each other? If you have a short in one of those it should show up when testing between that wire and ground (pump disconnected of course).


#3

I agree with OK4450. It sounds like you tested the wires end to end and found no trouble there. You need to check the power wire with the meter reference lead tied to chassis ground and then check the power wire to the pump while the pump is disconnected from the circuit.


#4

The wiring diagram, through my public library account, at ARRC (Auto Repair Reference Center) Web link, has a little better wiring diagram. That diagram shows that, after fuse #18 (20A), the power splits to: 1. the fuel pump–red / yellow wire; 2. the oxygen sensor heaters-- red / white wire.
Either set of wires could have a low resistance to ground short…including inside the oxygen sensors.
Your technique for finding a low-resistant ground short seems a bit rusty. The Auto Zone Web site has a poor tutorial for checking electrical circuits.
Those ohm measurements you made, are they of the fuel pump with the wiring harness disconnected? You need to make your checks on the wiring harness from fuse #18 up to the connector at the fuel pump.


#5

The plot thickens. I hadn’t realized that fuse 18 was part of the O2 heater circuit (the label on the fuse box just says “fuel pump”).

This car has been having oxygen sensor problems for while now (See “jetta oxygen sensor” discussion). The wisdom is that it is a wiring problem, but my mechanic has not been able to find it.

If there are mashed wires going to the O2 sensor, can anyone suggest a likely place to look?


#6

More data: I followed the O2 wires back to a connector in the engine compartment and unhooked them, so the sensor is now out of the circuit entirely (fuse still blows)


#7

And yet more resistance data (with fuse removed):

Fuse socket (top plug) to brown on pump connector 19 ohms
Fuse socket (bottom plug) to brown on pump connector 550 ohms

fuse top to red/yellow on pump connector .1 ohms
fuse bottom to red/yellow on pump connector 490 ohms

fuse top to chassis 6 ohms
fuse bottom to chassis 500 ohms

I have a Bentley manual, if it is to be trusted :slight_smile:


#8

It looks like all the wires are shorted together. You probably have a wiring diagram which shows where the other wires go…maybe the fuel level sender, and ___.
Unless you have a short finder, it can be fun finding the exact place in the wiring harness where the wires are shorted together. It’s possible, the wires may be shorted together in the electrical connector by dirt and gunk collected in the connector.
Use a spray cleaner, liberally, on the connector–inside and outside. Retest.
You might find this video of some interest: http://www.monkeysee.com/play/7777-how-to-fix-electrical-shorts-in-your-car


#9

Ok, here’s the latest. I surmised that the short must be in the brown wire bringing power to the pump, or in the wire to the O2 heater somewhere between the fuse panel and the connector I unplugged in the engine compartment. So I ran a new wire, with an inline fuse, from the bottom connector of fuse slot 18 to the pump. After much mucking about, I got the car to run briefly, but only haltingly. It is not running now.

The pump is showing a resistance of about 1.2 ohms-is this okay? Is there some kind of “priming” that is required with a new pump?

Also, when starting the car, I don’t get 12 volts at the pump until the starter is engaged. I was expecting voltage once the ignition was on.


#10

You stated earlier that the short was still happening even with the pump disconnected so I wouldn’t worry about the pump being a problem. Since the O2 sensors are on the same circuit the problem could very will be in that part of the circuit. Perhaps the wires got shorted by touching the exhaust pipes somewhere.

As far as resistance measurements go, the only one that is real important right now is the resistance between the protected side of the fuse (the side of the fuse that ties to the load)and ground. Measuring the resistances of the wire leads between the fuse and each load doesn’t tell us much about the short problem. The short is between the fuse and ground.

One of the last readings you took showed a resistance of 6 ohms to ground. I assume that reading was taken from the fuse position that blows. If so, then it looks like the short has been removed somehow. A short should show a reading of less than 1 ohm. In order for the resistance reading to be valid you will most likely have to turn on the ignition. There are probably relays circuits involved between the fuse and loads so if they aren’t set the readings are not valid.


#11

Right now, I do not have a short at all. Running a new power line to the pump appears to have circumvented the short. The pump doesn’t seem to be running. See Nov 8 post.


#12

I’d look for a place where all of those systems ground. I’m guessing you will find a bad ground, and the current “looks” for a better way to ground, which turns out to be through some other system. Peugeots were notorious for this sort of thing, though I had a related problem even on a BMW 740 (the main ground strap became loose, and the starter circuit sought a path to ground through the engine computer, with predictably dreary results).


#13

Ok - I ran direct 12 volt power and ground to the pump. It whirs, but no fuel comes out. I am returning to the “VW Jetta Fuel Pump” thread in the hopes I have a workaround for the wiring part of this headache. See latest post.


#14

Okay. I reread post 8, lets see if I understand this correctly.

You ran a new wire to the pump and you stated in post 8 that you also disconnected a connector in the engine compartment. Now the short is gone and the pump doesn’t run even with the new power connection. It was also previously stated that the short was still present when the pump was disconnected from the circuit. Do I have that right?

Since the pump motor was proved to not be the problem I don’t see how just running a new power wire to the fuel pump will clear a short problem unless the connector you disconnected in the engine compartment is still disconnected and the shorted wire is on that leg of the circuit. If the pump isn’t running and you have verified that power is getting to it along with a proper ground then the pump motor is bad or jammed. Measure the voltage to the pump directly across the leads to the pump. The ground leg to the pump may be controlled by the ECU and the pump isn’t getting a ground connection. You could also try running power and ground directly to the motor to see what that does.


#15

The fuel pump runs when you hot wire it, you said. Can you be certain that it isn’t running backwards? Can you be sure that enough fuel is in the tank for the fuel pump to pick up? Did you change the fuel filter / strainer (aka “the sock”) when you changed the fuel pump?
It looks like you are at that point where you need a mechanic to look at it and determine what the problem is. We’ve all been there, when we had a problem and the cause, to a fresh set of eyes, was obvious. We, then, marveled at our own lack of vision.


#16

There is plenty of fuel in the tank. I don’t think the pump is spinning backwards because it did run the car at one point. The Bently manual doesn’t mention a “sock” and I didn’t see anything sock-line when I took the old pump out (did the '97 have it?). My immediate problem is that I cannot get the new pump out: the twist/lock fixture on the base isn’t releasing.