December diagnostic challenge! (SOLVED)

[Jeremy airjer W]

Here's one that I helped another tech figure out.

The vehicle is a mid to late 90’s olds 98 with a 3.8 liter. If I remember right it was a vin K 3.8.

Vehicle comes for a crank no start. Found no fuel pressure. We have 1 volt at the pump with everything connected and 12 volts at the vehicle side of the pump connector with the connector unplugged from the pump. There is no corrosion on the power side of the circuit. In fact if we put a load on the power supply at the pump with the connector unplugged we can trace 12 volts all the way back to the fuse, so we know the supply circuit is fine.

So, what's the problems?

I'm going to guess that McGuyver has run into this before.

[Macgyver55]

More than once...

[Eric Wettschreck]

I'll take a stab at this, and remember, this is purely a wild arsed guess.

I've heard on GM's a fella shouldn't just replace a fuel pump, or pump cartridge. He should replace the little wiring harness that goes with it, also. Could this be the issue? Or, somehow, could the pump or pump wiring itself be grounding out?

Again, just a wild guess.

[Whoaru99]

If there is 12 at the connector but only 1 actually at the pump then it would seem a bad connection somewhere between the connector and the pump.

[Jeremy airjer W]

We are making our checks for power at the connector for the fuel pump sending unit.

Here's a hint.

There are two reasons why we would lose voltage. It is NOT added resistance in the power supply circuit.

[Jeremy airjer W]

GM decided it was a good idea to have a splice pack, multiple grounds connected to a grounding bar that connects to a main ground wire, right under the drivers front carpet. The main ground wire had melted at its connection to the ground bar which caused all kinds of things not to work (which wasn't brought to our attention). We snipped the main ground and soldered it back onto the grounding bar. Once the main ground was restored there was no longer any back-feeding of power through the ground and the fuel pump along with several other things one again started working.

[Whoaru99]

Bad grounds cause all sort of weird problems because any circuit sharing that common ground will try to source it via paths it shouldn't.

Normally when the voltage is correct when unloaded but drops when you apply the load it's a high resistance (bad connection) that restricts adequate current flow thus the voltage drops. Bad ground = high resistance in the circuit = voltage drop.

These sorts of things are often best isolated by doing voltage drop tests on the hot wire and the ground wire to isolate the bad connection, moving either up or down the circuit with reference to the source ground point or source voltage point. Voltage drop on the good sections will be minimal. Once you get the multimeter across the bad section you'll see a (relatively) high voltage. The same principle as why measuring in-circuit voltage across the terminals of a good fuse reads essentially zero volts but a blown fuse will show 12V (or system voltage) across it.

[Tom7227]

So using a bigger hammer doesn't solve this one?

[shizzy]

as long as it's grounded properly.

[Whoaru99]

So using a bigger hammer doesn't solve this one?

A multimeter (and schematic/wiring diagram) helps, but it doesn't have to be an expensive meter for non-critical voltage testing.

[Tom7227]

I am a long way past being able to do much with my vehicles short of oil changes and air in the tires. The stuff you guys write about makes me realize why service bays are clean and not cold wet grease covered dungeons any longer. But the problem with this Diagnosis thing is that there seems to be little chance of anyone short of a working professional from being able to even begin to solve it. Maybe something could be used where your reaction when you get the vehicle is "why couldn't the dumb a-- figure it out himself" That may be more educational to the average reader. I'm sure each of you pros have at least a dozen things you see with regularity that would fall into the 'duh' category but yet you see these things time and again. Vehicles have become so complicated that very few even begin to learn the basics.

For example my 33 year old daughter needed new wiper blades. We bought them and then put the car in the garage and showed her how the first one went on. Took her way to long to do the second. While the hood was up I asked her what each of the fluid reservoirs were for and she had no clue. IMO maintaining fluid levels are the responsibility of the vehicle owner not some kid at a Jiffy Lube.

Guess what I am suggesting is that you aim this a bit more towards the regular gal/guy and apply K.I.S.S.

[Jeremy airjer W]

I hear you Tom. I post this stuff to make you think. Everything has a reasonable solution if you are able to understand the puzzle. I love puzzles. Doesn't matter if its a jigsaw, soduku, othello, or plowing (yes plowing is a puzzle since you need to figure out how to move snow from point a to point b in the least number of moves! ). Hopefully the next time somebody takes a voltmeter to a circuit and they see a number they where not expecting the diagnostic challenge pops into there head and they figure out what the next two steps are are move on to those.

Look for the next one soon! It's definitely a palm forehead moment!