Why Fuel Pump Fuse is Blowing

[vetteoz]

I was using terminal A as the ground, with the positive on G. Using the probe that way I had resistance .

If you had resistance then you have a short to ground somewhere

but no voltage.

Not going to work , there is no voltage on G
check out the diagram I posted up ; G is a straight connection to pump when key is off with no power on it
terminal is only there for test purposes so you can put 12V on the pump to see if it works

 

[Schrade]

Using the probe that way I had resistance but no voltage.

.............................................

Not going to work , ...

I wouldn't tell him something doesn't work, if he has tools that measure resistance AND voltage ... I don't know how ya' keep up with THAT! And what are the UNITS called?

I don't think even an o-scope will do that???

 

[manic]

It lit. I am actaully using a voltmeter with positive on "g" and com connected to the negative post of the battery. It read 9+ volts. How do I locate the short? Should I just run new wire. I know where it comes in at the fuel pump, but how do I trace it back to the fuse?

I just tried the same test with the fuse removed and it still read 9+ volts. Am I doing this right?

 

[Schrade]

1) Test on the battery first. See if you get 12V (or more).

2) Then test the 'A' socket for grounding capacity - Voltmeter connected to the (+) DIRECTLY from the battery, and the other voltmeter probe into 'A'. Should show about the same voltage as the battery directly.

3) Then put voltmeter probe into 'G', and the other voltmeter probe to battery (-) post. Should show about same voltage 12V+...

Post back each test...

Test 3 is key 'ON', I think... Anybody here?

 

[manic]

1. 12.14v
2. 11.34v From '-' battery to 'A'. I tried from '+' but got no reading.
3. 9.6v From '-' battery to 'G'. Key not turned on. FP disconnected.

 

[Schrade]

1. 12.14v
2. 11.34v From '-' battery to 'A'. I tried from '+' but got no reading.
3. 9.6v From '-' battery to 'G'. Key not turned on. FP disconnected.

'2' is not right. Are you using jumper cables to reach from the battery to the ALDL?

One probe directly from the (+) battery post (or with a jumper cable so you can reach), and the other probe in the 'A' socket, should give you the same voltage as '1' - 12.14V.

If it does not, first check that the jumper connection, to extend the voltmeter to the ALDL is good. DO this by checking directly to the battery again, using the jumper cable (or whatever you're using to reach).

And let's make sure that the 'A' socket is not a 'Switched' ground... ANYBODY???



ed.:
We still have to confirm that 'A' ground is not a 'Switched' ground (I've never heard of that tho').
But here's a test schematic from 'Understanding electronics 101':

 

[manic]

I'll try again. Maybe my connection was bad. My voltmeter can reach from the ALDL to the battery. I won't be able to test till this evening.

 

[vetteoz]

. My voltmeter can reach from the ALDL to the battery. .

You are looking for a SHORT
Put meter on OHMS scale
One probe on G , the other to ( working ) ground .
If you get ANY reading you have a path to ground on that wire ( a short ) if tank connector unplugged

FWIW.
You only use the Volt scale when looking for availability of power.
You already know you have power on the pump circuit because pump runs OK until fuse blows

 

[Schrade]

Maybe test for voltage first?

Regardless of being analog or digital, an ohmmeter should never be connected to an electrical unit that has its own source of voltage. For one thing, the instrument is designed to measure resistance based on the flow of current produced by its own battery. Any interference from another current source will undermine its function and produce false readings. In addition, if the secondary source of voltage is high enough, it may irreversibly damage the ohmmeter.

Never use an ohmmeter to check resistance when battery voltage is present in the circuit. Eliminate the power source by disconnecting the battery or removing the fuse so there is no voltage in the circuit. You may damage the ohmmeter by connecting it to live voltage.

It is possible to damage some sensitive circuits when taking an ohmmeter reading because there is a voltage from the internal battery applied to the Unit Under Test (UUT).

Be certain not to place the ohmmeter leads across an energized circuit

A common error when operating a multimeter is to set the meter to measure resistance or current and then connect it directly to a low-impedance voltage source; meters without protection are quickly destroyed by such errors.

I think 'ohms' is just for testing continuity, not for voltage.

 

[vetteoz]

I think 'ohms' is just for testing continuity,

EXACTLY
He is chasing a possibly short so he is looking for a unwanted connection (continuity ) between the pump wiring and ground which is causing his fuse to blow
Checking voltage will prove nothing

Maybe test for voltage first?

Look at the circuit diagram.
With key off there is no power supplied to the pump circuit .It is a "dead" circuit.
If pump circuit had power say from the relay or OP switch being in the closed position then the pump would run full time with key off

The whole point of the G terminal is so you can easily put 12V direct to the pump ( key off ) to test pump operation

 

[Schrade]

I'll try again. Maybe my connection was bad. My voltmeter can reach from the ALDL to the battery. I won't be able to test till this evening.

Did you try the VOLTAGE measurement again? Go to step '2' again, and confirm improper VOLTAGE.

We need to FOCUS on those VOLTAGE readings of 11.34V, AND 9.6V. Those circuits have something wrong, to show VOLTAGE like that. Could be the short we're looking for. And this will give you basics of testing too, since you said you weren't sure how.

Do NOT put the multimeter on Ohms. If you do, and the circuit has VOLTAGE, you could toast the multimeter. Google 'damage multimeter', and read to your hearts' content.

 

[manic]

Too late. I already tested the 'ohms'. With the meter on 20k, fuel pump disconnected, and connecting "G" to "A" it read .05. Same reading whether the fuse was in or not.

I also rechecked the 'volts' and confirmed what I has said earlier.

 

[vetteoz]

Do NOT put the multimeter on Ohms. If you do, and the circuit has VOLTAGE, you could toast the multimeter.

:bash
LOOK at the fuel pump circuit diagram .
WHERE can it get power from with KEY OFF ??????

 

[vetteoz]

Same reading whether the fuse was in or not.

I suspect from your 1st results your ground problem is between the relay and the fuse because removing the fuse did not change results
Remove the pump relay and ohm test terminal A to ground with fuse out and see what you get
Fault can't be between G and relay because that circuit is cut off ( dead ) when pump running

Edit
:duh I take that back.
If fuse is blowing , the short must be somewhere down steam of the fuse
If short was up stream ( OP switch or relay wiring ) then it would take out the fusible link on the pump supply cable

 

[Schrade]

Too late. I already tested the 'ohms'. With the meter on 20k, fuel pump disconnected, and connecting "G" to "A" it read .05. Same reading whether the fuse was in or not.

That doesn't mean anything to me (I doubt that it means anything to anybody else too). If it does, I want to hear about it

I also rechecked the 'volts' and confirmed what I has said earlier.

2. 11.34v From '-' battery to 'A'.

You confirmed that you have 11.34 VOLTS, with the VOLTMETER probes - one on the battery (-) post, and the other probe on Socket 'A' ground socket?

That is not how it should be. You're absolutely sure of this?

If so, lets go after that... VOLTMETER one probe on the battery (-) post, and the other probe on the frame next to the battery. Should be 0.0V

Then VOLTMETER probe one on frame, and the other probe on Socket 'A'; again should be 0.0V.

Post back on that.

 

[boomdriver]

I suspect from your 1st results your ground problem is between the relay and the fuse because removing the fuse did not change results
Remove the pump relay and ohm test terminal A to ground with fuse out and see what you get
Fault can't be between G and relay because that circuit is cut off ( dead ) when pump running

I'm going to guess that his car suffers from the exact same disease that all 80's C4's will catch sooner or later....Wire rot in the engine harness behind the driver side cyl head.
This is where many things happen...the grounds for every major system on the car and ALL the engine mngt collect there and drop to their "bolt". This is also where several harness sections are spliced together with little more from the factory than black tape. The splices get corroded and cause all form of trouble,. Think of it as a tree with several branches taped in place...The branches cannot take any stress or they break contact with the tree.....

This is where the FPR pig tail is, where the fuel injectors split off, and where the oil press switch and pressure sender have their short section of the harness...all related to the FPR by ground or the power loop.
I'd be willing to bet that if he were to have someone hold the meter or use aligator clips and he reached in and wiggled that harness section, the meter would react......

Those multiple splices in the harness and the grounds being in the same place make for lots of opportunities for shorts and PP connections and elevated resistance thru the harness. Once in a while the firewall bulkhead connector will suffer from some bad insulation or physical damage that can cause mystery electrical ailments. Look at that gently with a good lite and a pick to seperate the wires one at a time.

 

[Schrade]

The 10amp fp fuse is blowing. I am only able to drive a short distance before it blows. (2x now) what could be the cause? ...

Short distance... Hmmm.

Short distance = warmed up = Closed Loop = circuits energized that aren't energized at Open Loop = blown fuse.

 

[vetteoz]

That doesn't mean anything to me (I doubt that it means anything to anybody else too). If it does, I want to hear about it

Auto electrics 101
An ohmmeter measures the electrical resistance of a circuit. If you have no resistance in a circuit, the ohmmeter will read 0.
If you have an open in a circuit, it will read infinite. An ohmmeter uses its own battery to conduct a resistance test.

When a component is tested, current from the meter battery will flow through the component and the meter will determine the resistance
by how much the voltage drops.

If the component has an open the meter will flash "1.000" or "OL" to show an open or infinite resistance.
A reading of 0 ohms indicates that there is no resistance in the component and it is shorted.



If you get ANY reading other than OL then there is a connection ( continuity ) somewhere ; in this case a circuit to ground from the pump circuit

 

[vetteoz]

Short distance... Hmmm.

Short distance = warmed up = Closed Loop = circuits energized that aren't energized at Open Loop = blown fuse.

CL only means the ECM uses the signal from O2 sensor to adjust fueling , nothing else changes

 

[vetteoz]

I'm going to guess that his car suffers from the exact same disease that all 80's C4's will catch sooner or later....Wire rot in the engine harness behind the driver side cyl head.

I agree but for different reasons
If he has bad insulation on the pump wiring then it could be giving a intermittent short to ground with vibration after driving ?