Fuel pump: Voltage present at connectors, no voltage when connected???

[Url]

I am troubleshooting a problem with the new fuel pump which I installed lately. The fuel pump relay has been bypassed and when I check the voltage between the connectors it is measuring +- 10-11V which seems ok.
However, when I connect the connectors to the new fuel pump there is no voltage anymore?!?
Any ideas?

[joeblow]

Tap the pump lightly with something then hook up the power again, seems seized up. What kind of pump is it?

[Url]

OEM Bosch pump.
What has the pump to do with the voltage at the connectors?
I should get a reading anyway, no?

[nesslar]

Originally Posted by joeblow

Tap the pump lightly with something then hook up the power again, seems seized up. What kind of pump is it?

Might not the problem show itself again later anyway? I'd question the pump...sounds like a lemon.

[crustychief]

Across the terminals or to ground from each one? Is this on a 911? what year?

[356RS]

The only way voltage can disappear when connected to the pump would be a very bad ground connection. Just enough current to show 10 - 11 volts on your meter and not enough to run the pumps electric motor. Of course the same could happen on the positive wire to but it's usually the ground.

[Url]

Car is a 1978 SC.

I have checked across the terminals.

1. Ground wire is connected to the pump, + wire is disconnected from the pump -> 11V
2. Both wires connected to the pump -> No voltage...

So the ground connection didn't change...

[john walker's workshop]

crappy electrical connection somewhere. start at the battery. check and clean the posts and clamps and the ground at the body. what's the battery voltage? 13.V?

[boyt911sc]

Originally Posted by Url

I am troubleshooting a problem with the new fuel pump which I installed lately. The fuel pump relay has been bypassed and when I check the voltage between the connectors it is measuring +- 10-11V which seems ok.
However, when I connect the connectors to the new fuel pump there is no voltage anymore?!?
Any ideas?

Url,

Determine when and where the absence of the voltage occured. First confirm the power with the connectors with a test light. Connect electrical connectors to FP terminals. Test separately again for power at the connectors (electrical) and at the FP terminals. Check which of the two (2) wires is causing the problem. It is unlikely that both wires are bad. But who knows until test is completed.

This method will isolate the problem at the electrical connectors and at the terminals of the FP. Wiggle and move the wires to test if the connections are stable. After doing the test light investigation you could continue using a multi-tester if you wished. Most of the time, it is a case of loose or poor contact. HTH.

Tony

[911pcars]

If you had checked for source voltage at the fuel pump output wire at the relay the correct way;

VOM connection to the circuit okay? Doesn't sound like it.

Connect the + wire (red) of the voltmeter to the output connector of the relay. Connect the – wire (black) to a good ground. Voltmeter should indicate source voltage; same as checking at the battery posts.

Switching the fuel pump ON should not affect the source voltage available to the fuel pump.

Can you sketch, scan and send a diagram of the connections?

Sherwood

[T77911S]

use the test light as tony said. the TL provides a load, unlike a MM. a MM can measure voltage even if the current available is low, a TL will show dimm/er (compared to across the battery) if you have a bad connection limiting the current.

you can also measure the resistance from the ground wire at the FP to ground.

check the fuse connection. clean it. remove the wire and screw at the fuse block, clean both. in fact, it does not hurt to remove each wire and screw and clean them, one at a time, with the battery disconnected. any tarnish or corrosion increases resisance which creates a voltage drop.
clean and check all connections, including grounds.

[Url]

Originally Posted by john walker's workshop

crappy electrical connection somewhere. start at the battery. check and clean the posts and clamps and the ground at the body. what's the battery voltage? 13.V?

Wrapup:
Battery voltage is indeed 13.V, voltage at jumpered 87a on the fuel pump relay is the same.
Voltage at the connectors when disconnected is +-11V. When connected to the fuel pump, no voltage anymore.

When I crank, I can hear the starter + see the air sensor plate move. Lifting air sensor plate also breaks ground at port 85 on fuel pump relay.

I have cleaned ground + all connectors so that should be fine as well.

What else could prevent current going to the fuel pump?

[john walker's workshop]

Crappy Electrical Connection. how was the fuel pump relay bypassed and why? is there a hidden inline fuse installed somewhere? like in behind the fuse board? a classic indication of a CEC is when you lose power at a wire when it's connected to something.

[T77911S]

11v?? should be 13v. voltage at pin 30 should be battery, if not, problem is towards the batt, if it is good, problem is towards the FP. measure voltage with respect to bare metal ground, not the other wire.

[911pcars]

Record the numbers next to each connector on the relay. Perhaps the incorrect connection.

Sherwood

[boyt911sc]

Originally Posted by Url

Wrapup:
Battery voltage is indeed 13.V, voltage at jumpered 87a on the fuel pump relay is the same.
Voltage at the connectors when disconnected is +-11V. When connected to the fuel pump, no voltage anymore.

When I crank, I can hear the starter + see the air sensor plate move. Lifting air sensor plate also breaks ground at port 85 on fuel pump relay.

I have cleaned ground + all connectors so that should be fine as well.

What else could prevent current going to the fuel pump?

URL,

This is a 10 sec. job to determine where the anomaly occurs!!!!! A test light will pin point the exact location and place where your problem starts. It is not how hard your work but how you do it. The presence of power (13 volts) is confirmed. So determine where and when this is lost!!!!

Tony

[911pcars]

Originally Posted by Url

Wrapup:
Battery voltage is indeed 13.V, voltage at jumpered 87a on the fuel pump relay is the same.
Voltage at the connectors when disconnected is +-11V. When connected to the fuel pump, no voltage anymore.

When I crank, I can hear the starter + see the air sensor plate move. Lifting air sensor plate also breaks ground at port 85 on fuel pump relay.

I have cleaned ground + all connectors so that should be fine as well.

What else could prevent current going to the fuel pump?

Where is your voltmeter negative test lead connected? It doesn't sound like it's grounded to the chassis.

Provide a sketch if possible.

Sherwood

[boyt911sc]

Originally Posted by Url

Car is a 1978 SC.

I have checked across the terminals.

1. Ground wire is connected to the pump, + wire is disconnected from the pump -> 11V
2. Both wires connected to the pump -> No voltage...

So the ground connection didn't change...

Url,

With both wires connected to the FP have you checked the following:
a). Does the (-) negative wire have good ground contact?
b). Does the (+) positive wire have voltage reading from the wire connector (not FP terminal). Do not use the negative wire (a) for grounding unless it is confirmed grounded!!!! Try using a different ground source.

Unless you get both YES from above tests, FP will not RUN!!!!! You need both power and ground at the same time. When you had the 13 volts reading, what ground source did you use?

Maybe I'm missing something. Reading conditions #1 and #2 tell me they are the same or identical. So what's the difference between the two? Keep us posted.

Tony

[john walker's workshop]

the suspense is killing me.

[boyt911sc]

Originally Posted by john walker's workshop

the suspense is killing me.

John,

I think you don't have anyone in your shop that could make a work like this turned to a box office thriller!!!!The drama continues as the world turn.........

Tony