Fuel pump won't run with 12v from car,but does from external

[4737 Carlin]

(I've tried posting this on the ClioSport forums but it's dead quiet over there - help me Pistonheads!)

Trying to get my Clio 172 (Mk2 Phase 1) started but the fuel pump won't run.

When I turn the ignition on I can measure 12v for a second or so at the fuel pump plug. But the pump doesn't prime or do anything.

If I bridge the fuel pump relay, like the bottom left hand side relay socket in this pic (not my pic but same thing)

then I can measure a constant 12v at the fuel pump plug - but the fuel pump still doesn't do anything.

However, if I connect an external power supply (from a battery charger) to the pump if runs fine.

How can this be possible? What am I missing here?

I've bought another (second hand) fuel pump and FP hanger assembly but still the same problem.

I'm pulling my hair out with frustration!

Any ideas?

[TheEnd]

Pump ground point?

[4737 Carlin]

TheEnd said:

Pump ground point?

It doesn't have a ground other than the earth/neutral wire. It's sat in a plastic hanger so no other grounding.

[TheEnd]

Well, 12v is 12v, so it should run.

The only way I'd imagine it not to work is a bad connection somewhere that limits the current, and the only way I can think of car wiring limiting current to that degree is either a bad earth connection, or the live is shorting against something.

[aka_kerrly]

I presume Clios run the same kind of way as other 80/90s cars I've worked on, usually when you turn the key to the second position it primes the pump.

Then when you are cranking the ecu requires a confirmation feed from the coil that ensures the pump continues to run once the engine has fired. This means that if your fuel pump relay is fine/you have power when you bridge the terminals and have power to the pump at the rear of the car then the problem may be between the fuse box and the engine/ecu loom.

Have you checked the voltage going to the ECU? a dodgy earth or damaged wire to the ECU could well mean it is receiving some power but not the full 12v> that it requires.

[PositronicRay]

A damaged wire can show 12v but hardly any current. Have you tried with a test lamp instead of a multimeter.

[Mr2Mike]

PositronicRay said:

A damaged wire can show 12v but hardly any current. Have you tried with a test lamp instead of a multimeter.

Exactly this. With the plug disconnected from the pump you won't be drawing any current, so if you measure using a high impedance DMM you'd still be seeing 12v even if there was a very high resistance in the circuit.

Try keeping the plug connected to the pump and probe the terminals from the back of the plug housing if possible.

[E-bmw]

As above, it may have 12v on it, but if there isn't enough current to run the motor, then it won't run.

Rewire it & it should be fine, as you have proved it works.

[4737 Carlin]

Thanks for all the suggestions.

I'll do some further investigating at the weekend.

[Mojocvh]

"When I turn the ignition on I can measure 12v for a second or so at the fuel pump plug"

Immobiliser??

[4737 Carlin]

Mojocvh said:

"When I turn the ignition on I can measure 12v for a second or so at the fuel pump plug"

Immobiliser??

I don't think it's an immobiliser problem as the dash light for that is flashing and going off at the correct times, which apparently it wouldn't if there was a fault (According to my research on the Clio forums). I believe the "12v for a second or so" is to prime the pump before the engine starts.

[imagineifyeswill]

I would suggest that the pump gets its earth from the ECU and the earth is the problem not the live feed. Peugeot 206s suffer a similar problem where the wire from ECU giving the pump its earth corrodes and breaks in the loom, on checking the fuel pump fuse its live leading you to believe that part of the circuit is fine when in reality it should be an earth at the fuse. Try googling it and you,ll probably find its reasonably common and there will be answer as to what the fault is.

[4737 Carlin]

imagineifyeswill said:

I would suggest that the pump gets its earth from the ECU and the earth is the problem not the live feed. Peugeot 206s suffer a similar problem where the wire from ECU giving the pump its earth corrodes and breaks in the loom, on checking the fuel pump fuse its live leading you to believe that part of the circuit is fine when in reality it should be an earth at the fuse. Try googling it and you,ll probably find its reasonably common and there will be answer as to what the fault is.

I had another look at this today and I think I've proved that its the positive wire that's the problem....
I connected jump leads from the battery and trailed them to the inside of the car. Disconnected the fuel pump plug. With one wire going from the positive side of the fuel pump plug to the pump and one wire from the Neg jump lead to the pump; the pump doesn't run.
But the other way around; a wire from the negative side of the fuel pump plug and a wire from the Positive jump lead = the pump runs.

That proves the problem is with the positive wire to the fuel pump plug, correct?

Even though I'm measuring 12.5v at the fuel pump plug when probing its two terminals. Weird.
What does fact that there is 12.5v at the plug, but the pump doesn't run, tell us? Is it that there's not enough Amps?

Any ideas what I can do to progress this?

[PositronicRay]

4737 Carlin said:

imagineifyeswill said:

I would suggest that the pump gets its earth from the ECU and the earth is the problem not the live feed. Peugeot 206s suffer a similar problem where the wire from ECU giving the pump its earth corrodes and breaks in the loom, on checking the fuel pump fuse its live leading you to believe that part of the circuit is fine when in reality it should be an earth at the fuse. Try googling it and you,ll probably find its reasonably common and there will be answer as to what the fault is.

I had another look at this today and I think I've proved that its the positive wire that's the problem....
I connected jump leads from the battery and trailed them to the inside of the car. Disconnected the fuel pump plug. With one wire going from the positive side of the fuel pump plug to the pump and one wire from the Neg jump lead to the pump; the pump doesn't run.
But the other way around; a wire from the negative side of the fuel pump plug and a wire from the Positive jump lead = the pump runs.

That proves the problem is with the positive wire to the fuel pump plug, correct?

Even though I'm measuring 12.5v at the fuel pump plug when probing its two terminals. Weird.
What does fact that there is 12.5v at the plug, but the pump doesn't run, tell us? Is it that there's not enough Amps?

Any ideas what I can do to progress this?

Use a test lamp on the suspect wire, sounds like it won't light. Could be a wire just hanging on by a couple of strands. If you can't find the break just run a new cable.

[E-bmw]

The end, Positronic Ray, Mr2Mike & I have all told you on Friday the issue will almost certainly be a poor connection/dodgy wire not allowing enough current through & the answer (or at least a way of confirming this) is to rewire the pump.

Why you no rishen? (said in mock Chinese accent as per Benny Hill)

You proved yourself before your original post on Friday that it seems to be the wiring, and then again, on Sunday, that it seems to be the positive wire, but still you won't believe your own findings for some reason.

Edited by E-bmw on Monday 19th October 11:09

[Oldred_V8S]

TheEnd said:

Well, 12v is 12v, so it should run.

The only way I'd imagine it not to work is a bad connection somewhere that limits the current, and the only way I can think of car wiring limiting current to that degree is either a bad earth connection, or the live is shorting against something.

This upends many people who say "I have 12v so the power side is OK."

12v only shows there is 12v present, but you could get 12v through a wet finger, (honestly) the presence of 12v does not tell you the cables can support the current required. Stick a 25 ohm resistor between your test lead probe and the outlet and check if there is still 12v present, if yes do the same with the red probe (and the resistor) and the ground lead of the pump supply and the black lead of your DVM on a ground.

[Royce44]

12v also doesnt mean you have "12v" as such. The meter is measuring a potential difference of 12v which is a different kettle of fish.



Quick lesson, for example, a live could be at 230v and the earth at 220v, the meter would show 10v, despite the fact both cables are dangerously live

[B'stard Child]

Royce44 said:

12v also doesnt mean you have "12v" as such. The meter is measuring a potential difference of 12v which is a different kettle of fish.



Quick lesson, for example, a live could be at 230v and the earth at 220v, the meter would show 10v, despite the fact both cables are dangerously live

Oh that's why I hate electrickery...........

[Matt Seabrook]

It's not that bad when you get your head around it

[ADEuk]

Have you tripped a fuel saftey switch?