Fuel pump voltage

[voucht]

Just before I started fitted the bodywork I tried the engine on and it was working.
Now that I want to start it up again, it doesn't
Everything else works (starter, etc.) but the fuel pump doesn't run. Normally, I can here it turning a few seconds when I turn the ignition key on, to pre-load the engine, then it cuts until I turn the key to startup the started motor. Now, I hear the the relay "click", so it is working, but I can't hear the pump running.
I checked the connection with the ohmmeter, and everything is OK from the fuse to the fuel pump connector on the tank closure plate (in-tank original pump from ford Sierra 2.0i DOHC)
I checked the voltage, during the pre-load, ad after, when the started engine is running (the fuel pump is suppose to run then) and everywhere I take it, I only have only 0.14 volts, whether I take it at the fuse, the relay, or the fuel pump connector itself. Of course 0 volts when the pump is not supposed to run (ignition off and after the few seconds pre-load, before switching on the started motor)

Is it normal that the voltage at the fuel pump is so low? Can anyone help me identifying the problem ?

FYI:
- battery is fully charged,
- I removed the inertia switch and connected the wires together,
- fuse and relay are OK, I have tested the circuit with another relay of the same sort, and it is the same.

Thank you very much in advance for your help.

[Enoch]

Just work backwards from the pump to the power supply and you will find your problem. It sounds like a bad connection, the load of the pump is pulling the supply voltage down. This happens because the bad connection is effectively a resistance. The Ecu seems to be switching the relay as you can hear a click from it so look at the 12 Volts going in to the relay. You need to check it when the pump is trying to run, otherwise there may appear to be 12V there but it will kind of disappear as the pump demands current. Check at the pump terminal, at the relay output terminal, relay input terminal, fuse in and fuse out. I would bet you will find at one of those points there will be a bad connection. The fact that you see some voltage suggests the problem is not an open circuit.
Hope that helps,
Dave.

[Enoch]

Another trick is to take a long wire from the +12 of your battery and connect it direct to the pump - does it run? If so try it at the output pin of the relay, does it run now? At any point after the relay the pump should run at any time. If you use this trick before the relay then obviously the relay needs to be energised for it to work.
Hope that makes sense, am doing this while cooking dinner

[Enoch]

Oh, and be real carefull not to short your live lead down to anything, to be on the safe side make up a long lead with a fuse in it somewhere.

[voucht]

Thank you very much for the replies on Saturday.
Still haven't figured out what the problem is, but thanks to you advices, I could check that my fuel pump works, and it does.
By working backward, I could also get closer to the cause of the problem. But still don't know what to do about it.

Originally Posted by Enoch

Hope that makes sense, am doing this while cooking dinner

Yes it does, and I hope you dinner was good


I've made a sketch to illustrate the problem:


FuelPumpProblemSchematic par Voucht71, sur Flickr

The fuel pump (1) feed wire comes, through its fuse (2), from the engine management relay (3). While testing the connection, I discover that the tension at the engine management relay main feed was only +10v, where is should be +12v. To check if it is the reason it does not switch on the fuel pump, I connected the port directly to a +12v feed. And the fuel pump started to run !

So it is because I actually have only 10v coming to the engine management relay that the relay does not switch on (I have tried with spare relays, and it is the same), and so does not power the fuel pump.

Then I tracked the main feed up to the source. Everywhere I take the tension with my voltmeter, I have only 10v. But, at the level of the connector “5”, it is very weird, because I got 12v when this connector is unconnected, but as soon as I connect it, I just have 10v. So there something somewhere that is “taking” 2 volts, but I don't know what. I uncovered the wires coming from this wire of the connector, and 3 elements are connected to this 12v feed:
- the engine management relay (where my problem is)
- the port #1 of the ECU
- a relay that I can't identify, but is working, as well as the connected fuse (all yellow, originally attached to the ECU frame, not with the other relays and fuses in the fusebox)
Nothing in between, no bad contact, nothing. Just this.

Does anyone has any idea of what the problem is?

I'm very tempted to connect a 12v cable from another 12v feed to the engine management relay (3), because when I do it, everything works : the fuel pump runs, the engine start up instantly, and I let it run a couple of minutes, push the throttle pedal, and everything works perfectly.

Would there be a major problem if I did that?

Thanks.

[Enoch]

Hi Sylvain, looks very much like the wiring from the ignition switch to the connector at 5 is too thin and you are getting a voltage drop as a result. It's very strange that your pump stops working at 10 Volts, I think maybe that it has some electronics built in to it as normally a pump would just run a bit slower at reduced voltage. Anyway, there is nothing to stop you running an extra wire as long as the pump remains under ecu control. You do not show an inertia switch in your diagram - are you running injection? It is vital that you have some device to turn off the pump if you have a crash, otherwise you could end up pumping fuel in to a burning engine bay. Anyway, putting an extra, thicker wire from the ignition key to the right hand side of connector 5 will do no harm and should fix your problem.
Best of luck, let us know how you get on.
Jaevlar Huite

[Numplumb]

Have you got a flat battery because the would give you 12v but drop when under load from the relay

[Enoch]

Unlikely to be a flat or weak battery as it works when a wire is taken direct to the pump. Could be a contributory factor though, although Sylvain says it cranks and starts ok when the bypass wire is used.

[voucht]

Thank you very much for the replies. I've been very busy this week and haven't spent a lot of time in the garage. But now I can come back to you.

Originally Posted by Numplumb

Have you got a flat battery because the would give you 12v but drop when under load from the relay

Thanks for the input. I knew my problem could look like the battery is flat, and that is reason why I specifically wrote in the first post that the battery is fully charged, together with other "obvious" thing I had checked in the first place. But thanks!

Originally Posted by Enoch

Hi Sylvain, looks very much like the wiring from the ignition switch to the connector at 5 is too thin and you are getting a voltage drop as a result. It's very strange that your pump stops working at 10 Volts, I think maybe that it has some electronics built in to it as normally a pump would just run a bit slower at reduced voltage.

Actually, the 10v-12v problem is not on the fuel pump, it is on the relay which controls the fuel pump (engine management relay). I've been really going backward on this feed wire, until I couldn't understand anything. Hours of undoing what I had done, really pissed me off. I could never really find out what the problem was exactly.

Originally Posted by Enoch

Anyway, putting an extra, thicker wire from the ignition key to the right hand side of connector 5 will do no harm and should fix your problem.

So, thanks to this answer, I finally gave up and put an extra wire from a +12v source on this relay feed cable, and all the other connections to the ECU etc. haven't been altered. The engine now starts up and works perfectly

Originally Posted by Enoch

You do not show an inertia switch in your diagram - are you running injection? It is vital that you have some device to turn off the pump if you have a crash, otherwise you could end up pumping fuel in to a burning engine bay.

I do run injection, and you are right, the inertia switch is not on the schematics because I removed it. I always thought that if the roadster flips over, in spite of the roll bar, my head will have more to worry about than fire . But I got your point, you made me think, and I put the inertia switch back on line, and works perfectly. Thanks!

I have been putting all the cables back together, and I'm almost back to the point I was 10 days ago. Finally!
Will finish to trim and fit the bodywork now (minor adjustment so it should be quick), and will install the lightning I should receive from CBS next week. After that I'll have to fabricate the front wing stays, then I will be close to call the inspector for the final inspection... and by the time it is done and the car is registered, it is going to be winter time great !

Thank you very much again for the help.

[Enoch]

Well at least you are making progress and you have a beautiful country in which to drive your car when it is done. About 20 years back I had to take a piece of telecommunication equipment from Solentuna (can't remember how to spell it) near Stockholm to Boras. I asked the company I was working for to hire a vehicle and they got me this crappy little Rascal type van that would only do 50MPH with the foot to the floor. I got lots of time to see the scenery, which was outstanding. So you have lots to look forward to and I know that Sweden can be a wonderful place in the summer.
All the best,
Dave