Fuel pump voltage

[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.