Lotus Elan

Switched 12V to Boot for fuel Pump

PostPost by: ElliottN » Fri Nov 20, 2015 11:32 pm

Recently installed an electric fuel pump in my rhd S4 dhc. The loom has the wiring for the interior lights fitted on the pillars of the fhc so we picked up that cable (purple/white) adjacent to/behind the passenger seat and soldered in (and suitably insulated) an extension cable to the boot to power the fuel pump. As it is switched off the ignition it's perfect.
Everything was fine for a few days until the car died on me with what turned out to be a lack of fuel to the carbs. Diagnosis quickly proved that the pump wasn't running. As the cable was showing 12V I suspected the new pump. Jumped the pump off the battery and it roared into life. Rechecked the cable for the pump - multimeter showed 12V but it couldn't power a test lamp.

I'm assuming I have a poor connection somewhere with resultant voltage drop.

What I'm looking to know is where does the purple/white cable terminate up front?

The wiring diagram shows it runs to a connector strip/block?
So where's that? behind the dash on the bulkhead somewhere?
Elliott - 70 S4 dhc
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PostPost by: mbell » Sat Nov 21, 2015 12:03 am

Did you use a relay?

I would have just used the switch live as input to a relay with a fused 12v supply from the battery. I wouldn't have powered it directly from the switch live.
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PostPost by: ElliottN » Sat Nov 21, 2015 12:22 am

No relay.
Good point. I can do that!
Cheers.
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PostPost by: lotusfan » Sat Nov 21, 2015 10:33 am

Elliot

Your connector strip/block is very probably in that wonderfully accessible position adjacent to the wiper motor behind the glove box!!
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PostPost by: billwill » Sat Nov 21, 2015 2:06 pm

You really should make some provision for switching off the fuel pump automatically in the event of a smash.

There are 3 main methods (at least)

1. Intertia switch (an decellereation in excess of about 3G causes the switch to trip off)

2. Volkvagen fuel pump relay. (this is kept engaged by a series of pulses derived from the engine ignition circuit. If the engine stalls the relay disconnects after a short delay)

3. Oil pressure (a circuit can be devised with a relay so that when the oil pressure ceases because the engine has stopped after a short delay the relay opens stopping the fuel pump)
lotus-carbs-f40/electric-fuel-pump-noisy-idle-t20354-15.html#p117904

Further discussion:
lotus-carbs-f40/electric-fuel-pump-noisy-idle-t20354.html
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PostPost by: TroonSprint » Sat Nov 21, 2015 2:48 pm

I have used both 1 and 2, an inertia switch and the VW ignition-pulse relay. There really is no delay with the VW relay. If the igntion pulses stop coming, I think the relay throws the circuit open in less than a second. The inertia switch I used can be turned off by hand by pulling up the plunger. This gives a crude ant-theft provision as the car will only drive for as far as the fuel in the float chambers will take it.
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PostPost by: john.p.clegg » Sat Nov 21, 2015 4:28 pm

I had the inertia type fitted...didn't like "spirited" driving round bends...

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PostPost by: lotusS2guy » Sat Nov 21, 2015 6:59 pm

When my Elan has been left for a while, I prime the carbs by turning on the ignition and listening for the fuel pump to slow down before starting the engine. It would seem that the VW relay would prevent this. Is there a timer associated with the relay that waits a predetermined length of time before requiring pulses from the ignition to keep the relay closed?
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PostPost by: TroonSprint » Sat Nov 21, 2015 7:29 pm

Herb, that is a drawback of the VW relay. I have put in the circuit a momentary action push button which by-passes the relay and allows me to prime the float chambers should I need to. I have found that even if the car isn't used for 3 or 4 weeks, there is still enough good fuel in the carbs to start it without priming. But then I live in cool Scotland. :D

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PostPost by: lotusS2guy » Sat Nov 21, 2015 7:42 pm

D'oh. :oops: Why didn't I think of that. I've found a timer module that will provide power to the pump for a selected time and then shut off, but a push button is so much more reliable! Our fuel here is junk....when it evaporates from the carb, there is a varnish remaining that is difficult to remove. OTOH, it's pleasant 85 degrees F. today and there isn't a cloud in the sky. :D
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PostPost by: stugilmour » Sun Nov 22, 2015 6:03 pm

Elliott, please understand I have a Plus 2, and it may be completely different than your S4 Coupe. Anyway, just going to toss this info in for you to check. Appologies if this just confuses things as folks with a proper S4 Coupe car and diagram will know better.

Purple / White is the wire from the courtesy lamp to the switch. The switches are to ground. Typical locations for the switches are the two doors and some sort of dash switch. They form a daisy chain loop that is connected to all of the interior lamps and all of the switches.

Each interior lamp has a power supply to the other side of the lamp bulb. In the Plus 2 these are Purple, Hot at all times, Fused. Looking at the only S4 diagram I have to hand, it looks like they may be from the window lift switches Red / White, which is supplied by White, Hot in Run & Start, Unfused,

When testing with your voltmeter the Purple / White loop could actually indicate close to 12v potential to ground with all of the doors shut and the courtesy lamps not switched on by whatever dash switch is used. The Purple / White loop is kind of a floating ground with the lights off. However, if any of the courtesy light switches are electrically closed (by either opening a door or turning on the dash switch), purple / white goes to ground. Depending on the total resistance thru the bulb filaments (they are in parallel) to the Purple / White loop there might be a voltage drop if any material current is drawn by either a test lamp or the pump..

So, do check for 12 v supply while opening and closing the doors etc. Not being able to fully light a test lamp sounds consistent with this, but why the pump would work I am not sure. :) Question; do the window lifts work only with the key on? Really reaching here, but if one or more of the interior bulb filaments failed, the resistance from the bulb supply to the Purple / White floating loop would increase, which might account for the pump failing after a bit of use.

On the S4 diagram I do see a White / Purple cable, which is a completely different deal but could easily be confused with the courtesy light Purple / White. The White / Purple runs from the key to one of the fuses, supplying the Purple, Hot at all Times, Fused circuits. However, I would not expect this White / Purple wire to be any where near the back of the car. Probably more coincidence than anything, but White / Purple is actually the colour reserved for electric fuel pump supply under BS-AU7 Colour Codes, but as an electric pump was not stock I guess they could use the standard colour as they chose. :)

http://www.mgb-stuff.org.uk/colourcodes.htm

I think I ran my electric pump with a dedicated Purple, Hot at all times, Fused using it's own fuse without a relay. I only did it this way because I ran out of relay slots during my re-wire. :) Agree it would be preferable to use a relay to lighten the load on the ignition switch, but it should work either way if correctly supplied. In my case there was no ignition switched power at the rear of the car, so I had to add it.

HTH

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