TurboTommy

Anybody know what "normal" voltage readings should be at the fuel pump, engine running?
I'm guessing current needs to go through a relay. Do relays build up resistance over time, like wires?

MooreBoost

12v

i would replace the relay and if that doesnt fix it i think you got some wiring problems

beentherebaby

There should be very little voltage drop from the battery to the fuel pump under load/operation, perhaps a few tenths of a volt using a DVM.

Relay contacts do wear/pit/oxidize and increase resistance. Fuel pump relays malfunction or completely fail on ocassion as they carry a significant current. They are relatively inexpensive to replace and it's a good practice in my experience to do so after 3-5 years of use on most vehicles.

theedge

Voltage drop is directly related to how much current (amps) you are drawing. Do you have an 044 pump installed? If so, voltage drop is likely over 1 volt at idle.

Edit: Relays shouldnt build up much resistance, unless there is corrosion or a bad connection.

elargentino

Yes - the notorious "DME" relay sends the current through the its second set of points to the fuel pump after the DME has determined that all other systems are deserving of fuel, and this occurs after the closing of the first set of DME relay points.
Try a new relay.
What is the voltage at the pump and what type of pump do you have?

Willard Bridgham 3

Feel the relays with your hand.....if it's hot, it's the source of the voltage drop and needs to be replaced.

TurboTommy

Last time I checked I had a .4 v drop using the Lindsey 'HV' pump. You mentioned "at idle" Would there even be a difference whether at idle or running the car at power (pump is always running full with various amounts of fuel returning to the tank)?

beentherebaby

Voltage drop is due to circuit resistance. (See Ohm's Law if you care). If the resistance in the FP circuit is correct the voltage drop will only be a few tenths of a volt to the fuel pump. A .4 V drop would be normal and proper. A 1 V drop is usually excessive for a FP.

The power supply wiring needs to be matched for the current draw to prevent excessive resistance, voltage drop and wire overheating. Failing electric FP's draw excessive current and run hot.

theedge

The Lindsey HV pump is a lot smaller than the 044, but bigger than stock. Larger fuel pumps will draw more current, which will produce a larger voltage drop in the stock wiring.

Yes, there would be a difference on boost. The voltage drop should be the same from pump running by itself with engine off up to atmospheric pressure (no boost) since it will be running the same fuel pressure over that whole range. But once you get into boost, the fuel pressure will go up and thus the pumps current draw will go up. Once current goes up, voltage drop goes up, which is very hard on the pump since theres more load on it (higher fuel pressure) and less voltage.

On stock wiring with an 044 I was seeing over 1 volt drop at 3 bar fuel pressure. I think it was around 1.5 volts. So I added another relay with 12 or 14 gauge wiring and a ground on the engine block ground point. That eliminated the voltage drop and the pump ran better (I could hear a difference, with the engine off).

IMHO, anyone with an upgraded pump fuel pump (ESPECIALLY an 044) should be running additional wiring to it.

TurboTommy

Are you saying that you replaced the stock wire from the relay to the pump, with a larger gauge wire?

theedge

I augmented it, so there is two circuits. So stock wiring going through the stock relays, and a 14 gauge I think in addition.

TurboTommy

I wonder if we could just take power directly from the alternator, like some of the headlight kits.
What do some of you think?

elargentino

NO!
This is EXTREMELY DANGEROUS!
The DME relay/circuit is there specifically to keep you from becoming cooked in the event of rollover/accident.
The Dme cannot open the power circuit to the fuel pump - it will keep pumping.
This is why the relay supplies power only once all other criteria are met.

If you use the 044, you may run heavier cable to the pump from relay and it will help.
Also check to see that you have a very good ground.
As always, if you are making a new ground on a zinc machine, clean the metal first with a wire wheel to ensure contact.
-Gabriel

shortyboy

Just purchased this fuel pump relay kit https://stmtuned.com/products/stm-fuel-pump-rewire-kit to add to my 044 pump. Any idea how to wire to dme relay? Instructions indicated I run the main power with fuse to battery then to relay. But instructions here who have done it ran the power wire to dme relay. Really confused. Any help is appreciated.

V2Rocket

easiest way to do this is:
1) run a fat new wire from battery back to the fuel pump.
2) mount new relay near fuel pump
3) attach fat new wire to relay terminal 30
4) add new wire from relay terminal 87 to fuel pump +
5) use stock Porsche fuel pump wiring on terminals 85 and 86 of new relay
6) add new ground wire from fuel pump to chassis

this way the DME relay is intact and functional with its safety features built in.
stock fuel pump wiring is just used to trigger the new fuel pump relay in the back. very minimal current flow now.