924Board.org

Maybe924 · Joined: 14 Aug 2007 Posts: 412 Location: New London, CT

Hello all,
While working to solve a rich running condition, misfiring and the unfortunate side effect of a glowing exhaust I came upon a couple of anomalies.

Testing my pressure numbers I found a system pressure of 5.4 bar or about 79psi which is under spec. My warm control pressure was 3.5bar

Also, while checking my voltage to the fuel pumps I found 10 volts at the pump. If I remove the wires from the pump and test voltage I get a solid 12 volts. I assume this is not normal... does this indicate a fuel pump is kaput?

And lastly, if this fuel pump is in fact a goner what could be burning up my pumps??? I've gone through 3 in the past 2 years.
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1981 931 GT (CGT Intercooled)
1985 911 Targa Carrera (Sold)

bnoon · Posted: Tue Nov 17, 2009 8:17 am Post subject:

A poor power and/or ground connection could cause the amperage to spike, causing blown pumps and low voltage at the pump. Without proper voltage to the pump, pressure could be low... I would clean all connections at the fuel pump relay, power source, and ground, or even run new wires if the condition of the wires is bad enough.

What does voltage at the battery read when the car is running with the fuel pumps running/and not? How about at the power terminal for the fuel pump relay running/and not? The power loss is either between the battery and the fuse block, or the fuse block and the pumps.
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'80 924 Turbo - SOLD!
1986 Porsche 944 Turbo - SOLD!
Porscheless

Maybe924 · Joined: 14 Aug 2007 Posts: 412 Location: New London, CT

I've cleaned the grounds and run new wire from the fuse board to the pump. I do get 12 volts on the wires without the pump hooked up. However when I check voltage across the pump terminals when its hooked up its only 10 volts.

The battery is good and the alternator is doing its job showing 13.5 volts running and I get 12 volts at the fuse when the pump is hooked up also.
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1981 931 GT (CGT Intercooled)
1985 911 Targa Carrera (Sold)

Maybe924 · Joined: 14 Aug 2007 Posts: 412 Location: New London, CT

Almost forgot. I also bypassed everything and hooked up directly to the battery and still only get the 10 volts hooked up to the pump, testing across the terminals.
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1981 931 GT (CGT Intercooled)
1985 911 Targa Carrera (Sold)

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

Nevermind. I need to read all of the replies before posting

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erstwhile owner of just about every 924 variant ever made

Maybe924 · Joined: 14 Aug 2007 Posts: 412 Location: New London, CT

I guess I'll try running directly off batt again and ground it straight back to the negative terminal. Although I tested resistance on the fuel pump ground and it check out.

If the fuel pump still drops in voltage then I guess its got to be the pump itself.
_________________
1981 931 GT (CGT Intercooled)
1985 911 Targa Carrera (Sold)

Maybe924 · Joined: 14 Aug 2007 Posts: 412 Location: New London, CT

I'm gonna proceed with this theory in mind.

I have too much resistance somewhere in the circuit from the battery to the fuel pump.

This is burning out my pumps prematurely.

Once burnt the pump is only drawing 10 volts.

So, I'll order a new pump, check how many amps its drawing after install, if its outta spec I'll then locate the path of resistance and replace.
_________________
1981 931 GT (CGT Intercooled)
1985 911 Targa Carrera (Sold)

!tom · Posted: Tue Nov 17, 2009 11:26 am Post subject:

Your theory doesn't make sense to me.

Current, voltage and resistance are related. Increase the resistance in the circuit, the voltage AND amperage go down at the motor.

Your 10 volts at the motor is due to voltage drop between your battery and your motor. There's no other explination. This could be caused by any number of poor connections, including the battery terminals themselves.

Running your pump at 10 volts is NOT burning it out. It will impede the performance of the pump, though.

What size wire are you using for your tests? You can expect almost a half-volt loss through 14 gauge wire from the front to the back of the car.

Again, as Smoothie often points out, devices don't draw volts, just like alternators don't push amps. Given an input voltage, a device draws a particular current. This current causes losses in things such as connections and wiring, which is seen as both heat and a voltage loss on the other side of the component you're looking at. You're seeing 10 volts at the pump at the same time as 13.5 volts at the battery because the 3.5 other volts are losses through the intermediate wiring. Good wiring should be around half a volt, so your other 3 volts are due to undersized wiring, or poor connections. There's no other possibility, worn pump or not.

A good test would be to measure the current draw of the pump.
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78 924 NA
5-lug

Maybe924 · Joined: 14 Aug 2007 Posts: 412 Location: New London, CT

I'll get some heavier gauge wire, make certain the connections are good and test direct from battery again.
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1981 931 GT (CGT Intercooled)
1985 911 Targa Carrera (Sold)

Rich H · Posted: Tue Nov 17, 2009 6:16 pm Post subject:

What he said, the pumps draw loads of current so voltage drop can be significant, especially if your connections are not 100% clean and the wire is thin. I imagine if the wire is thin it might gets a bit warm...

Remember if your battery is old it might be struggling to provide voltage but if it starts OK this is unlikely.
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec

Maybe924 · Joined: 14 Aug 2007 Posts: 412 Location: New London, CT

Of course you guys were right. I did a bunch of different tests today. I found the pump currently installed didn't flow enough and switched out to another one I had on the shelf. That pump managed to pump 800ml in 30secs. Both pumps were only receiving around 11volts when the car was running and as it warmed up the voltage dropped a bit more.

So I hooked up directly to battery power but left ground as is.... viola! I got 13 volts while the car was running.

So it looks like the ground is fine but the existing wiring providing power is shot. I'll have to investigate where the problem lies. I assume the fault could be in these spots.

1. Batt to relay board.
2. Bad solder on relay board.
3. Relay itself is ruled out since jumpering it changes nothing.
4. Wire from relay board to fuse board.
5. Solders at fuse board.
6. and lastly wire from fuse board to pump.

I can say this much... the jumper gets hot enough to burn my fingers, must be a lot of resistance somewhere. Wondering if this suggests that the resistance problem is ahead of the jumper?

Lastly, after running the pump directly off batt power my system pressure went from 5.3 bar to 5.7 bar.

Is it possible that this wiring problem is causing the rich running and misfiring?
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1981 931 GT (CGT Intercooled)
1985 911 Targa Carrera (Sold)

Smoothie · Posted: Thu Nov 19, 2009 2:02 am Post subject:

tbramich · Joined: 02 Dec 2009 Posts: 25 Location: charlotte nc USA

if the motor is only running on 10v and was designed to run on 12v (which means 12-14v) then YES it can burn out quicker. i see it all the time at work. when the electric motors have wiring issues and get less voltage than required it causes the motor to get hot and therefore break down at a faster interval.
i'd say bad power or ground. check the resistance in your ground wire from the end to the relay.

Maybe924 · Joined: 14 Aug 2007 Posts: 412 Location: New London, CT

Forgot to update.

I found the source of the problem and now get 12 volts at the pump while its running. It turn out to be from the fuse board to the pump. Fuel pressure reading are within spec now.
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1981 931 GT (CGT Intercooled)
1985 911 Targa Carrera (Sold)

stevekat · Joined: 19 Jan 2008 Posts: 719 Location: Los Angeles, CA

What was the specific problem? Bad solder, crimp, corrosion?
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"Never get out of the boat. Absolutely goddamn right. Unless you are goin' all the way."

Gone to new home: '81 924T, US version, CGT Intercooler, UTCIS-PT, Euro DITC, Greddy Trust MBC, Forged Fuchs Flat Dish 6's, Factory Recaro's.