1973.5T CIS/ Fuel Pump Issue. Frustrating!

Sunroof · 01-21-2012, 08:23 PM

Can someone advise just how the fuel pump in the 1973.5T/CIS is set up? I know that the pump is wired to fuse number seven as too the CD unit, but just what happens when you turn on the key before starting relative to the pump? Does the Micro-Switch, WUR or cold start valve come into play as part of the fuel pump activation circuit? Can any of these components stop the pump from operating?

Here is the issue:

I turn the key on (not engaging the starter) and I hear the pump hiss and fill the injectors. Remember the pump is in the rear left wheel so I am pulling fuel. Turn the key and the car starts. Once in say five or so times, the fuel pump will not turn on although the CD unit will humm. No fuse and no relay I am aware of in the system. An intermittent pain in the butt! I cleaned the contacts on the fuse box and checked the wires to the pump (fairly new unit) and still the same issue.

Whats going on here? Can someone please explain the electrical circuit for the fuel pump or care to guess what the problem is?

Thanks
Bob

mixer911 · 01-21-2012, 08:58 PM

I'll take a shot seeing no one else has,
When the problem happens again,give the pump a bump and see if that does anything.
If that doesnt work pull the wire off and check for power,when not working.
They will run if their getting fuel or not
I took one apart once,pretty simple couple of brushes,not really service-able
thats my best guess,mine was a 74, goodluck

E Sully · E Sully's Garage

The fuel pump gets power directly from the ignition switch, rd/bk wire ign sw to fuse block, rd/grn fuse block to fuel pump. It is powered in run and start unfused and the pump should run as long as the key is on. If you clean and check all the electrical connections and still have a problem, the fuel pump should be the problem. The cdi is powered by the same wire from the ignition switch. If it is humming, power is going to the fuse block from the ignition switch.
The micro switch wur and cold start valve are separate circuits.
Porsche uses straps on the back of the fuse block to link certain positions together. It is posssible the riveted link needs to be checked.

ossiblue · 01-22-2012, 07:10 AM

Ed described it perfectly. There is also a connector to the fuel pump inside the engine compartment, just before the wires pass through the bulkhead to the pump itself. Be sure to check all the connections if you missed this one.

Sunroof · 01-22-2012, 07:37 AM

I did clean the contacts to fuse number seven and checked the connections to the fuel pump. I cannot see any connector inside the engine compartment as the fuel pump is under the wheel well and the lines come directly to it from the tunnel not through the engine compartment.

Whats strange is that the problem is intermittent! The car of late will start up a majority of the time and the warm up cycle and engine run is excellent. I could go for miles and no issues. BUT, when you park and turn the engine off and try a restart, NOTHING! No pump operation. The problem can be cold or hot start. I thought it must be an electrical short somewhere or a relay is not doing its thing, but where? Cleaning the contacts at the fuse did not help overall.

It must be in the circuit somewhere and following electrical schematics is not my specialty. But, if the pump and CD are on the same circuit the CD unit always works when the key is on,. That would discount the electrical portion of the ignition switch. I do not believe I have a failing pump since its a new Pierburg and when it runs its just fine.

Can their be some rely in the circuit, a switch, junction box, etc, etc, where I get no power connection? Can anyone take a look at the electrical schematic and let me know if we are missing something here?

The electrical gremlin.....dreaded and ugly. Keep the ideas, theories, guesses coming, please.

Thanks fellas,

Bob
73.5T

ossiblue · 08:01 AM

Since the CDI and pump are on the same power circuit, and the CDI works, that leaves only the wiring to the pump. Two suggestions: First, get prepared for the next failure by having a test light or multimeter ready. When the pump fails to run with the key "on", immediately check the power to the pump with your test device. Second, you may have a bad ground circuit to the pump (the "other" wire.) If your power feed is good during failure mode, it could be the ground wire. If both check out, that leaves the pump itself even though it is new.

BTW, what color are the wires that connect directly to the fuel pump--blue and black? I ask because on my 73.5, and the diagrams, the wires from the fuse block (red/green) and ground (brown), meet at a connector in the engine compartment and become the blue (power) and black (ground) for the pump. You describe wiring directly from the tunnel so there may be a difference.

E Sully · E Sully's Garage

In this picture is the connector from the main harness to the fuel pump lead. As Ossi stated, the wiring changes to blue and black from the connector to the fuel pump. Check the ground wires to the chassis by the mounting bracket for the filter.

Sunroof · 01-22-2012, 08:24 AM

Thanks Sully,

I went on the the electrical diagram and started tracing, but despite the hot wire coming from the fuse box to the connector and then pump (yes a blue wire) I am not sure about the brown wire since it looks like it goes through several spots like a multi-connector and even a few switches along the way. That brings up where the ground wire may be on the chassis. Do you know where the wire gets attached? This would be very helpful. Ossi mentioned near the fuel filter.

Thanks for the great photo

Bob

ossiblue · 01-22-2012, 08:37 AM

Bob,

Ed posted a great pic, and that's how I remember the connection to the fuel pump. As I recall, the ground wire to that connector (the brown wire that is on the left of the connector in the photo) is grounded to the chassis or the electrical panel (can't remember which) along with two or three other brown, ground wires from other components, somewhere just out of range in the picture.

If your set up is similar to the one in the pic (you've found the connector) then you can unplug just the black lead to the fuel pump and connect it to a good, separate ground as a test to see if that clears up your intermittent problem

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by ossiblue

Bob,

Ed posted a great pic, and that's how I remember the connection to the fuel pump. As I recall, the ground wire to that connector (the brown wire that is on the left of the connector in the photo) is grounded to the chassis or the electrical panel (can't remember which) along with two or three other brown, ground wires from other components, somewhere just out of range in the picture.

If your set up is similar to the one in the pic (you've found the connector) then you can unplug just the black lead to the fuel pump and connect it to a good, separate ground as a test to see if that clears up your intermittent problem

LJ,

Do you remember when we tried to diagnose Steve's (another member) electrical problems that took us days if not weeks to locate the culprit? After a series of tests, it pointed to a loose electrical connection between the ignition switch and FP. The car was an SC. I would suggest to do same analysis.

Tony

E Sully · E Sully's Garage

Bob,
If you look at the upper left corner of the picture I borrowed, you will see the stud. It should have 2 flat ribbon ground wires going to the panels in the electrical compartment, and a series of brown ground wires.