FUEL PUMP doesnt get full voltage...
12.5 at battery, 11.5v to relay board - 7.5v out to fuel pump.

a local porsche repair shop i do business with asked me if i could find out if any of you guys does or knows of someone who rebuilds the relay board. as shown above there is 4v voltage drop somewhere in the relay board to the fuel pump. interestingly, according to keith who worked for porsche in the early 70's, they had a problem with the bottoms getting corroded. in 72ish they "tarred" the bottom, in real time that just trapped water/corrosion in... i'd have to assume he cleaned corroded fittings as he's pretty thourogh

any relay geru's out there?

i've seen relay boards for sale for $10-150. my recommendation is to buy 1 or 2 and swap. i remember something about the copper tracks are riveted and sometimes come apart.

someone was doing it.
Ask Jeff Bowlsby, he may remember the guy.
It was over on rennlist.

Rich

But... have you checked all the grounds for dirt and corrosion? Even at the age of our cars, rarely does the relay board itself seem to be a problem.

The underside of the board is sealed with potting compound. I remember an email about refurbishing the board yourself.... Lemme look around.

Okay... this was posted by Jeff Keyzer on Rennlist in 2003. Here is how he repotted his board:

Don't assume the relays are still good. Relay contacts corrode over time and wear out. If you have to replace the points in your distributor regularly, the contacts in the relay are just the same albeit with much less opening and closing. Try swapping around some relays to see if the voltage drop changes with the relay. Also, consider every connection in the path you are measuring; each will contribute a little to the overall loss.

thats what i'm thinking, total voltage drop (combined). possible resistance through pitted connections on the relay points. easy enough fix to swap known good relay and check to iliminate that one issue. usually relays fail totally to power, even if intermittantly though.

does anyone have a quick fuel pump and relay board diagram they could post?

thanks, mike

Click to view attachment
check for a voltage drop across the fuse

What year/engine is this?
I'm most familiar with the 73/74 models.

Just a thought, isn't 12.5 volts at the battery a little low?
I'd be kind of concerned about the 1 volt drop just to get to the relay board.

Where exactly did you measure the 11.5v?
Connector T14 pin 14 is the best spot to measure what is going into
the board that feeds the fuel pump. If you can't measure it there
probe the right side of the S13 fuse (red in the photo).
(See Fuel Pump for Carbs article for connector references)
NOTE: pin 14 is directly under pin 13 on the left side of the T14 connector in the photo of the relay board.

From where power enters the board at connecter T14 Pin 14 to
connecter T14 pin 13 where the fuel pump +12v signal exits, there are only 3 traces
of copper.

The traces themselves won't fail. They are pretty stout pieces of copper.
What I have seen (and it happened to me) is that the trace fails to make
full connection at a rivet point or actually cracks at the rivet point.
Mine cracked directly under the S13 fuse. (red in the carb article)

Have you taken a voltage measurement directly at the board vs out at
the pump? Connector T14 pin 13 or Connector T12 pin 12.


Another thing to check is perhaps your heater blower or rear window defroster.
Both of those pull power from the same circuit. If they start pulling lots of current,
it might cause a drop out at your fuel pump.

Other than that I'd start probing the relay board with your volt meter while
things are running to see where there drop is.

Get a good ground, directly at battery if possible then
start at T14 pin 14, then check each side of the S13 fuse
(red on in the picture in the Carb tech article).
Then check T14 pin 13 or T12 pin 12.

I've also seen relay contacts go bad or corrode. If you see the drop
at T14 pin 13, it could be something related to a trace or it could be a bad
fuel pump relay contact.

--- bill

thanks all, good input i'll let you know what he finds.