Electrical problem - fuel pump

[Porsche-O-Phile]

For 85.5 n/a.

Very weird problem related to "no start" condition. Checked the fuel rail - no fuel pressure. Jumpered terminals 87b & 30 on the DME relay receptacle - nothin'. No fuel pump operation. Okay, suspected bad fuel pump. Put 12v across fuel pump at the plug connector at the pump itself and the pump started up and ran fine - no weird noises or anything. Double, triple and quadruple-checked for fuel pump operation by jumpering terminals 30 & 87b again to no avail. Fuse #34 is fine (and proper value - 15A). WTF?

I shined a light up under the dash looking for any obviously loose wires dangling under the main electrical block but didn't see any. Far as I can tell everything is fine. Haven't done much with the car in a couple of weeks and it ran fine then.

Very odd. Suggestions?

Edit: I also checked the DME harness at the DME and it's fine - installed tightly and securely. I suppose I'll check for continuity betwen the DME relay receptacle, fuse terminal #34 and the connector at the fuel pump at the rear of the car tomorrow.

Bad ground perhaps? What else shares that ground point at the rear of the car that I could use as a double-check of the ground?

[SoCal Driver]

Factory alarm?

[Porsche-O-Phile]

Yea the car has one but it is in the "off" position - don't have a key to it. It's on my list of things to do to yank the cylinder out and have it re-keyed so I can actually use it. So yea, I thought about that but I'm pretty certain that isn't it since I haven't messed with it (had no way to) since the last time I ran the car. I s'pose I could disconnect the leads to the factory alarm box just to eliminate that possiblity though seeing as I have the DME and all that stuff exposed and just sitting on the floor right now.

This one's a bugger. I still like "bad ground" but like I said I did have the car running before. . .

[SoCal Driver]

87a to 30 (battery) to run the fuel pump. There is a sub b but it's 85b (grounding through the DME for the fuel contact coil).

87 is power to the DME and injectors.

85 is to ground. (Brando's favorite terminal)

86 is the power from the ignition switch. This is interupted by the alarm. (I was thinking it was the fuel pump circuit) This is "fixed" by jumping pins 1 to 4 on the alarm plug. Jumping pins 2 to 8 reconnects the alternator warning light.

Note that power for the later idle control vavle (servo actually) comes off of the injectors. While the early idle control valve comes off of the fuel pump power.

[Porsche-O-Phile]

I checked the factory wiring diagrams - oddly the 85/2 diagrams don't show terminal 87b present on the DME relay (it shows "87a", same as the early cars). The terminal 87b appears only in the 86+ factory wiring diagrams for both the n/a and turbo. Odd. Anyway (based on the 86 diagrams for an n/a) it shows the circuit as pretty simple. Terminal #30 on the DME relay (E31 on the central electrical block) goes to the battery via a black & green wire and provides the positive 12v connection to the DME relay, which you stated (correctly).

Terminal #87b goes comes off the DME relay to fuse #34 (15A fuse) and from there "tees". One line (E24 on the central electrical block) goes to DME connection plug #4 and from there to the ignition switch, etc. The other line (F25 on the central electrical block) goes to the fuel pump via ANOTHER green & black wire (you'd have thought they could use a different color here. . .) which presumably is the +12v connection to the pump - meaning that one would expect the pump to be a continuous-ground type with the "control"/switch on the positive end of the circuit - facilitated by the DME relay.

Coming out the other side of the pump is a brown wire, which the factory wiring diagram shows going to point H36 (a "weld connection point" (?)) and from there to "passenger room wire harness", which I assume means bundle of wires running along passenger's side. The Haynes manual diagram shows this wire (the brown one) going to ground point #9 (underneath the rear of the car) which is more simple and which I'm more inclined to believe unless I'm simply getting confused by the factory wiring diagram (always a possibility - I'm not an EE).

Anyway, it sounds like I need to find this ground point #9. Doesn't look like anything else goes to it. . . I'll also check the continuity of the circuit and report back.

[armandodiaz]

Before you start going crazy chasing electrical circuts, have you checked to see if you have 12V AND ground at the fuel pump and at the other end of the fuel pump harness?

[SoCal Driver]

87 a or b (what ever) fuel pump contact output does not go to the ignition switch or terminal #4 on the DME. After the fuse it goes to the fuel pump and to the pre heater for the later O2 sensor. If you have a turbo it also goes to turbo water pump relay. The other side of the fuel pump goes to a grounding point.

Misreading the schematics (not that impossible to do) will get you into a world of hurt.

[Porsche-O-Phile]

Thanks for that Hugh - I just re-read 'em and I concur.

Here's tonight's update:

I checked for continuity and it's present as would be expected. I yanked the master electrical block out and testing the green & black wire from both E24 and F25 yielded zero ohms (well, almost zero ohms. . . My 944 isn't superconductive) from that point at the MEB to the black & green wire at the fuel pump. Ditto on the brown wire to the - battery terminal.

I also was able to run the fuel pump by supplying +12v to the #4 terminal on the underhood DME master connector. I was NOT, however, able to get the fuel pump to run by jumpering terminals 30 & 87b. I suspected that the contacts might have been bad so I pulled the entire master electrical block apart (a HUMONGOUS pain in the ass) and cleaned up all the terminals where the relay spades plug in. I'm pretty confident that the terminals should be fine now.

Should DME relay terminal #30 yield +12v? It didn't. I'll have to check those diagrams again. . .

Gawd I hate electrical problems. . .