No power to fuel pump

[FastWoman]

Hi all,

I was out on errands a couple of days ago and had my '78 fail to start at the post office. Had it towed home a few blocks. Thank god for AAA.

After a bit of diagnostics, I found I had no fuel pressure. I pulled out the FSM and started fault tracing. I found that the ignition relay, the fuel pump control relay, and the fuel pump relay were all working fine. I had +12 to the fuel pump relay with the ignition switch in the start position. Also when I jumpered across the plug to the fuel pump relay, the pump kicked in. Cool. So I plugged the fuel pump relay back in, and everything worked fine. I figured it was just a dirty contact somewhere. (Lots of dirt in that box>)

So I spent the next half hour cleaning contacts with WD-40, working them well, etc. Then when I put everything back together, SAME PROBLEM AS BEFORE! Thus is was my most favorite type of problem -- an intermittent one.

This morning I went back to fault tracing. I jumpered across the fuel pump relay connection again, and no fuel pressure. I then did a resistance check from the fuel pump relay connection, presumably through a fuel pump, and then to ground. There was no conductivity. Something was open. So I figured it might be an intermittently bad fuel pump.

I then jacked up the rear end of my car, chipped off some of the insulation painted over the fuel pump connections, and measured voltage across the terminals. I couldn't measure any voltage with the key in the start position, and there was obviously no fuel pressure either. I also did a conductivity test across the terminals of the fuel pump. (The reading was 1.1 ohms. Normal, or somewhat low?) Then wondering whether the unit was grounded, I did a measurement from the two terminals to the bumper. That was about 4-5 ohms, depending on the terminal, and whether the motor was in series with the measurement. Seems a bit high, but then again, I was measuring at the bumper, and someone else was holding the probe to the bumper for me.

So I'm guessing my relays are good, and my pump is good. The fault lies somewhere in the wiring between the relay and the pump. But where? The FSM doesn't seem to indicate a connection between those two points. Is there one? Any common failure points I should be looking for?

Frustrated in Virginia,

Sarah

Edited August 22, 2009 by FastWoman

[FastWoman]

Could use some help here -- an "uh huh" that I'm not on the wrong track, before I start tearing into body parts and wiring harnesses! Better still, perhaps someone knows a trouble area or two where I should start looking.

After the rain let up, I was able to continue testing. I was finally able to break all that epoxy (?) off of the + terminal on the fuel pump and lift that wire -- a green wire with a light blue trace. I was similarly able to pull the wire from the plug to the fuel pump relay. Same color code. Both connections were clean and bright. No question there's contact between the connector and the wire.

I quickly jumpered between the battery and the + terminal on the fuel pump. It pumped.

I then did a conductivity test on the wire between the engine compartment and the fuel pump. I read about 366 kOhm. From the fuel pump side of the wire to ground was 65 kOhms. Not surprisingly, the resistance between the relay side and ground is about 366 kOhm.

One might think the wire has been severed somewhere, with the pump side of the wire lightly touching ground (65 kOhms) and the other side not. This hypothesis seems inconsistent with the fact that the pump worked for a few moments a few days ago before it failed again. The rubber grommet is in good condition where the wire enters the body beside the fuel pump, so the problem isn't there.

Any recommendations where to look? Locations where wiring harnesses might get bound up, rubbed, or cut?

Any information would be greatly appreciated!

Thanks,

Sarah

Edited August 23, 2009 by FastWoman

[FastWoman]

FOUND IT!

I did find diagrams in the FSM where the wiring harnesses were strung. (I wish there were an index in the thing, because I would have found it earlier.) There I saw that there were large junction block connectors between harnesses just inside the firewall. My G/B wire was in one of those connectors. I pried apart the connector with some difficulty, only to find a crusty, somewhat melty connection inside. There was good continuity from the respective connectors to the ends of the wires. I cleaned up that connector and the adjacent one with WD-40, working the connections pretty liberally. Then I connected it all back up and started my engine. Yipee!

The weird resistance readings (which I corrected in the above post, as I couldn't see the "k" on my auto-ranging multimeter from where it was perched atop the exhaust pipe beneath my car -- thought it was 65 ohms, not 65 kOhms) were due to a half-cell reaction in the corroded connector. There was a tiiiiiny voltage between the two ends of the wire and between the corroded connector and ground. That was enough to trick out my multimeter into reading the wrong resistances -- 65k in one direction and 366k in the other. Had I reversed my probes, I would have found the opposite. Lesson to be learned: When troubleshooting a large wiring harness in a possibly corroded piece of equipment, don't use the ultra-sensitive, ultra-low current multimeter. Instead, use one of those cheap mechanical ones that puts out a higher test voltage (and won't be tricked out as much). And then when you get a measurement, try reversing the probes to see if you get the same measurement. If you don't, there's a tiny voltage inserted somewhere, and there may even be some rectification going on -- good evidence of corrosion between dissimilar metals.

Anyway, I'm running again. Yipee! BTW, I do appreciate anyone who might have taken time to do some head scratching on this problem, even though they didn't think of anything to post.

Peace,

Sarah

[73 240Z Man]

Did you ever get an answer on this....?

[sblake01]

It would appear that she solved it herself.

FOUND IT!..........Anyway, I'm running again. Yipee!

[diduzthat]

Sarah, I'm having the same type of problem on my 78z. No power to the fuel pump or to the connectors under the passenger seat, although the pump works fine when jumped. Where are the connectors you mentioned in your post? Are they under the dash on the passenger side? If you don't mind.

Thanks,

Ron

[olzed]

Sarah, I'm most impressed in your problem solving skills.

I could not handle the stress of problems like this.

Thank goodness I have a 240.

[SteveJ]

Ron,

Take a look at the BE (Body Electrical) section in the factory service manual (FSM). That will show you what you want to know.

[FastWoman]

Sorry I haven't been around much. Hurricane Irene.

Ron, Take a look at where the big wiring harness dives through the firewall on the passenger side of the engine compartment. You're going to need to find the two large connectors right on the other side of the firewall, just inches away. You'll need to remove a small, plastic trim piece to uncover the harness and connectors. The fuel pump contact is right in the corner position of one of those connectors.

[grantf]

Usually we turn to FastWoman to solve our fuel problems, So If I saw this thread earlier I would suggest that she check with FastWoman as she has done, and FastWoman solved Fastwomans problem. But really, I have noticed a lot of the interior firewall terminals on my car have the same corosion problems. Perhaps it is prudent to clean them all before a similar problem occurs.