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Posted by: robkammer Jun 6 2025, 05:21 AM

All: The intermittent fuel pump issue still lingers on our BB. Can anyone tell me where the wire (or wires) that feed pins 12 and 14 on the 14 pin connector are routed from the battery? It's obvious that it goes into, and returns from the hole in the firewall. But does it go all the way to the dash, or just under the passenger seat?

And the color, is it red/white or red/yellow?

This seems to be the cause of the problem. And check out my OBD 914 version below. Only way I could figure how to determine what was failing.
Rob

Posted by: JeffBowlsby Jun 6 2025, 08:04 AM

https://bowlsby.net/914/WiringHarnesses/docs/914_MAIN_1974.pdf

Posted by: robkammer Jun 6 2025, 12:52 PM

Thanks Jeff: I have those prints I believe, but your files will print much better. I'll print them out and tape together for an old time spread sheet. I'm still interested in where the wires physically run through the car. And the color. The Haynes I have lists red but does not include a 74 diagram.
Today the car started right up and idled well for about 15 minutes. Tomorrow? Who knows. It's been frustrating.
Rob

Posted by: Superhawk996 Jun 6 2025, 01:46 PM

Rob

Everything you need is right here



It’s been at least 4 years since I’ve messed with my harness but my recollection is the red battery feeds from the battery to the relay board never leave the engine compartment.

I believe they go down toward and into the firewall boot but then go up to the relay board on the other side of the engine bay to the relay board under the shelf.

DMM is your friend.

Thought you had this solved by spreading the fuel pump relay (J17 on schematic) pins and tightening the relay board sockets - am I mistaken?

Posted by: robkammer Jun 8 2025, 08:17 AM

Thanks for the print. I thought I had it solved as well. Headed out for a 10 mile ride but turned around when I realized I had left my phone behind. Car died three times on the way back but restarted each time.

My home made OBD confirms the fuel pump circuit is the issue. during more testing we discovered that pin 14 and 12 do not always have 12V. And when they do we can't wiggle, jiggle, or bump anything to recreate the disconnect.

We were at Stoddard yesterday and I picked up three new relays. Only because all the relays I have are old, or URO crap. ( all pass the headlight test) Obviously not a relay issue as the disconnect is leading to the relays. But I figured, why not?
Friday and yesterday the car stats and idled just fine. I'm hesitant to drive unless I have someone following with my truck for a pull home. Frustrating.

I'll look at the wire routing again today, but it seems like it's not direct.

Posted by: didenpx Jun 8 2025, 08:42 AM

Can you describe the dropout to the fuel pump please? Is it random while running? Does it drop out only long enough to cause a stumble? Is it associated with anything like shifting into reverse or honking horn?

Posted by: JamesM Jun 8 2025, 12:32 PM

Old relay boards can cause a lot of problems which is a real pain given no replacement is available.

One issue that I have seen a few times that causes intermittent fuel pump failure that was difficult to track down was the connection between the spade on the board for the fuel pump relay ground trigger and its trace to the relay on the back side. These two components are basically press fit together and the combination of pressure from the connecter being installed combined with heat and vibration will cause it to fail intermittently while driving but test fine on the bench. To find the failure you have to back probe the forward left side pin on the 4 pin d-jet harness connector while it is plugged in, and pin 86 (driver side most pin) on the fuel pump relay socket. While doing this press on the connector and wire/pin to see if you lose continuity. This has to be done with the connector on, and potentially while everything is hot as it is the pressure on the pin that causes the connection break.


The spade that says "Ground to enabled fuel pump relay" in the first picture. To repair it you have to get to it under the potting material on the back side of the board. Be sure to test again after repair as a good repair can be difficult. Its tough to get enough heat into the junction to avoid a cold joint without melting the board. I like to use a deoxidizer on the metal prior to solder as well.



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Posted by: robkammer Jun 9 2025, 04:24 AM

Thanks James, great drawings. I'll be back at it again today. When I left it Friday it was running and idling just fine. Went out yesterday expecting it to start right up, but no power to the fuel pump again.
I didn't have a lot of time yesterday but I did check power at pin 14 and 12, all good there. I'll run the 22 step test AGAIN today, and double check the contacts you point out.
I've attached a shot of the wiring harness from the battery to show that it hoes through the firewall to an as yes determined point of return. That may be moot as the #14 pin seems to be fine.
Pulled out the Stoddard relays and put the old working relays back in. Hew relays are disappointing. No pin numbers in the casting, and no diode in either the black or red version.
Update later today!

Posted by: robkammer Jun 10 2025, 03:47 AM

Here's the latest: Ran the tests again yesterday, car starts and runs fine, until it quits. The only anomaly the test showed was at pin #8, which should show 12 volts with the ket on and I only show 3.3 V. Used two different meters a couple of times and receive the same reading. This points to the ignition switch. I was suspect of the switch even though it had read 12V earlier.I have a new one in hand. That's the project for this morning.
Cheers and thanks for reading!
Rob

Posted by: Superhawk996 Jun 10 2025, 11:19 AM

keep using that meter.

Verify it after ignition switch swap again to make sure it isn’t bad wiring and excess voltage drop in the wire.

Posted by: robkammer Jun 10 2025, 03:46 PM

All: The switch is in, and all seems good. Tested #8 and I have a full 12.5V. I did find some sketchy wiring under the dash, but it was the windshield wiper harness that needed attention. Hindsight being as clear as it is, I should have just put the switch I suspected in at the onset. Thanks for watching! Test drive with a chase car is soon to come.
Rob