Fuel Pump Relay issue...maybe?

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doncaruana

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So a while back I had an issue I posted in here where my truck would hard start every so often. It will be a long crank and then it will turn over and run for a second and then stop. But the next crank will be perfect. And it never runs rough. Never figured it out and neither did a couple of places that I took it.

Had it at another shop for a different issue and asked them to take a look at it. They were stumped because the starting is so intermittent. Eventually on a whim, they said they swapped the fuel pump relay with the A/C relay, which is right next to it and magically it had started about 15 times in a row after that at last count (probably every 3-4 starts the original issue would happen).

Now, there are a number of things with this that have me scratching my head. First, in all the reference stuff I have, I can't find anything that definitively identifies the actual relay that is supposed to go there (I had a hard enough time finding a labeled block for the underhood relay/fuse center while they still have my truck!). There are 3 relays there that are the same size. BUT...
1. I replaced the fuel pump a little over a year ago for a different problem with a brand new AC-Delco relay, as recommended by Autozone.
2. Based on using Autozone's system, the fuel pump relay is 15-81106 (a 5 pin relay). But the A/C relay (and starter relay) are D1741C and are 4 pin relays.

So, assuming I heard them right on the phone (was a little distracted cause I was at my dad's helping him out), what does all this mean?

Now the really weird part - how would this even work?? Here's a pic I grabbed online of the underside of the 4 pin relay. The only difference between them is 87a isn't there on the 4 pin. From what I understand, this is what the pins do:
30 - Power
85 - Ground
87 - Switched power to circuit
86 - Switch that controls the circuit
87a - Always on except when switched to 87

So, if I'm understanding what he said and the way all this is working, by swapping the relays, he removed the pin that is supposed to always supply power to the fuel pump (87a). Does this even make sense? And is it indicative of some other weird problem? I am completely befuddled...

relay_pins.JPG
 

east302

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87A looks to be for the fuel pump test connection, intended to operate the pump directly, bypassing the oil pressure switch and VCM controls. Put 12V on it and the pump will run.

The 4-pin would just remove the ability to use the test connector which is tucked in between the fuse box and firewall.

9FD43F15-C49F-46CD-92F4-27200D2E56F9.jpeg
E26FDA85-959D-446A-A287-EE6FB7359209.jpeg
 
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doncaruana

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87A looks to be for the fuel pump test connection, intended to operate the pump directly, bypassing the oil pressure switch and VCM controls. Put 12V on it and the pump will run.

The 4-pin would just remove the ability to use the test connector which is tucked in between the fuse box and firewall.
@east302 - you're a damned treasure!
 
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doncaruana

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Update...see if this makes sense to anyone...
With the original type 5-pin relay in for the fuel pump, major starting issues. Even worse than before.
With a regular 4-pin relay (ie the same as the AC relay), it appears to work perfectly.
 
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doncaruana

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Did you try (another) new 5-pin relay?
Yep. Brand new. When I said "original" I meant the one that's supposed to go there. Bought a new 4 pin and put it in there and it seems to work fine. At least for now. We'll have to see if it holds up but the shop and I are scratching our heads trying to figure out why that would be the case.
 

east302

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Yeah, that is strange. The only difference is that pin connecting to the test connector. Perhaps that terminal in the panel is damaged or something is going on under there and causing a short.

Well, if the 4-pin works then I’d run with it.
 
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doncaruana

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Yeah, that is strange. The only difference is that pin connecting to the test connector. Perhaps that terminal in the panel is damaged or something is going on under there and causing a short.

Well, if the 4-pin works then I’d run with it.
Yeah. Only thing that makes sense to me is some kind of weird short on that test line or even in the panel itself. The next thing to see is if this resolves my p1351 issue from last year!
 

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