Testing fuel pumps on TBI, but different - The 1947 - Present Chevrolet & GMC Truck Message Board Network

D.FENS · 11-13-2015, 11:59 AM

Just about reached the end of the interweb on this one. If the info is out there Google can't find it and neither can I. Both Haynes and Chilton CD-ROM manuals are super vague about the subject. Do not have a dealer service manual on my truck, if anybody has one I would be interested.

I'm thinking one or both of the fuel pumps on my '88 crewcab have played out. 7.4L TBI, dual tanks. Pumps are SUPPOSED to be in the tanks, I guess they are because I can't find a main pump underneath the truck. Just a giant oil-filter looking deal rigged up in place of the factory fuel filter.

Google searching tells me there is (again) SUPPOSED to be a fuel pump test circuit on the ALDL plug under the dash. My truck does not have a pin in position G like the interweb says it's supposed to. I'm guessing that may not be used for dual-tank and / or big block trucks, don't know.

So, with the common test method not available here, is there an easy way to test both pumps independently?

There is a red / white wire, with a grey connector, plugged into the fuse box adjacent to the ECU power wire. Is that the one I'm looking for? I did find a couple of forum posts suggesting you can test a dual tank truck out under the hood but did not say where.

csdineley · 11-13-2015, 12:21 PM

Attach 12v from battery to red wire on fuel pump relay on firewall. The selected fuel pump will turn on. Flip the tank switch on dash to change between tanks.

D.FENS · 11-21-2015, 12:13 PM

Wanted to follow up in the event it helps anybody else.

The jumper wire was where csdienely said it would be, but it was wrapped up in the harness and I had to dig for it.

With power to the jumper wire, and toggling back and forth on the dash switch, one pump runs (and the gas gauge moves), the other side nothing.

Still won't start, so I will be checking for fuel pressure at the filter and fuel rail connections shortly. And maybe buying a gauge kit if a visual check doesn't help.

wildwilly4x4 · 11-21-2015, 11:37 PM

you might check that the tank switch valve is working as well. IIRC the pumps actually swap by the tank valve making contacts in itself when it moves to open one tank or the other. Since I swapped to an LS style fuel system, I just have the pump and return both from the driver side tank. I welded 1/4 inch pipe fittings to the bottom of each tank, and ran a 3/8" SS tubing to tie them together. I then built a fuel fill x-over (like the dual fill that used to be available) that fills both tanks from the driver side. Mine just acts like a big 32 gal single tank now.

11-13-2015, 11:59 AM	#1
D.FENS Registered User Join Date: Sep 2008 Location: Huntsville AL Posts: 284	Testing fuel pumps on TBI, but different Just about reached the end of the interweb on this one. If the info is out there Google can't find it and neither can I. Both Haynes and Chilton CD-ROM manuals are super vague about the subject. Do not have a dealer service manual on my truck, if anybody has one I would be interested. I'm thinking one or both of the fuel pumps on my '88 crewcab have played out. 7.4L TBI, dual tanks. Pumps are SUPPOSED to be in the tanks, I guess they are because I can't find a main pump underneath the truck. Just a giant oil-filter looking deal rigged up in place of the factory fuel filter. Google searching tells me there is (again) SUPPOSED to be a fuel pump test circuit on the ALDL plug under the dash. My truck does not have a pin in position G like the interweb says it's supposed to. I'm guessing that may not be used for dual-tank and / or big block trucks, don't know. So, with the common test method not available here, is there an easy way to test both pumps independently? There is a red / white wire, with a grey connector, plugged into the fuse box adjacent to the ECU power wire. Is that the one I'm looking for? I did find a couple of forum posts suggesting you can test a dual tank truck out under the hood but did not say where.

11-13-2015, 12:21 PM	#2
csdineley Registered User Join Date: Feb 2012 Location: Morgantown, Wv Posts: 294	Re: Testing fuel pumps on TBI, but different Attach 12v from battery to red wire on fuel pump relay on firewall. The selected fuel pump will turn on. Flip the tank switch on dash to change between tanks. Attached Images __________________ 87 V20, Vortec 350, Sm465, Np205, Dana 44, 14bff 4.10's

11-21-2015, 12:13 PM	#3
D.FENS Registered User Join Date: Sep 2008 Location: Huntsville AL Posts: 284	Re: Testing fuel pumps on TBI, but different Wanted to follow up in the event it helps anybody else. The jumper wire was where csdienely said it would be, but it was wrapped up in the harness and I had to dig for it. With power to the jumper wire, and toggling back and forth on the dash switch, one pump runs (and the gas gauge moves), the other side nothing. Still won't start, so I will be checking for fuel pressure at the filter and fuel rail connections shortly. And maybe buying a gauge kit if a visual check doesn't help.

11-21-2015, 11:37 PM	#4
wildwilly4x4 Registered User Join Date: Aug 2009 Location: Sunray,TX Posts: 251	Re: Testing fuel pumps on TBI, but different you might check that the tank switch valve is working as well. IIRC the pumps actually swap by the tank valve making contacts in itself when it moves to open one tank or the other. Since I swapped to an LS style fuel system, I just have the pump and return both from the driver side tank. I welded 1/4 inch pipe fittings to the bottom of each tank, and ran a 3/8" SS tubing to tie them together. I then built a fuel fill x-over (like the dual fill that used to be available) that fills both tanks from the driver side. Mine just acts like a big 32 gal single tank now.



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