 Jumper relay board T4b fuel injection port, why would you? |
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| nditiz1 |
 Jun 27 2020, 10:16 AM
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#1
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Senior Member  Group: Members Posts: 1,188 Joined: 26-May 15 From: Mount Airy, Maryland Member No.: 18,763 Region Association: MidAtlantic Region 
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So I'm going through the electronics and fixing things that are not working or might be wrong and I came across this jumper in the relay board. This has been converted to carbs but the fuel pump was directly hooked to the positive side of the coil - will change that. Is there a reason to jumper the FI ports on the relay board?
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| sixnotfour |
Jun 27 2020, 10:37 AM
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914 Wizard Group: Members Posts: 10,432 Joined: 12-September 04 From: Life Elevated..planet UT. Member No.: 2,744 Region Association: Rocky Mountains 
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you have carbs ..yes
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| Spoke |
Jun 27 2020, 11:09 AM
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Jerry Group: Members Posts: 6,978 Joined: 29-October 04 From: Allentown, PA Member No.: 3,031 Region Association: None
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I've seen this connection before. It will work but it is a really weird setup. What you have connected is labeled as pins III and IV going to the fuel injection system which is removed when running carbs.
Pin III is the fuel pump relay and normally for carbs pin III is connected to ground to turn the fuel pump relay on any time the ignition key is in the RUN or START position. Pin IV is the starter bendix pin and the voltage on it is zero when the key is in the RUN position and is 12V when the key is in the START position. With pins III and IV tied together, the fuel pump relay gets its "ground" voltage through the bendix of the starter when the bendix is not powered. This happens with the key in the RUN position. However, when the key is in the START position, the bendix is powered and the starter is spinning, pin IV is 12V and the fuel pump relay is turned off as the starter is spinning the engine. When the key is moved from START to RUN then the fuel pump turns back on. The correct connection here is to ground pin III to a local chassis ground stud like the one behind the relay board. Pin III is the leftmost pin on that connector in the above picture. |
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| SirAndy |
Jun 27 2020, 11:40 AM
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Resident German Group: Admin Posts: 41,641 Joined: 21-January 03 From: Oakland, Kalifornia Member No.: 179 Region Association: Northern California |
QUOTE(Spoke @ Jun 27 2020, 10:09 AM)  The correct connection here is to ground pin III to a local chassis ground stud like the one behind the relay board. Pin III is the leftmost pin on that connector in the above picture. http://www.914world.com/specs/SirAndyCarbFuelPumpRelay.php (IMG:style_emoticons/default/bye1.gif) |
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| rhodyguy |
Jun 27 2020, 12:27 PM
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#5
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Chimp Sanctuary NW. Check it out. Group: Members Posts: 22,080 Joined: 2-March 03 From: Orion's Bell. The BELL! Member No.: 378 Region Association: Galt's Gulch |
Plus, if the stock pump harness is functional there's no need for additional wires. Like pulling pump power off the coil. Very bad plan. I learned the hard way. Aftermarket pumps can require alteration of the pump end of their harnesses. Save the stock end.
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| nditiz1 |
Jun 27 2020, 12:54 PM
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#6
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Senior Member Group: Members Posts: 1,188 Joined: 26-May 15 From: Mount Airy, Maryland Member No.: 18,763 Region Association: MidAtlantic Region
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I feel like they did this mod and then totally went away from it by going directly to the pos side of the coil.
So if I remove this and then ground pin 3 to the chassis, @SirAndy (I was already following your guidance in the link you posted (IMG:style_emoticons/default/beerchug.gif) ) then I should hook in the hot wire from the pump (previously coil) into one of the two 14 pin connector sports? OR should i be locating the stock fuel pump wire in the wiring harness and connecting it to that as @rhodyguy suggested? |
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| Spoke |
Jun 27 2020, 01:20 PM
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#7
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Jerry Group: Members Posts: 6,978 Joined: 29-October 04 From: Allentown, PA Member No.: 3,031 Region Association: None
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QUOTE(nditiz1 @ Jun 27 2020, 02:54 PM) I feel like they did this mod and then totally went away from it by going directly to the pos side of the coil. So if I remove this and then ground pin 3 to the chassis, @SirAndy (I was already following your guidance in the link you posted (IMG:style_emoticons/default/beerchug.gif) ) then I should hook in the hot wire from the pump (previously coil) into one of the two 14 pin connector sports? OR should i be locating the stock fuel pump wire in the wiring harness and connecting it to that as @rhodyguy suggested? Pin 13 on the relay board connector going towards the cabin is where the fuel pump picks up power. The 25A fuse protects the fuel pump wiring from damage in case of a short circuit. Attached thumbnail(s) 
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| nditiz1 |
Jun 27 2020, 01:39 PM
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#8
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Senior Member Group: Members Posts: 1,188 Joined: 26-May 15 From: Mount Airy, Maryland Member No.: 18,763 Region Association: MidAtlantic Region
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QUOTE Pin 13 on the relay board connector going towards the cabin is where the fuel pump picks up power. The 25A fuse protects the fuel pump wiring from damage in case of a short circuit. Thanks Spoke. Looks like someone jumped off the B/R wire once before as there is an aftermarket connector near the 14 pin connector. I need to find where the B/R wire would normally terminate. It should be where the new Carb fuel pump is at correct? OR did the factory move the pump up underneath the tank in 74? |
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