760 fuel system electrical bypass

1990 760 T b230ft lh2.4

Hey yall, sorry ive been posting so much, you guys have been super helpful and I very much appreciate it. Hopefully this will be my last post for a while, I just have one question which I was hoping to get opinions on before my dumbass tries to do it later today. I traced down my current problems (no pun intended) and think I have more or less figured out the electrical gremlin and have a solution in mind.

Heres the problem and stuff Ive checked through
-spark is fine
-battery is fine
-distributor is fine
-radio suppression relay is fine
-Fuel pumps arent getting juiced up.
-Relay is 100% fine, I have two good ones and fixed the bad one by reflowing it.
-Fuses are all good.
-Grounds have been cleaned and checked.
-test lamp and ohm meter showed fuel relay not getting enough juice
- ECU is maybe fine. I have a second one I will try if my "solution" fails.
-fuse slots for fuel injection and pumps are burnt up, probably due to poor wiring or grounds.
This picture depicts what the PO did to try and fix the problem that I believe has been ongoing and that I am now dealing with today (crap wiring)
https://drive.google.com/file/d/1FXxGE1FdMqyVLqcdEfUDU4IwUSzC-gOW/view?usp=sharing

Here is my plan:
-Totally ignore the fuel relay and all its wiring
-Rig up my own wiring from both fuel pumps to my own relay, fuse and switch.
This will allow me to hop in the car, flip a switch and turn both pumps on manually. If I do it right, I will know that the wiring is good, grounds are solid and I will have a sweet switch to flip to make me feel cool. Back on the road.

I have a buddy who knows a great deal more than I do about electrical and this was mostly his suggestion. I have a relay and switch at the ready and im about to track down the wiring that comes from the pumps.

So, how dumb, how cool, how turbobricks is this idea? Obviously I will have to remember to flip the switch back off and it wont be quite as convenient as having it set up properly, but I am at the end of my wits with this whole wiring nonsense and this solution seems doable. Let me know and thank you all again for being so knowledgable and willing to lend words.

I am slowly starting to understand that having this car as a daily is more of an undertaking than my noob brain can handle, and it is time for me to buckle down and read myself silly. If anybody has a "volvos for noobs" book lying around or something similar let me know. My kingdom for knowledge.

If you're going to try bypassing the fuel relay be aware that the fuel pumps can run your battery dead in about 30 minutes if you forget to turn the pumps off. Consider Installing an oil pressure switch that will ground your fuel pump relay, this will then shut the pumps off if there's no oil pressure. You would need a 5 terminal relay that has 2 outputs on the same circuit (thinking 87/1 and 87/2). Check Dave Barton site for good info on relays.

lummert said:

If you're going to try bypassing the fuel relay be aware that the fuel pumps can run your battery dead in about 30 minutes if you forget to turn the pumps off. Consider Installing an oil pressure switch that will ground your fuel pump relay, this will then shut the pumps off if there's no oil pressure. You would need a 5 terminal relay that has 2 outputs on the same circuit (thinking 87/1 and 87/2). Check Dave Barton site for good info on relays.

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Excellent, thanks for this. I have a 5 terminal one in sight and good idea with the oil pressure thing. My main concern which Zvolv mentioned is that if I were to crash, theres a good chance fuel will still be pouring out from the pumps and I could burn up and dieeee

lummert said:

If you're going to try bypassing the fuel relay be aware that the fuel pumps can run your battery dead in about 30 minutes if you forget to turn the pumps off. Consider Installing an oil pressure switch that will ground your fuel pump relay, this will then shut the pumps off if there's no oil pressure. You would need a 5 terminal relay that has 2 outputs on the same circuit (thinking 87/1 and 87/2). Check Dave Barton site for good info on relays.

Click to expand...

is there anything else I will need to wire in to make it actually start? simply jumping the relay socket triggers the pump, but still cant start it like that. Radio suppression relay?

Bypassing stuff

Do you guys smell that?







Smells like CAR B Q

Before you go created a future fire bomb, have you tried jumpering the fuel pumps at the relay connector to see if they even turn on? Remove the fuel pump relay and place a jumper (any small piece of wire with both ends stripped will do) between pins 30 and 87/2.

If they key is in the on position, you should hear both pumps running. There should be power at pin 30, if there is not then the relay has been bypassed. The pumps should be connected to 87/2, at least they are in my 85 740.

nickvolvo746 said:

is there anything else I will need to wire in to make it actually start? simply jumping the relay socket triggers the pump, but still cant start it like that. Radio suppression relay?

Click to expand...

Fuel pump relay triggers the radio suppression relay which supplies power to the injectors. So, temporarily bypassing the fuel relay and starting the engine requires a jumper with one input (30) and 2 outputs (87/1 and 87/2).

Thanks man, I did end up trying that. The solution has arrived. I am a total dumbas$ and didnt clean the engine bay grounds nearly enough. took everything off and took a wire wheel and sand paper to every last screw and contact and after putting it all back on, the thing started right up. Thank you again for all your input, reading all the stuff ive read over the last few days and months has taught me a lot, especially about how much I dont know.

ZVOLV said:

Grounds are checked with a voltage drop test. Google it.

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Glad you got it going.

I keep repeating voltage drop test around here, but nobody listens. If you had bad grounds, a voltage drop test would have shown it.

ZVOLV said:

Glad you got it going.

I keep repeating voltage drop test around here, but nobody listens. If you had bad grounds, a voltage drop test would have shown it.

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Yeah ill admit it im not always a good listener. Also barely know how to test for that **** when my smart car friend isnt around, lol