I have a 1986 740 wagon with the bosch system. Today it died in transit and would not start for hours at all. The engine is flooding out with gas. The relay for the fuel pump is making prominent noise that it haddnt before. The relay from my other 740 clicks loud in my wagon too but both work. I get a limited response where I can drive for 15 seconds after I dry start the car without the relay then push it in real quick when she turns over.
I can floor it in this state but the RPMs max out at 1500, fluctuate, and then she stalls out.

Taking out the fuel regulator line did get a longer start at one point and got me a block down the road but that method worked twice only. Fuel line has steady fuel but monitoring we did at somepoint observe inconsistent priming and no buzz from the fuel pump at times.

Can the fuel tank on an 86 wagon be accessed the same way in the cab as my 91 sedan can be through the trunk?

 

with the relay out, your entire fuel injection system is not powered, so I fail to see how the car could run at all. taking the fuel regulator out would cause fuel to be sprayed all over the engine compartment, and no fuel pressure at all, or if you plugged up the outlet end of the fuel rail, would cause way too much fuel pressure.

1986 is right in the middle of the 'biodegradable wiring harness' years, the wire insulation fails inside the black tubes so you can't see it, like this...



if your harness hasn't been replaced with an updated one, good luck.

 

The issue i see now are the injectors either not firing or me for example turning the key ten times and they only firing the 2nd and 9th times. Im thinking computer or relay but i CAN NOT find the injector/noise relay on my 86 wagon and i can not find it on my 91 740. I looked on the strut towers and center console but did not see any matching the relat i found in my internet searches it looks like a recktangular relay with a cluster of four cylinder shaped connectors on one end. That however matches none of mine.

What are their locations and does a 91 740 ecm innerchange with an 86?5

 

there is no 'injector relay'. there is the main relay which also includes a fuel pump relay (ts two relays in one). on a 740, this is in the main relay panel behind the ashtray, while in a 240, its behind/under the glove box.

on both cars, the ECU is just forward of the lower right side door hinge, behind the interior liner panel.

an 86 is LH 2.2, while a 91 is LH2.4, they are very different ECU's.

 

Yea i know which relay is the fuel pump but the pump is pushing fuel now but just not always activating the injectors in unison. I know fp relay works because it does on my 91 i took it off with. Whatelse would cause inconsistent signal to injectors?

 

oh, a 740/940 has another relay under the hood, known as the 'Radio Suppression Relay', this provides the power to the fuel injectors. It is turned on by the main relay, and stays on until the main relay is turned off. on a turbo, the power output of the RSR goes through a pack of 2 resistors, and each of those resistors is connected to two of the injectors.

the injectors are fired by the ECU, all 4 are fired together from the same signal. the ECU output 'grounds' the injectors...

 

Can a faulty RSR also cause an engine to flood out with fuel though? Or for injectors to behave erratic?

 

a faulty RSR would cause the injectors not to switch on at all, since they'd get no power.

 

Can i get a picture of the relay because i dont see it on my '86 and not sure if its been taken off my '91. Autozone, advance, and oreilly's really couldnt find a relay matching the pics i found.

 

is typical on a later 740 or a 940 turbo. I think the non-turbos are on the other side near the coolant..

the relay by itself looks like


and here's a lot more about them, and the right kind to get (I wouldn't use any other brand than Stribel).
Volvo Relays

 

The problem now is this radio relay was used from 1987-95. I am assuming halfway thru '86 they start using it. What was used in place of it on models prior to 1986? Were earlier 740s just controlled by the computer period and if so which wire?

 

the original design powered the fuel injectors off the main relay (often called the fuel pump relay, but its actually two relays in one). On the 240 this wasn't a problem, that relay was over behind the glovebox, but on the 740 they centralized all the relays behind the ashtray and directly under the radio.... the fuel injectors draw significant current spikes twice per engine rotation, the close proximity of the radio created a 'buzz' on the radio, mostly on AM channels, so they added the RSR.

I don't have 740 wiring diagrams (other than 1991/1992 when they are shared with 940), so I can't say what year this started. typically changes are by model year, a model year 1987 car is usually one made after about july or august 1986.

you can verify the injectors are getting power with a volt meter. unplug an injector, turn the ignition on, and use the volt meter, in DC Volts, black lead on the ground rail near the fuel injectors, and red lead on one of the two pins of the plug that fits onto the injector. one pin should be +12.x V (battery voltage), the other pin is 'floating' and may read erratic voltage.

an even better test is to get some 'noid lights', and hook them up to the injectors, then crank the car, the noid lights will flash if everything is working.

 

Okay i know the reason for the radio relay but then why does the fp relay for my 1991 fit my 1986 ? Doesnt the 91 relay have half as many connections to bridge meaning less connectors.

But is the "main relay" identicle to the fp relay on my 1991 and in the same spot or one of the other ones? If thats the case i might be able to buy it but i dont know its appearence.

 

the main/fuel pump relay is the same for all 240/740/940 from mid 80s forward, EXCEPT 94-95 non-turbo 940's, which used a different relay configuration.

if your engine is flooding with gas, I doubt its the fuel pump relays fault. bad fuel pump or fuel injection or radio suppression relays result in NO fuel, not too much fuel.

instead, I'd be checking the fuel pressure regulator, via a fuel pressure gauge connected to the fuel rail. too much fuel pressure results in too much fuel.

also, a open circuit Engine Coolant Temperature Sensor (ECT) will cause it to run way too rich.

 

Lookin at it now the injectors dont do anything but the pump does so with my new regulator in place i believe the pressure just got built up so high that it may have forced the valves in the injectors open at a certain point. Because my 1991 floods out and we calm that down with removing and inserting the relay to get a start. Same trick works on my '86 but for different reasons.

But the fuel injector portion of that relay in the 1986, what was it then used for on my 1991? Id like to know that just to know that.

A 1991 fuel pump relay has the same part number as a 1986 injector/radio relay at the stores. I am assuming they go in the same slot in the same relay console too.

 

on the 240 and the very first 740, the main relay powered the ECU and injectors and other FI related stuff, and the fuel pump relay (within the same relay module) powered the fuel pumps.

on the later 740s and most 940s, they added the radio suppression relay under the hood... the main relay turns this on also, and the RSR provides the injector power directly from the battery.

on the final years of 940s, they realized the main relay was redundant, and used the RSR to also power the ECU and went to a single simple fuel pump relay in a different slot in the relay panel.

 

What should the psi be for the fuel line? And can you get away with a bad intank pump if you have a strong main pump?

 

LH 2.4 fuel pressure is 42-44 PSI above manifold vacuum

with a dead in-tank pump, my main pump struggled when the fuel tank is below about 1/3rd full.

 

Thats what it does when it starts and when i take my relay out. Video should had been longer but it basically done that for four minutes yesterday and the pump kept going with no relay to let it pump, prime, struggle, do anything period..

 

removing the main ('fuel pump') relay should render the engine inoperable. no fuel pressure, no ECU, no injection == no run.