I have a 1986 gle 740. Today i lost fuel. When i checked to see why the fuel pump was no longer pumping i discovered the main relay was not functioning. Switching it out with several new relays did not help. I began checking wires and discovered the red wire that powered the relay was not energized.I then followed the wire back all the way to the ECM. Pin 6 on the ECM was not sending any juice out to the main relay. The computer is getting juice, its 3 grounds are all functioning(with key off too), i get the same results with multiple ECMs though. What else should be checked to make sure the computer knows when to juice up the main relay and what can possibly bypass this pin and possibly be used as an alternate in its place?

 

the fuel system relay should be getting its /power/ via a fuse thats powered directly from the battery via the positive terminal.

the ECM just does relay control. I don't have a wiring diagram for a 86 740 so I can't be much more specific.

 

Thoseare some decent schematics.
It says the red wire coming from pin 6 on the ecm eventually ends up feeding the relay if i am reading it right. The relay wont click anymore, the spare relays diddnt help either, there is no juice coming from that red wire.

 

The brown wire from fuse 1 to relay pin 3 is the power to that relay. the ECU grounds the red wire to turn on that top coil, which turns on the main ECU power. the blue-yellow wires from the upper relay contact are the ECU power

the green wire from the lower half of the relay provides the power to the fuel pumps, fuel injectors, etc.

 

So the brown wire should be checked to make sure juice ia getting to the relay and the blue yellow wire should be traced all the way to the ecm and then checked to confirm its sending juice to it?

 

Im still not completely perfect at reading schematics but im trying to figure out if the brown wire feeds into a another wire after fuse 1 that links it to a "bus bar" or not? I still cant force any of the relays or spare new ones for the fuel/main relay to work even when hitting the brown wire with battery power. Where does the brown wire originate from once we leave the fuse area?

 

i hate those schematics, they are horribly drawn. the ones in the greenbooks are much clearer, I just don't have those for 740's other than the 91/92 740 that are equiv to a 940

the battery is wired directly to a 'positive terminal' bus bar that on a 7/9 is located behind the dashboard on the firewall, somewhere above the pedal cluster (on a 240 its under the hood on the left fender). a red wire from this positive terminal goes to a bus bar under the fuse bay where fuses 1-6 get their power.

 

Those actually are the schematics from the green book for the 2.2 system. If the bus bar is not getting a good connection though is it safe to just go ahead and directly wire a new wire from a direct positive source or from a wire that's only hot when the ignition is in the on position?

And thank you so much for ur usual top notch help

 

um, if the fuse panel isn't getting power? or the postive terminal behind the dash? eeek


p.s. those are the Mitchell wiring diagrams from their generic all-cars data service. the greenbook volvo diagrams look more like this (1992 740/940 non-turbo LH 2.4 fuel injection)

 

oh. if the positive terminal is not getting good power, I would repair the wires, they go direct from the battery positive terminal, via one or two heavy gauge red wires. the positive terminal, labeled 78 in my diagram above, and also referred to as 15/1 in other diagrams I've seen, is a flat rectangular black plastic box, maybe 2x3", that has a pile of all red wires coming out of the bottom. the cover on this box unsnaps, and inside, its a brass plate with a bunch of 1/4" spades that all those brass wires are plugged into.

if the bus bar feeding fuse 1-6 is not getting good power, I would repair/replace that wire from the positive terminal. again, fairly heavy gauge red wire.

 

ah, found this greenbook online for a 1987 740.. I believe they are VERY close to identical.

http://www.volvowiringdiagrams.com/v...20Diagrams.pdf

 

The 1986 is a transitional year so an early 86 and a late year 86 have differences.. I dont have a computer to open that up with so ill just trace the Brown wire's origins and any other relays that might not be getting powered correctly right now

 

that 87 diagram (and also other stuff I browsed online) suggested that fuse 1 could be getting its power EITHER from its own brown wire, or a red wire, OR from the bus in the fuse panel thats on a red wire, and there was no clear guidance as to which config was used when. in some configurations, its powered directly off the battery, while in others its off that positive terminal.... and on a 740 the positive terminal might be on the passenger side, location not clear but somewhere near the glovebox (on my 92 it was on the drivers side).

 

oh yeah. 86 is also in the 'biodegradable' / 'ecojunk' wiring era (approximately 82 or 83 to mid year 87), where ALL the wiring in the vehicle is subject to crumbling insulation with age and heat. probably a good reason why there aren't that many early 740's left on the road.

 

Yea i did have to purchase a newer wire harness. I did directly wire the brown wire to the battery and that diddnt activate the relay. Hitting the blue yellow wire with my hot wire did get a click but the i injectors diddnt fire and fuel pump diddnt turn on. Ran my hot to the blue yellow wire and pump wire simultaneously and still got no squirt from injectors.

 

instead of hitting random wires with juice, I'd be in there with a volt meter trying to figure out whats going on.

does yours have a 'radio suppression relay' under the hood? later 740's did, I'm not sure when they started... this relay is slaved to the 'system' half of the fuel relay, and provides the power to the injectors. the 'system' half of the fuel relay should come on with the ignition key, while the pump part only comes on when the engine is actually turning over, as detected by the hall sensor in the distributor which goes through the ICU ignition control unit.

 

I looked awhile ago for other reasons and i think the radio relay starts in like 1990 with these 740s. I hooked the hot to the brown just to see if since the relay was grounding the red that goes to the ecm at its end if that would force the relay to connect.

The reaaon i hit the pump wire and the green wire is because i knew green is the injectors feed but i guess the test was incomplete without a computer grounding the injectors.

 

Ignitor sends power to the DISTRIBUTOR.

HALL sensor in the DISTRIBUTOR sends a square wave electrical signal that varies from 5 to 0 volts back to the Ignitor/i.c.u. As the signal rises from 0 to 5V the control unit starts to compute the timing of the next ignition pulse. As the signal falls to 0V the control unit commences ignition countdown and delivers ignition pulse as computed

ECM collects signals from the AIR MASS METER, COOLANT TEMPERATURE SENSOR, THROTTLE SWITCH, O2 and:

grounds the FP relay/pumps..

That's according to volvoclub.org so basically make sure the ICU is getting good input from the hall sensor and the volt signal will prompt the fp to activate along side spark.

Opens the INJECTORS for the appropriate times.so hall effect and icu have to absolutely be working before ecm can even decide to allow the fp relay to be grounded. Thats what i took from it.assuming data coming from 02, air meter, and etc i think it bt default will just put the vehicle in a type of limp mode.

 

1987 per that greenbook I linked has the radio interference suppression relay

 

Been out of the state so i had to leave it alone for a few weeks but i guess my new question involves the order of events. The hall sensor is communicating via the 5 v or 0 v found in the blue wire used for signal. Before any of the signal is sent back does the computer for ignition fire up the 12v wire going to the hall sensor then ground it out with the black wire/third wire on the hall sensor? Is the reference volt from the blue wire basically just the voltage from the red wire being broken up after passing between the walls within the distributor? Is that reference volt wire basically the wire that will after being boosted in the powerstage feed the coil ?