1987 740 GLE is blowing fuel pump fuses when the ignition is turned on. We have replace the fuel relay, both fuel pumps and it still blows fuses. The wiring from the pumps is fine. We checked each wire from plug to fuse. The fuse blows with the pumps unplugged completely. The fuse does not blow with the fuel pump relay unplugged. I took apart the new and the old relay and they are both fine. Solder joints and wires are all in great shape. I have the dash partially removed and I all the wiring harness exposed. I cannot find any wiring insulation cut.

Could the ignition switch be causing the short? Is there something under the hood causing a short? All the other fuses have no problem. The only other thing not working is the overdrive. I think it may need a new relay. Could that be related to this? My son and I are new to Volvos, but experienced car guys. Does anyone have any new ideas for us? Is there another components that I'm unaware of that I could cause this? I have a Haynes manual, but don't know what to look for. Thanks.

 

ideally, you want the 1987 740 greenbook wiring diagrams, they are the only thing that's accurate.

on a 92 740 (thats all I have a wiring diagram for), the same fuse #1 powers the main fuel pump and the ECU and a bunch of other related stuff. if you're blowing fuses, there's a short to ground somewhere. on my 92, the fuel pump relay is also the ECU power relay (its a double wide with two relays in one unit), so I think I'd pull that relay, and check and see if any of its power outputs are shorted to ground, on mine the outputs are relay pins 87/2 (fuel pumps) and 87/1 (ECU etc)

In addition to the ECU itself, 87/1 on the 92 740 w/ the b230f engine also powers the idle valve, it powers the 'radio suppression relay' coil but not the switched side, and it powers the mass air flow sensor... Most all of this is blue/yellow wires on my 92 diagrams, except for the purple wire to the fuel pump. on this diagram the aux fuel pump in the gas tank and the Lamdasond O2 sensor is on a separate fuse 11 thats powered off the same 87/2 pin as the main fuel pump.

this diagram, http://www.k-jet.org/files/greenbook...g_diagrams.pdf is of an 1988 760 and the LH2.2 B230FT turbo version also shows the fuel pump powering as pretty much the same as my 92 LH2.4 B230F diagram except the 760 has different fuse numbers(!!).

sooo. you could pull the fuel pump/injection relay and I believe I'd use an digital multimeter in 'ohms' between pin 87/1 and ground, and between 87/2 and ground, and see if there's a dead short on either one. if its at least a couple ohms, it shouldn't be blowing fuses. Note that there's no pin on the relay socket thats connected to ground, you'll need to get the ground for this test from somewhere else, like the main dashboard ground point where all the wires are bolted to the firewall, I believe behind the glovebox (been awhile since I was in there, different cars blur together).

The OD is pretty much an entirely separate circuit. if the OD light never comes on, it could be a OD switch, wiring, or relay problem. note the OD 'up arrow' light means that OD is DISABLED, not that you're in overdrive. with the light out, the transmission should shift into 4th aka OD on its own at speeds above about 40MPH with no-to-light throttle.

 

Our fuel rely does have two relays inside. And, with the key turned on, the fuse will not blow with the relay out. You can leave the key on, plug in the relay and watch the fuse blow. I will check for grounding. I didn't know that fuse ran other things. Maybe the ECU is bad. Thank you very much for your ideas. I will try them tonight and report back. I appreciate the quick response. I like Volvo people.

 

which fuse is blowing? per my wiring diagrams, one fuse is the ECU + main Fuel Pump plus some other stuff, while the other fuse is the in-tank aux fuel pump, and the lamdasond o2 sensor.

 

It is fuse #1, listed as fuel pump/engine control/ETC, that keels blowing. We put a digital multimeter on the 87/2 power out on the fuel relay socket and a good ground. The meter read 4.3 ohms. The same test on the 87/1 had no resistance. So I think that means follow the wire out from 87/2?

 

no 4.3 ohms is reasonable, 12 volts into 4.3 ohms is about 3 amps, which sounds like a typical normal load for the fuel pumps to me. 0 ohms would be bad, thats a short.... follow 87/1 and the blue/yellow wires which go various places....

ok, here's the schematic I'm using, which is, as I said before, for a 1992 740 LH2.4, you have an LH2.2, but at least as far as I can surmise, the fuel pump and this stuff is all pretty much the same.

(click for bigger)

I'm putting 50-50 its a wiring short somewhere, hopefully, this list of connectors will let you isolate it. all the wires are blue/yellow per my diagram.

your relay is 2/13. leave it out for these tests, each test point I describe, check for ohms to ground.... if pin 87/1 is shorted 0 ohms to ground, I'd first try and seperate connector 'A' and see if the short goes away., this is a big rectangular block connector with a lot of pins (like 4 rows and 12 or something columns) at the right A post under the dash.... this signal is pin 9 on that connector (blue-yellow). after unplugging it, check both the plug and socket side of pin 9 for a short.

4/23 is the ECU, and this signal goes to pin 9 on the ECU, so I'd unplug the ECU and see if the short goes away. The power goes through pin 3 of connector "D" in the back right corner of the engine compartment, this goes to the idle valve 8/5. the signal goes to the 'radio suppression relay' 2/12 in the engine compartment, which I think is behind the battery (pin 2, blue/yellow), you could disconnect this relay and see if the short stops. it also goes to pin 4 of connector "Q", a 2x2 connector in the back right corner of the engine compartment and thence onto pin 5 of connector "R", a 4x2 block in the back LEFT of the engine compartment which is connected to the mass air flow sensor 7/17 in the intake air pipe (also blue/yellow wire)... you could unplug the MAF and see if the short stops.

 

Ok. I think i'm on to the problem. I was reading the manual on the relay locations and noticed the fuel pump relay is also called the injection system relay (similar to what you said). That made me think about following the injector wires into the engine compartment to the fuel rail. On the surface they looked great. But when I followers them down to the large diameter sheath and pulled them away from the block, the wires were exposed... not just the ground wires, but the injector leads too. As I started pulling the harness away, it fell apart literally. The rubber turned to dust. So I need a new harness before going any further. I'm relatively certain this is the source of the problem. I priced an engine harness online and found them from $361 to $440. Wow. That is too much to spend on outlet $1000 car. So unless you know another source, we will just use this as a pattern and rebuild it reusing the connectors. If that doesn't solve the issue, we'll at least be on the right track. I wouldn't be surprised if this whole thing may have ruined the ECU. We'll cross that if we get there. Hopefully the fuses did their job and its ok. So tank you for now. We really appreciate all your advice. You are awesome! Any ideas for a new cheaper engine wiring harness? Take it easy.

 

you have one of the biodegradable wiring harnesses

volvo had that problem in the early 80s, when Europe first started with the Reduction of Hazardous Substances (RoHS) laws. The new formula plastic insulations proved to not be as durable as the old plastics... They sorted this out and found better materials. My 87 240 hasn't had any wiring problems, but earlier years did.

Mercedes apparently had this problem right into the 90s (they also continued to use KE-Jetronic CIS systems long after Volvo had switched to LH Jetronic, eeek)

re: harnesses, phew. I think I'd get the greenbook specific to your year 740, these are like $30 or something direct from Volvo, it has the complete wiring diagrams for every inch of your car. then I'd get a bunch of different colors of automotive grade hookup wires in different gauges as appropriate... I believe you -can- get the crimpon connectors for all the different sorts of connectors, so you use a slender tool to pull the old pin out of the shell, then crimp a new pin onto your new wire and reinsert it into the shell. rinse and repeat.

crimp-on pins and sockets...

(click for larger)

I do believe I'd go through the wiring diagrams in the greenbook, and make a wire list. the greenbook shows the general layout of the factory harnesses too as well as the location of all the connectors, ground points, etc, so you can use that as a guide for how to route the wires in your homemade harness. record the colors of the wires you use for future reference. try and use red wire for red stuff, brown for brown, black for black, and wing it with the other colors. wrap it all in spiral wiring loom when you're done (electrical tape is a poor choice in a car, it will go gooey with age).

but thats all theory, I've never done this to a car (I have an electronics engineering background)

 

here's your harnesses if you want to double check their prices, look up the part numbers on TascaParts.com ...



Harness 1 (engine) is only $400, harness 3 (main harness) is $530 for a 87 740 B230F

 

Thank you so much for your help. We were getting a little discouraged and feeling like we needed to go back to a more familiar type of car. But we like this Volvo so much. What I'm trying to teach him is to figure things out and find the resources to fix things himself. But I'm excited now to find this type of forum and help. These solutions are exactly what I wanted my son to go through. So when we have it all done he can appreciate what we did to make it happen. That is how car guys come to be. I was hooked the first time I had to rebuild an engine after troubleshooting for months. It was one of my most satisfying accomplishments. So thank you for your help. I will post more and a picture of the harness when we are finished. So we will post with you then. You've been a great help.

 

Pierce... our problem has bee solved. After removing the engine wiring harness we laid it out on the table and started cutting out 2' sections of bare wire that had insulation falling apart. We used our fingers to dig back all bad insulation until we reached insulation that can't be pulled off. We bought assorted 14 gauge wire and crimp connectors and replace the bad wires one by one. A lot of wires were ready to break away from their plugs, so we pulled the plugs apart and ran the wire all the way. Then we bought some heat resistant wire channel tubing and a buck of zip ties and went to town. In the end it almost looked new. We installed it, tightened a bunch of other connections we found. It fired right up like a charm! Now we are back on track reassembling the interior and dash. I have a cool pic of the process, but couldn't figure out how to add a photo. So thanks again for your help. Take care.