Hiya guys

Okay so, I have this 1991 245 that has developed an intermittent stalling/no start issue. I have so far figured out that it is the fuel pump...when I put in the key and don't hear it hum, the car won't start. Sometimes it hums, sometimes it doesn't. Sometimes it hums and then it starts and runs for a couple of seconds and then it stalls out and I'm not hearing the hum anymore.

"Jumper the #4 and #6 fuses!" the internet helpfully offers. "If the fuel pump kicks in, your fuel pump relay is bad!" Well sure enough, when I attach alligator clips to those two the fuel pump kicks on and I can drive it just fine with no stalling. So I bought a brand-new relay from DaveBarton.com and...the problem persists.

At this point I am suspecting the ECU, but that is a new level of expense with the whole "replace the part and see what happens" technique, so I thought I'd ask y'all if that's very likely to be it, or if there are other elements in the chain between those two fuses that could be causing it. This is the first car I've ever worked on and my troubleshooting ability is limited so I am wondering if this is the right point to let a mechanic have at it...or if it really will be simply plug-n-play ECU replacement.

Thoughts? Thanks.

 

if the ECU was bad, the car wouldn't run ever.

make sure your 25A fuse under the hood is good. your jumper is bypassing that. those fuse holders rot away with age, I replaced mine with a waterproof marine one I got at a boating store.

the control sequence goes like this: key on, and the ECU (fuel injection control unit) turns the fuel pumps on about 1 second, then off again and waits for pulses... the crank position sensor is connected to the ICU (ignition control unit), and tells it the engine is turning, the ICU in turn passes those pulses on to the ECU so it knows its turning and it turns the fuel pumps back on.

 

This is great info, I will check out the 25A fuse holder first, thank you.
If that's not it, where would you look next?

 

hmmm. I think I'd be in there with my multimeter probing the pins of the fuel pump relay socket. It *IS* conceivable you got a bad one as a replacement, but both being intermittent the same way is rather suspicious, however the fact that the car drives with the fuel pump relay bypassed, really does suggest that relay, or maybe the fuse 6-4 wiring...

OH, just opened the 1991 240 wiring diagram to double check this. NO 25A single fuse holder under the hood, it uses fuse 6 in the main fuse panel as the injection fuse.

that relay is actually TWO relays in one package, one powers the ECU etc, and the other powers the fuel pumps.

• at the socket, pin 30 [3] (red wire) should always be hot, this comes directly from the 25A fuse, and powers the /1 coil and both contact sets.
• pin 86/1 [4] (yellow/black) is the control signal from the ECU telling the main FI relay to come on
• pin 87/1 [1] (orange and/or brown wires) powers the ECU, the injectors, the idle air controller, and the MAF, and the coil to the other relay in there ...
• pin 85 [6] is said /2 relay coil power, wired to 87/1 via an orange wire
• pin 86/2 [2] (blue-green) comes from the ECU telling the /2 fuel pump half of the relay to turn on
• pin 87/1 [5] (yellow-red) is the fuel pump power, goes to fuse 4 and the main pump, fuse 4 goes to the tank pump.

the [] numbers are an alternate socket numbering scheme shown on some diagrams.

so I might just slip wires around pins 85 and 86/2 and leave a 12V test light hooked up (if its an LED, put the + side on 85 and the - side on 86/2)... when that light is lit, your fuel pump SHOULD be powered if the relay is working.

 

I eventually found a poor connection at the fuse box. After pulling it out I discovered a loose cobbled-together jumper cable linking the #4 fuse (fuel pump) and the #3 fuse (heater fan). A yel/red wire that should have occupied the jumpered lug was just hanging loose.

I tightened & resecured the jumper cable and now the car is running as it should without stalling. However I can't help but wonder why someone did this in the first place and what it's a workaround for?

 

You never know why people do crazy things to cars, then complain about the results. I see it every day. Volvos especially have the reputation of being easy, and this on top of them getting cheap and being the last resort for many, makes people attempt imroper repairs--sometimes the car tolerates the incompetence/stupidity, sometimes not...