Won't Start, Problem Diagnosis - ECU??

Hey volvo enthusiasts,

I'm helping my buddy out with his car, it's a 1991 240 automatic but it's having a few odd problems. To start off it doesn't run, and we figured out that the fuel pumps don't run or prime, so we replaced both the relay and the main in-line pump, but it didn't help. We found out that the pumps aren't getting 12v, but their fuses are. The pumps will run when I jump fuse 4 to fuse 6. When we jump fuse 4 to 6 and crank the engine, it still won't start. We later found out that the fuel rail is getting fuel, but the injectors aren't getting power. We tried jumping 12v to one of the injectors but it still wouldn't fire. We know that it gets spark at least part of the time because it ran on starting spray. We also pulled a plug and it looked normal, but when we tested for spark on the spot with the plug out of the engine, we were getting nothing. None of the plugs had spark. What could cause it to have spark only part of the time??
We were thinking it could be the ECU since it has the notoriously problematic "pink label" ecu, the one ending in 651 I believe. Does anyone think that this is a problem or does anyone have any other ideas? Thanks!

Also if it is the ECU, can I swap in an ecu from a 92 or 93 240?

Hey nick! The pink label 561 ecu is notoriously unreliable, however your problem could be a couple different things. For starters, all of the 561 ecus I have seen do NOT prime the fuel pumps in KP2. They only prime when they get a crank signal from the crank sensor. I’ll bet both of your pumps are fine. Your problem, to me, seems like it could be the crankshaft position sensor. The harness on them is often tattered.

<strike>Have you tried starting the car on starting fluid? That helps a lot.</strike> oops missed that part. How often will it run on starting fluid? Every time? I have seen intermittent/weak spark from a humid climate. Removing the distributor cap and drying it out very well fixed a crank no start on a friend 245. But if you don’t have an injector pulse that won’t help.

I believe a failed crank sensor will not pulse the injectors and may not let you have spark either, but I’m not 100% sure on that.

And yes, you can swap in a newer lh2.4 ecu. The one ending in 951 is the best I believe.

A no spark condition definitely keeps the ECU from running the fuel pumps. It could be either the crank position sensor or the ignition module. It could be many things, however, those are the 2 most common causes of a no spark condition. As old as the crank sensor is, if it is the original sensor, that is where I would start.

A failing crank sensor would cause a no spark and no fuel injector pulse situation. Or intermittent operation of both.

Pull codes with the obd black box which will confirm both computers are powered up.

Noid light for injector pulse verification.

Thanks for all of the responses everyone. I'll replace the CPS and see what happens. As far as the onboard OBD1 system goes, I couldn't get it to show any codes at all, not even the everything is normal code (1-1-1). Could this point to ECU or is the box itself faulty?

Nickagriffin said:

Thanks for all of the responses everyone. I'll replace the CPS and see what happens. As far as the onboard OBD1 system goes, I couldn't get it to show any codes at all, not even the everything is normal code (1-1-1). Could this point to ECU or is the box itself faulty?

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Absolutely. Assuming you’re doing it correctly with the vehicle either running or in KP2, you need to be able to retrieve codes. Sometimes it’s a poor connection under the diagnostics box but I personally had a ‘failed’ fuel ecu that would not power up the fuel pump relay (and pumps and RSR) when it was below freezing. If I waited long enough, the vehicle would start, especially if it warmed up outside. Reason being is apparently because there’s a cold solder joint that fails on the board. Keep in mind that on a 7 series, the fuel pump relay also energizes the radio suppression relay. So when jumping the fuel pumps at the relay on a 7 series car, it still would not start because the RSR was not powered up. A 240 does not use an RSR so it *should* start when the pump is jumped assuming everything else checks out. Be sure you’re jumping the main pump and not just the tank pump. You could also jump the main pump at the pump itself with 12v and see what happens.

The ecu I had fail was a 556 instead of the 561 you have. Both pink label trash IMO. I was not able to pull codes from the fuel ecu, but I was able to pull codes from the ignition computer. So that pointed directly to an ecu issue. Especially since once I swapped an identical 556 ecu in from a friends car, I was able to pull codes. My 556 in his car was not able to pull codes. So, the problem followed the ecu. I replaced the 556 with a 951 I got for $19, problem solved.

Sorry for the book. I went through a hell of a time reviving this 740 (still am actually) but I’ve learned a lot in the process.

Nickagriffin said:

Thanks for all of the responses everyone. I'll replace the CPS and see what happens. As far as the onboard OBD1 system goes, I couldn't get it to show any codes at all, not even the everything is normal code (1-1-1). Could this point to ECU or is the box itself faulty?

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That means no power to the computers, probably. Or a bunk OBD box. Or you just did it wrong.

Instead of swapping the crank sensor, get a $5 test light and learn to use a wiring diagram.

With test light, key on, power to coil? Blink between coil neg and ground while cranking? Blink between 12v positive and injector ground? It's a very basic tool.

Test light to computer/s power wire? Etcetera.

Do you have power to the MAF?

Hey guys, quick update. We got it running finally by swapping in an ECU from my Volvo (a 1992) and it fired up but took a bunch of tries. Now it has a problem where it dies when we accelerate and will sometimes randomly stall or not start at all. The original problem is definitely fixed though, so thanks for everyone's help!

Your 92 is lh2.4 and not lh3.1, correct? Not sure if it would even start with the wrong ecu but it’s worth asking.

PromiseRing said:

Your 92 is lh2.4 and not lh3.1, correct? Not sure if it would even start with the wrong ecu but it?s worth asking.

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yup, it's a lh2.4. Oh, and the onboard OBD scanner started working with the new ecu, suprisingly didn't give me any codes, just 1-1-1.

With the stalling you want to carefully check for vacuum leaks. Be sure to check the hose that goes from the mass air sensor to the throttle body. That needs to be removed to check it. It cracks on the bottom where you can't see. Also be sure to check the flame trap hoses as the the large ones can crack and cause a big air leak. Check the intake gasket, and all the hoses.

dl242gt said:

With the stalling you want to carefully check for vacuum leaks. Be sure to check the hose that goes from the mass air sensor to the throttle body. That needs to be removed to check it. It cracks on the bottom where you can't see. Also be sure to check the flame trap hoses as the the large ones can crack and cause a big air leak. Check the intake gasket, and all the hoses.

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Thanks for the reply. I checked the ridged hose from the MAF to the throttle body, it was uncracked but had a black sludge in it?? Also the flame trap hoses were completely deteriorated so thanks for the tip there!

I noticed it also had a pretty bad exhaust leak, could that cause stalling?

Nickagriffin said:

Thanks for the reply. I checked the ridged hose from the MAF to the throttle body, it was uncracked but had a black sludge in it?? Also the flame trap hoses were completely deteriorated so thanks for the tip there!

I noticed it also had a pretty bad exhaust leak, could that cause stalling?

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I doubt it. A leak can cause an idle stumble and hesitation, but I doubt it would cause it to stall.