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LH 2.4 ECU interchange

CountOfNowhere

Active member
Joined
Mar 24, 2004
I have a 1991 940 16v that I am pretty sure has a dead ECU. According to the documentation I can find, it's a 5xx ECU.

Hoping someone can confirm a couple things I've pieced together:

1. 5xx and 9xx ECUs are wired the same and can be swapped
2. Chips do not swap between 5xx and 9xx ECUs

I read a comment that indicated 16v and NA 8v software was similar enough that I could run an 8v NA ECU on my 16v. Based on reliability reports, I'm thinking that is potentially a better call than finding a 5xx ECU and swapping my 16v chip in.

Does anyone have any thoughts or experiences? I haven't removed the ECU yet, and the ECU charts don't have the 940 16v listed (I'm just assuming it's the same as a 1991 740 16v).
 
Good to know you've got an ECU! I wanna double-check my diagnosis thought process and want to see if I can close out the couple of remaining items. The speedo jumping makes me think it's a bad ECU or bad crank position sensor, the lack of fuel pump power makes me think the ECU has failed (or I'm badly misdiagnosing and the fuel injection relay is bad).

Symptoms: Previous owner said it died one day. Crank, no start, speedometer jumps around on cranking.

Confirmed:
- no voltage at fuel pumps on initial key-on or crank
- there is power at the fuel injectors at key-on
- got spark on #1 on cranking
- Jumpering fuel relay powers on the fuel pumps, plenty of fuel on the return side of the regulator
- Confirmed trigger wire at radio suppression relay works (key on it has voltage, key off it doesnt)
- Jumpered the radio suppression relay and tried starting the car with FI relay in place as well as jumpered (no voltage at fuel pump when pump isn't jumpered)

Still to confirm:
- ECU is triggering FI relay?
- ECU is getting power?
- My understanding of the FI relay may be incorrect. Need to find a wiring diagram, maybe I need to jumper more than just the fuel pump pins?
- Crank position sensor is functional? (I really want me a scope)

Does the ECU sound like a logical conclusion, pending the remaining three checks? I am going to poke at it later today and see if I can't confirm the remaining items.

edit:// I know this is referring to Rex, but I suspect it applies in this instance: Is spark triggered by the crank position sensor? This thread makes me think it is, which would indicate (in this case) the crank position sensor is fine. http://forums.turbobricks.com/showthread.php?t=265164. I originally thought there was a hall sensor in the distributor that controlled spark but I'm often wrong about this sort of thing).
 
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The ECU controls the relay by grounding the control circuit of the relay. You can take a meter and check if the computer is grounding the relay when cranking. It also should ground the relay key-on for a quick prime too.

A noid light is the fastest tool for checking injector pulse. Or the screwdriver handle to your ear while cranking. Tip to injector.
 
Hang on....

You could check if there is current at the MAF blue yellow wire/key on. Use a test lamp.

If not current there, the relay is bad or the circuit, or the computer. You could use a meter to check if the computer is grounding each of the 86 terminals. Or make a Y shaped jumper wire and connect the juice from pin 30 to each of the 87 terminals and the car should run while you get a new relay or computer.
 
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The 16v ECU's and the 8v ECU's are different inside. Different board, everything on the board arranged differently. 5xx and the 9xx's. Not sure if you could put 16v chip in 8v ECU and it would work. I never tried that.

Was this that car in vancouver? The 16v 940gle that didn't run? I was thinking about trying to buy it. Snooze you loose. I also have extra 16v ECU's. Not selling them....
 
The 16v ECU's and the 8v ECU's are different inside. Different board, everything on the board arranged differently. 5xx and the 9xx's. Not sure if you could put 16v chip in 8v ECU and it would work. I never tried that.

Was this that car in vancouver? The 16v 940gle that didn't run? I was thinking about trying to buy it. Snooze you loose. I also have extra 16v ECU's. Not selling them....

That's the one! Ended up being a fuel pump relay :)

Funniest of ****, there was an OEM fuel pump relay in the radio pocket thing and bam, started right up.
 
:woowoo:

Glad you got it.

Me too, thanks for the diagnosis help! The relay was getting grounded and power was making it to the MAF as expected. Did I mention that there was an OEM Volvo relay in the radio pocket? Replaced the janky aftermarket one and the ****er started right up.
 
Checked for burned terminals. The fact that somebody was in there may mean its burned up a few of them before.
 
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