940 & 960 LH2.4 954 ECU running rough/rich

edited September 14 in General

This is kind of a follow-on from this thread where I asked about the chip that was in an 954 ECU that I was yet to test. I've since got the car up and running, and that ECU runs poorly and I'm trying to figure out why.

As of right now, I have two LH2.4 ECUs.

  • A green-label 563 ECU out of a '91 940 Turbo that was running when removed. Working well in idle, cruise and boost.
  • A white-label 954 ECU that came with the car, unknown condition. Barely idles, dies and runs mega rich (~10AFR at idle).

I have a feeling that the 954 ECU is just a dud, because regardless of the fuel chip, it doesn't appear to run correctly. I don't know the history on it. I've been running the 563 ECU on the car for about 1000KM now with zero issues, but I got some chips to swap into the 9xx ECU and would prefer to run as late an ECU where possible.

I haven't fully stripped it down to inspect it to see if there's any blown components but the few electrolytic caps I can see appear to be ok.

Additional info:

  • Running a Spartan 2 wideband O2 sensor with simulated narrowband output feeding back to LH2.4. Works perfectly with 563 ECU. Have run with the O2 sensor disconnected to disable narrowband output and made no difference.
  • Running either the original 954 chip (see other thread linked) or aftermarket chip both give the same rough and rich idle.
  • Can't pull any diagnostic codes or seem to reset codes. When attempting to reset on position 2, sometimes it won't respond, and sometimes after ~5 seconds the LED will come on again.

Startup and idle on both ECUs:


  • Video of startup and rough idle
  • Video of attempting to pull/reset codes and then the light stays on?
  • Video of doing the same, then running afterwards, no difference


  • After running the 954 so runs rougher than it normally would at start) but then idling no worries after that:

Is it possible there's something I'm missing? With exactly the same setup, the two ECUs should behave the same, right? I can't figure out why it runs so poorly on the 954 ECU and works perfectly on the 563.

If you've got a LH2.4 turbo ECU that you have and might wish to sell to me, PM me please! Also got thread here. 😀


  • Might be grasping at sraws here, but -

    • What type of EZ-K are you running? (I presume you're not changing it when swapping ECUs?)
    • What type of lamdasond sensor are you using? (IIRC there was a change of p/n in YM1994)
    • Does it make any difference with the 954 when the engine reaches operating temp?
  • edited September 14

    Wasn't swapping the EZK at all during those most recent tests above, though I have tried multiple different 148 EZK's, ranging from the '91 940 to 1997/1998 148 EZK's that have all been fine on the 563 ECU. They made no difference to the running on the 954 ECU though.

    Regarding lambda sensor, as mentioned in the first post I'm running a wideband with simulated narrowband output. While it works fine on the 563, I figured it would be worth disconnecting entirely to test if it's that causing issues. I disconnected the power for the wideband controller and it seemed to make no difference whether the O2 sensor was feeding it the narrowband signal or not. I think I might have some spare original O2 sensors lying about but could be a pain to revert some of the wiring. I get a 1-1-1 signal on the 563 so figured it doesn't have an issue with the O2.

    Haven't tried the 954 at operating temp as it ran so poorly, but perhaps I'll give that a go by running it on the 563 until it's warm then swapping it out. Thanks for the suggestion ;)

    Any idea why I can't pull the codes at all on the 954?

  • edited September 14

    Warmed it up on the 563 ECU, got it up to temp and then swapped in the 954 ECU. It runs even worse when warmed up. I didn't try to warm it up on the 954 because I'd basically have to sit there with my foot keeping the idle going running rich as hell until it warmed up.

    In the below video it's the car that's hunting except for when I rev it out just a little and later hold it around 2k to see if it doesn't run as lumpy (spoiler: it does).

    While I do get that it does self-learn, I still feel like it should idle at least as good as the 563, which idled no worries from the first crank.

    The only thing that I can think of that might affect things is that the wiring harness might be different and that perhaps it's missing an input somehow? I might need to go through each pin on the ECU and check that they correspond to the values/voltages/resistances that it expects. That said, the LH2.4 connector doesn't appear to have any revisions between ECU versions so... maybe not?

  • edited September 14

    Any idea why I can't pull the codes at all on the 954?

    Given that aspect, and also your followup from a warm engine I'm starting to suspect the 954 is cactus, especially if you can't pull any codes at all from it.

    I had a look at the IPdown site to see if there was any strange business with OBD2 on the 954 which might have disabled the simple ol' LED testing ability on late model ECUs, but that's not the case. So no codes makes the 954 suspect.

    Does the EZ-K throw any codes when the 954 is in use? If so, this may point to what exactly is cactus on the 954 if you're lucky, like, if it's missing or distorting any ignition signals.

    WRT to the lambdasond sensor, my vague recollection was that they changed from PTC to NTC, or vice versa. Details are in TP31361-1 as found in the OZ Volvo Tech Library.

  • I thought it was only the really late LH2.4 ECUs like the 986 that were OBD2, like those in the '97-98 940s, and that we didn't get those (unless it was a private import I suppose). Even then it seems that much like my 850, they weren't actually OBD2 compliant. 😓 I don't mind the OBD1 blinking light setup though.

    Good idea with the suggestion of checking EZ-K codes - I'll give that a go and see if I get anything.

    I've never seen that greenbook before - that's an awesome set of info in there! Thanks for the link, and thanks to James for hosting those files!

    I'm beginning to think more and more like there was a good reason this was found in the pile of spares with the car, with the lid already half-opened. I don't mind doing the troubleshooting but starting with a known-good unit is probably going to be the better choice moving forward. Hopefully I can find someone who is willing to part with their ECU and I won't have to sell a kidney to afford it 😅

  • edited September 14

    Alright so some progress, kinda.

    When I initially installed the aftermarket fuel chip, I placed the chip in the same orientation as the chip that was in there. First mistake, as I should have looked online for the orientation or just looked a bit more closely.

    As you can see in my image in the first post, I reversed it, thinking I had it the wrong way around somehow and the car ran, but it ran poorly. Surprised it ran at all, really.

    I just swapped it back so the notch faces the edge it's closest to, and:

    • Successfully able to get LH2.4 codes (says 1-1-1)
    • Able to get EZK codes also (also 1-1-1)
    • Also can hear the fuel pump prime for 2 seconds on KP2 like the 563 ECU does
    • Doesn't start. Smelled like it was just dumping fuel into it
    • Pulled injector plugs and 100% throttle in case it was flooded
    • Fired up after cranking for while and it seemed to run smooth until it ran out of the fuel sitting in there

    Doesn't want to run with the injectors re-connected. At the moment I'm still suspect of the ECU but at least it's doing more than it was. Hoping I haven't done damage to the chip as a result of having it backwards, but given that it does eventually run when it's clearing the flood, that's something at least.

    Edit: Interestingly, it seems to have the same symptoms as described for a car that has a destroyed injector driver as described here. Unfortunately the only place that might have a suitable transistor is closed. D'oh.

  • jamesincJames @jamesinc Oz Volvo Ice Fortress
    edited September 14

    Put a noid light on the injectors and see what's going on when you crank.

    You can also wire up any 12V lamp but for $10 I enjoy the convenience of this one ^

    I've had faulty LH-2.4 ECU hold my injectors at 100% duty cycle before.

  • That was a good suggestion with the noid light. Though there wasn't anywhere locally that had something, a suggestion I found online got me thinking about how I could rig up something to test it out with, and I remembered I had some LED indicator bulbs I bought years ago and never used. I grabbed it, bent out the pins and voila:

    At this point, I was pretty sure it had just dumped fuel into the crankcase and because I was planning on doing it anywayit was a good time to change the oil and then do a test fire with the LED connected and the injectors disconnected and see if the theory was right.

    There it is. What still baffles me though is that with the fuel chip incorrectly installed, what exactly is happening that is allowing it to run like it was, even if it was running like crap? Maybe it's a special 'feature' bosch implemented 😂

    I'll see if I can source one of those TIP122 transistors to try and get this thing going. Thanks for the tips and suggestions folks :)

  • jamesincJames @jamesinc Oz Volvo Ice Fortress

    I haven't dug deep enough to understand exactly what causes it, but my assumption is that something goes bad in the injector driver circuit (the fat transistors under the heat shield at the top left here):

    Who knows though, I don't have enough signal generators to make the ECU run on the bench. It could be related to the IC being inserted backwards, but the one time I saw this effect it was a bone stock ECU (well, as far as I knew).

  • You're right, it is the left-most transistor that drives the injectors. My best guess is that in a previous life this ECU may have been used with low-z injectors without a resistor pack. I'm in the process of replacing the transistor at the moment through following this article, though I don't have any flux for the desoldering braid and it just doesn't work without it. Can't find any flux locally so gonna pick up a solder sucker as I lost my old one.

    Hopefully then I'll be able to remove the old transistor without busting the pads/traces on the PCB and pop a new one in. It's only a few bucks to try it out and if it works, free ECU.. sorta.

    There's been other people that have inserted their chips backwards with no long-term adverse effects seemingly, but with stuff this old sometimes it's just luck of the draw if you damage things.

  • jamesincJames @jamesinc Oz Volvo Ice Fortress

    Nice, keen to see if it works!

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