Any harm in trying a 951 ECU in a 940?

[Brontes II]

Hi all,
I've got a super base model 94 940 automatic, and I'm looking at ECU options to increase my idle speed a bit.

Why? The car has a mechanical fan setup. Again why? The e-fan setup had failed/overheated under the previous owners care and blew the head gasket. The repair work on both were pretty sketchy and both the head and e-fan wiring ultimately had to be trashed. I made the decision to "downgrade" to an old-school clutch fan for simplicity and peace of mind.

As far as my idle speed goes, the car is currently equipped with either a 943 or 946 ECU. (I need to open up the bracket again to confirm which.) And it has a target idle of ~600 RPM. It's my understanding that this is an emissions thing and it's a common speed for e-fan equipped cars because the engine is under slightly less load than mechanical fan setups (like cars equiped with the 951) which often run in the ~750-900 RPM range.

I've been able to get away with the factory 600 RPM idle for a while, but it's always been a little anemic, and as I've started fixing other stuff like the A/C and power steering, the idle load has increased and it's gotten to the point were the engine gets a little stumbly when stopping hard or sitting at a light with the A/C going. After playing around with the throttle a bit and trying to cheat the base idle, I'm realizing just how good LH 2.4 systems are at maintaining their target idle speed. If I adjust the linkage to sit in the 800 RPM range it runs amazing and brakes great for a run around the block and then promptly compensates to bring things back to the 600 RPM target.

I'm thinking an ecu from a mechanical car would solve the problem, and I think with 951 is probably the best option since I don't have to worry about the e-fan signal, or cold-start injection. It's normally a 93/Classic 240 ECU, and/or a drop in replacement for the older pink lable ECUs in a bunch of older 240s. Is there anything from a pinout perspective, etc. that I'm overlooking would be an issue with dropping this into the 940? Or are there better options I should consider instead?

Any thoughts are appreciated. Thanks

 

[OVERDRIVE]

It would plug in and run fine, as you said no fan driver. I've ran a 933 (740 NA EGR, fan driver) and a 556 (240 EGR, no fan driver) and the idle speed was the same. In fact I could not tell any difference between them. I briefly had a 946 equipped 940 and it idled at a normal speed.

I think your tach is inaccurate or you have a vacuum leak.

 

[NLRegina]

Just a thought, but are you sure that your idle valve is functioning right, and your idle is set properly?

 

[Brontes II]

It would plug in and run fine, as you said no fan driver. I've ran a 933 (740 NA EGR, fan driver) and a 556 (240 EGR, no fan driver) and the idle speed was the same. In fact I could not tell any difference between them. I briefly had a 946 equipped 940 and it idled at a normal speed.

I think your tach is inaccurate or you have a vacuum leak.

Thanks for confirming that it's safe to try.

I've considered vacuum leaks. I mean, it's a 30-something year-old beater. It's got at least a few minor ones, somewhere. But most of that stuff has been addressed within the past few years. small manifold hoses are newish, main intake pipe is new, IAC and flame-trap hoses are new, heater valve vaccuum hardline is capped off for now. A/C vent selection works, but it can be a little slow to switch, and I do lose a little air from the dash under load but it doesn't go out completely. EGR stuff was blocked off be the previous owner. It's kind of janky looking but I think it's holding well enough. So yeah probably some small leaks, but nothing that I think would account for ~150 RPM of idle. I guess the brake booster could be worth looking at. But otherwise I think the vacuum situation is pretty good-ish right now.

Also, while trying to cheat the idle up a bit via the throttle linkage I tried inducing some intentional vacuum leaks just to see what would happen. The ECU immediately compensated and held the idle at ~600.

Tach is probably inaccurate too. I've just been using the in-dash one. I know, I know. Putting a meter on it and making sure everything is set back to factory speck linkage-wise is on the list for tomorrow. But I don't think it's super far off. The car sounds like its got a very laid back idle. it always has. At least compared to how I remember my older 240s idling. Am I misinformed about the idle speed on LH2.4 cars being more or less completely set by the ECU? When I read over stuff on setting the base idle on the later cars it seems like it just confirms no components have completely failed and that the cables and linkages are set within a comfortable window. I haven't seen any info on changing the actual ideal speed itself though.

 

[Brontes II]

Just a thought, but are you sure that your idle valve is functioning right, and your idle is set properly?

As far as I know. I picked up a reputable rebuilt one just so I could take my time cleaning and testing the one that was originally on there. The new valve might run ~50 RPM higher than the old valve. Maybe. Otherwise no noticeable difference. See the above reply for info on the idle speed settings. I guess the answer is kind of. Or it's at least in the factory ballpark. But, the more I mess with it, the more I'm thinking the ECU is just going to keep things were it wants them, unless there's a major, major issue it can't compensate for. I could absolutely be wrong about that though.

 

[OVERDRIVE]

The idle is 100% controlled by the ecu on LH2.4. I'd follow the greenbook procedure to reset the throttle body and make sure it's running correctly. In the pocket book it shows idle speed for all of the engines and if I'm remembering right, all 8V LH2.4 engines idle at 750rpm.

 

[NLRegina]

As far as I know. I picked up a reputable rebuilt one just so I could take my time cleaning and testing the one that was originally on there. The new valve might run ~50 RPM higher than the old valve. Maybe. Otherwise no noticeable difference. See the above reply for info on the idle speed settings. I guess the answer is kind of. Or it's at least in the factory ballpark. But, the more I mess with it, the more I'm thinking the ECU is just going to keep things were it wants them, unless there's a major, major issue it can't compensate for. I could absolutely be wrong about that though.

I think your idle should be 775+-50

W/ Bosch Electronics — 1994 Volvo 940 L4-2.3L SOHC VIN 88 B230F Service Manual | Operation CHARM

Detailed repair manual for the 1994 Volvo 940 L4-2.3L SOHC VIN 88 B230F.

charm.li

further, here are the correct idle adjustment specs:

Bosch — 1994 Volvo 940 L4-2.3L SOHC VIN 88 B230F Service Manual | Operation CHARM

Detailed repair manual for the 1994 Volvo 940 L4-2.3L SOHC VIN 88 B230F.

charm.li

I'm suspicious that your 600rpm idle is not in fact a normal condition and I would suspect a failure somewhere along the line. Ironically enough, if everything else is in spec and working, you could have a failed idle circuit on your fuel ecu in the passenger kick panel, which would be fixed with a replacement ecu. I had such a failure on my 1991 with Regina.

 

[bobxyz]

As has been suggested above, check that your tach is accurate and check that the idle valve rotates smoothly. You can clean it with MAF cleaner, but don't use carb cleaner/brake cleaner due to plastic parts inside.

LH2.4 idle in park is ~775 rpm. The ECU will bump up the idle speed slightly when shifting from park into gear. The ECU will also bump up the idle speed for the A/C.

You can use the special diagnostic test modes 2 and 3 to check the ECU signals and to cycle the idle valve. Unplug the injectors first to prevent dumping fuel into the cylinders.

Engine and OBD Diagnostic Codes<

Volvo Engine and OBD Diagnostic Codes<

www.volvoclub.org.uk

 

[2manyturbos]

Your 946 ECU is likely not the issue. The way you are attempting to control the idle is an issue. The ECU needs to do ALL the idle control. Go through the throttle plate and TPS adjustment proceedures and follow them to a T. Make sure your TPS is actually functioning correctly. You do not set the idle with the throttle plate stop screw. That is there to have the plate barely open so the throttle doesn't completely close when releasing the throttle.

 

[Brontes II]

Thanks for the response, everyone. Specifically, thanks for steering me away from the idea of an ECU swap magic bullet solution. Please excuse my ignorance. This is only my second LH2.4 car, and the first with a factory e-fan. Also, I'm kind of stupid. In theory, a difference in idle speed between ECUs seemed reasonable enough to me.

Also, please excuse the slow reply. After realizing how off-base my understanding was, I needed a day to read over the LH2.4 EX116K Greenbook posted on Oz, gather some supplies, and prep a couple things under the hood. (More specifically, I realized that the tach and dwell circuit on my ancient multimeter was dead and had to order a new one. And that my good feeler gauge set was too wide to fit around the throttle linkage, so I needed to buy and trim down a cheap set. And that the OBD connector on the car barely worked and needed to be cleaned and re-sleeved. General, annoying, not-being-fully-prepared stuff, etc.)

After spending a day messing with it, results are mixed bag. All linkages, screws, and cables adjusted back within spec. These were all a little out from me playing with them. TPS was readjusted per the book and now sits just a hair further counter-clockwise than before according to my original sharpie mark. Things audibly click when they should and don't when they shouldn't. As far as I can tell, all throttle-related things are as they're meant to be.

I'm still working my way through the OBD tests. So far, Test Group 1 (socket 2, single button press) doesn't show anything abnormal. I did try disconnecting the IAC while the engine was running, which caused the idle to surge to around ~1500, (according to the dash tach) and threw code 2-2-3, but this cleared with a battery reset and the test now reads 1-1-1. Otherwise, I'm still working my way through the Group 2 tests, but if I'm understanding the guidance docs, the ECU seems to at least know when the A/C is on. Which is a relief since some of that circuit has been modified. (More info on that to come.)

Regarding vacuum leaks: soapy water and tube-to-the-ear tests haven't revealed anything new. The obvious stuff has recently been done. Misc. small manifold hoses all replaced. IAC and PCV hose completely replaced. New oil separator, even. Throttle body and intake gaskets are maybe 3 years-old. Air box filter changed and AMM recently cleaned along with new main air duct install. I also temporarily capped-off the charcoal canister vacuum ports at the throttle body, and (likely permanently) capped off the cruise control vacuum hard lines. Still not saying there aren't any vacuum leaks. I just can't find them.

Regarding the in-dash tach readings: They're definitely off. The readings on the new multimeter are pretty jumpy, but by my estimate they started out averaging around ~780 RPM (before adjusting anything) and ended up averaging in the low 800s (after following the adjustment procedure). I had to use an over-wire inductive pickup between the coil and distributor to get a reading so there's probably some noise in the reading. But I can also still see and hear the idle speed adjust itself fairly constantly. Seems more stumbly than usual, but I'm also really watching it just now.

I sanded the quick-connect spades on the coil. (Though it just now occurs to me that I didn't remove and clean the entire 3-way connector where it mates to the coil.) Female connectors were similarly cleaned. Readings at the dash now range from ~500-750 RPM and will occasionally drop to zero when stopped. It did this dropping-to-zero thing for a while maybe a year or two ago. But that cleared up spontaneously on its own so I never really looked into it. I guess the next step there is to pull the cluster?

Around the neighborhood driving tests today were so-so. It feels a little less stumbly on hard stops with the A/C going. But you can definitely tell its struggling a bit somehow. I noticed when watching the tach (regardless of accuracy) and listening to the engine while driving, that it looks like things are trying to compensate for the A/C fan and/or condenser kicking on, but maybe not very well. The tach will tends to briefly tick up and then settle slightly lower than it started. As this repeats the reading slowly goes down. This obviously, isn't completely accurate, but the engine noise generally does change in synch with the tach.

Beyond all the incredibly long-winded stuff mentioned above. The only potentially related things I noticed today were 1) my crank pulley is starting to wobble and I'll need to replace it next timing belt change, if not sooner. And 2) the in-rush current from the condenser fan impacts the engine more than I remember, so I may need to rethink that. (It's a Four Seasons stock-equivalent fan mounted to a non-stock relay triggered by the original high-speed fan wiring circuit. So no soft-starting or wind down. Just full on, full off based on the high-pressure switch.)

At this point I'm thinking I'll drive it as-is a few days, work my way through the rest of the OBD tests, and probably take a closer look at the basic spark stuff since I haven't in a couple years, and the car has seen nothing but A to B city driving for most of that time.

As always, any additional thoughts are appreciated. Thanks again.

 

[redblockpowered]

Rule of thumb: it's NOT the ECU.

Obviously this isn't literally always the case but I would pretty much always rule it out to start.

 

[NLRegina]

Rule of thumb: it's NOT the ECU.

Obviously this isn't literally always the case but I would pretty much always rule it out to start.

It isn't until it is - I was driving myself insane trying to resolve my lack of functioning idle air control valve despite the valve working smoothly on a bench and the ecu detecting it in diagnostic modes, as well as it working on the first start after a battery disconnect