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I have a 1994 940 Turbo wagon. I bought the car in October and have not done a whole lot with it. I did install new plug wires, cap and rotor. When I first got it it started okay but did have a check engine light on. I have not had a chance to look into it. Lately it has had a difficult time starting and the only for sure way for me to get it to start is to unplug the air mass meter start it let it one for a couple of minutes and then plug the amm back in. It is then fine for the rest of the day running errands. Next morning same thing. I have plugged in a known ignition stage unit and tried both the coil and radio suppression relay in my 1992 740 turbo. I ran the code on it and got 113 which I think pointed to wiring for o2 sensor. I also checked the tps and was not able to make the one lights stop blinking when opening the throttle plate.
I was going to check the ect sensor. Any tips for getting to it? Thanks.
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I’m going to replace the tps as that is not working via the test on the diagnostics. I will report back after that.
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Hi,
That seems to be a logical move because if the ECU doesn’t know you want to idle or power to drive away it’s liable to do what ever it thinks it should do with the incoming RPM signal.
Something similar has to happen with the unplugged or missing pins in the AMM.
I guess limp mode is meant to keep you from being stranded but not as frustrated like a Throttle Position Sensor, a sticky Idle Air Control or really twisted as a half broken Crankshaft Position Sensor can.
At least you are finding something to keep us waiting in our armchairs for a resolution, so I will keep only one leg up on a my hassock. 🤫
Phil
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Disconnecting the AMM immediately forces the ECU into "limp home mode", which operates the engine under pre-programmed conditions for fuel mix and timing advance, ignoring a number of input sensor signals. The engine will run a bit rough at idle, will be hesitant on acceleration, run with less power at moderate rpms, and act like it's governed at anything much above 2,600 rpm.
AMM air flow sensing is the main input signal ignored. If the engine now starts and runs, albeit roughly and hesitantly, then a faulty AMM goes to the top of the suspect list, by far.
One thing you can try is using spray contact cleaner to thoroughy clean both ends of the AMM connector. Also when connected, peel back the rubber boot and making sure the wires are pushed in. The female contacts are supposed to be packed with dieletric silicone grease (spark plug lubricant), also with a wipe around inside the lip of the rubber boot, for moisture and corrosion protection. Certainly worth a try as you could have a marginal connection, although I somehow doubt it if your sympoms are as consistent as you indicate.
Off the top of my head, the only other input signals that might be forced as ignored in limp home mode are the ECT and O2 sensor. Neither is critical for engine starting and initial operation. OBD diagnostic codes pointing to faults such as fuel trim and the O2 sensor can often be red herrings after a no start, rough running, stalling situation. A failed ECT often results in a poor base idle before it starts affecting engine response. A failed O2 sensor by itself usually just affects fuel trim (rich/lean), not starting.
You could try spending time trying to diagnose, test, and confirm if the ECT and O2 sensor are healthy (which they may or may not be), but IMO, the fastest way to get on top of diagnosing your no-start is likely to swap in a known good AMM, reset the OBD diagnostic codes and take it from there. Can you beg/borrow one for a test? Your '94 B230FT is LH 2.4. Any Bosch AMM with the -016 part suffix will work from an '89-'95 940/740/240. It never hurts to have a spare anyway. Used is okay, especially for a spare.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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Thank you. I should have mentioned they the current airmass meter is known good one that worked perfectly in my 1992 740 turbo wagon.
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Your AMM having worked in your 740 is encouraging, but not a guarantee your current ECU is always getting a good signal, such as first thing in the morning when everything is cold and connections possibly damp. It could be the AMM has turned bad or, as I suggested, a marginal connection in the associated wiring between it and the ECU.
Time to start using the OBD system for more clues. Refer to the FAQ here for detailed instructions. I presume you're running around with the Check Engine lamp on. Forget any current codes stored, clear them (port #2 and #6) and wait for them to come back fresh so you know what it's currently seeing. You need to begin by clearing them when you've got a no-start situation happening, so first thing in the morning. Begin with the AMM connected, attempt starting a few times. Keep checking for the CE light, immediately read and record all new codes. Now unplug the AMM, start and run. You will immediately get a CE with a missing AMM signal code (now in limp home mode). It will continue collecting any additional codes. Now plug the AMM back in, clear the codes again, restart and carry on running and driving, collecting any further codes throughout the day, periodically checking for any new codes (until you see the 1-1-1 on both ports or the codes repeat when you reach the max 3 code limit). At the end of the day, do a final read. Please report back. BTW, you're not supposed to disconnect and connect the AMM with the engine running or the ignition on, else you risk arcing and possible electrical damage to the ECU or AMM -hope you haven't been doing that. Also, once it's running in limp home mode, I believe it stays in limp home mode until the engine is turned off.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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I thought my AMM was bad but it was a stuck O2 sensor. If you pull the fuel rail and the coil wire , connect the grounds up that are held by the fuel rail bolts, put some rags down , turn the key like you are trying to start, then go look at the injectors. make sure they arent; stuck open and letting the fuel rail dump into one of the cylinders.. I had one that would not shut off..
the car would run pretty well once it cleared out all the gas from that cylinder.
I have also seen green connectors at that AMM plug and cleaning fixed it.
i tried using that AMM spray cleaner but later realized its' not supposed to be used on heated element ones, which you have. it never killed mine but maybe It's not recommended. I noticed you can buy the wire part from rock auto or you can exchange them for rebuilt ones. Sounds like yours is tested and proved ok and it's an expensive part to buy just to swap and see.
probably not your issue but have a look at the wiring to your TDC sensor it might be rough if never replaced. If that wire looks hardened up I'd replace that as well.. same with the old O2 sensor.. those two aren't too hard to swap out.
check any wires around the ignition , coil wires, the wires to the transistor ( power stage) all the spade connectors in that area. also grounds like the ones for the fuel injectors and any other grounds.. battery cables, I had a no start issue and the coil wires ( the ones that screw on and have brass spade connectors, the wires were ok but the bolts on the distributor got rusty and that was killing my coil ..
My 240 has an engine fues in that area on the DS innner fender, not sure if your car does but if you find a fuse or a holder in that general area, check the contacts.
It's possible to upgrade to carbs which was a much more reliable system but you need the old style manifold, carbs and distributor and you can use a smaller electric fuel pump and keep the in tank pump as is. my brother did 3 cars and ditched the ECU's , a bit less power but way easier to keep going. older cars with carbs experienced way fewer of these difficult to solve electronics issues. If you do that you may also need to think about how to make the speedo work.
the problem isn't so much that the electronics were badly designed, they just added a lot of complexity and from there forward almost every car made has problems of becoming unique due to the automakers redesigning them so frequently. Its good for the dealers because people get stuck and go spend money but it's also the reason I'd never buy a new volvo.
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Hi,
Dave Steven has hit on all the very valid points of a no start with the AMM unplugged scenario.
It does exactly as he says the management system resorts to the very basics just as he says.
What you have is not enough other information to point you away from what you think is working each time.
While I was reading his response on the connector pins having moved UP in the plug.
It was a wise choice especially since you have already traded out the AMM.
There’s no telling how many times that has been done before you got the car.
I believe it is truly something else and if Dave had more information he would definitely changes lanes and look for another exit.
The man is a very sharp diagnostics person and conveys his thoughts to his posts.
In saying that, he is like the others on the BRICKBOARD and can be of limited use from an armchair.
We do not have eyes, hands and ears on the car and FaceTime still has its limitations.
The No Start scenario in the morning but does well the rest of the day is more of a lack of fuel since you should have a good tune going on with the ignition system. IF EITHER AMM works, the rest of the day, I say discount it as the problem. For the most part they, the 016s, are very reliable.
Refrain from using a spray cleaner, especially, if the AMM has the stretched fine wire, they are delicate.
Several Brickster’s have questioned if they could have ruin an AMM with that stuff.
On Other mechanical vane types or solid state sensors the AMM’s still can benefit from a spray.
Several Hondas have been fixed with it so I have a can of it but not the Honda cars.
Just my relatives or neighbors have seen it work for their Hondas but then later models use a decent identification code to help circumvent, going by tow, to a shop.
You need to try bump the starter a few times but not cranking it much to see if it starts easier when you tire of bumping it.
The fuel rail maybe losing volume overnight or the fuel pumps are hesitating to jump up the fuel pressure.
Next would be check the plugs for cylinder flooding or complete dryness.
The ECT, engine coolant temperature, sensor could be a fault and so can a throttle position sensor if it doesn’t say I want to idle while starting up.
You should never have to depress the gas pedal on these cars. It screws things up in the ECU and air mixtures that will throw you a curveball right there.
It destroys what you need into the cylinders as a troubleshooting reading.
Hope this helps you check other things to tell us.(:)
Phil
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I think I have resolved the issue. The computer was suggesting the tps was not working. I ended sup switching it out with a known hood used one. The problem seems to be resolved now. We will see what happens. Thanks for the help.
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Hi,
Thanks for the follow up or feedback. It sure helps to know what happened and it boosts the ego of those who try to give forth their good experiences.
Thanks from all of us.
Phil
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