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940 How to Cause Check Engine Light

Matt Sheldon

New member
Joined
Aug 14, 2015
Hi All,

First time poster here. I own a 91 940se wagon. Just had the heater core replaced by a well established (Seattle) Volvo mechanic. When I got the car back none of the dash lights were working as well as none of the dash warning lights. Took it back and they said they fixed it. Dash lights now working however I don't think any of the warning lights are working yet. They light up and go off when I start the car but I can't get the check engine light to come on due to a fault. I have disconnected the RPM sensor on the turbo inlet hose, MAF sensor, the O2 sensor and pulled a spark plug wire then started. None of these methods caused the Check Engine light to come on or stay on. My question is should any or all of these methods have caused the warning light to come on? They threw ODB codes (accept the O2 sensor for some weird reason). What's the best way to cause the Check Engine light to come on? I want to make sure I haven't missed anything before I take it back a 3rd time.

Thanks

Matt
 
Maybe disconnect the O2 sensor and drive for awhile. Does the check engine light come on with the rest of the idiot lights when the ignition is on, but engine not running?
 
Yes. The check engine light lights up when the ignition switch is in the acc position. It appears that all the bulbs are working. It's that the dash warning lights appears to not be getting fault signals after they put the dash back together.

Pulled the RPM sensor plug on the air inlet hose and got code 1-3-1

Pulled MAF plug and got code 1-2-1.

Pulled both plugs from the O2 sensor and got no code. Seems weird. I have a replacement but a bad O2 sensor doesn't explain why I didn't get an ODB code when unplugged. Different problem for a different time.

Strangely enough I noticed the problem because of a non-functioning brake light. The LR brake light was out and the dash warning light was working when I took the car in. Got the car back and the brake light was still faulty but no warning light on the dash as well as no dash lights. Returned it. Got it back the 2nd time with functioning dash light and no warning lights coming on. They told me they had fixed the problems as well as my non-functioning brake light. How nice of them. Realized last night that the brake light was still out and no dash warning light.

Fantastic! Was looking up the codes that the ODB system threw during testing in my Bentley manual. Just noticed that there is a sidebar in the ODB codes chart that tells you which codes will cause the Check Engine light to come on. The MAF sensor test should have caused it to light up. It would seem that the dash warning light are not getting the signal.

I'll redo the MAF test to reverify the problem and then it's time for a phone call.
 
Are there two check engine lights? There is the one on the right and then the real one on the left. You sure the left one lights up? All the dash warning lights come on when you turn the key?
 
Is your 940 using Regina or LH2.4? For LH2.4, I think the CEL can be turned on by either the LH2.4 ECU or by the EZ116K. Maybe you have a poor CEL connection on the LH2.4 connector but a good one on the EZK -- this might explain the CEL coming on and then going off, but not turning on when ECU sensors are disconnected.

Off hand, I think that the only dashboard light that will illuminate when you disconnect ECU sensors is the CEL. Oil pressure warning is direct from the pressure switch and doesn't go through the ECU. Likewise for alternator, parking brake, master cylinder warnings. For the bulb warning, try removing a brake light bulb and stepping on the pedal. Does the warning light illuminate?

Your original lack of any dashboard lights sounds like the dashboard connector just wasn't plugged back in after dash reassembly.
--> Edit: I haven't worked with 740/940 dashboard connectors and mistakenly thought they were similar to 240's. Thanks Art for the correction. This BrickBoard FAQ may be useful.
 
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Yes. The check engine light lights up when the ignition switch is in the acc position. It appears that all the bulbs are working. It's that the dash warning lights appears to not be getting fault signals after they put the dash back together.

Pulled the RPM sensor plug on the air inlet hose and got code 1-3-1

Pulled MAF plug and got code 1-2-1.

Pulled both plugs from the O2 sensor and got no code. Seems weird. I have a replacement but a bad O2 sensor doesn't explain why I didn't get an ODB code when unplugged. Different problem for a different time.

Strangely enough I noticed the problem because of a non-functioning brake light. The LR brake light was out and the dash warning light was working when I took the car in. Got the car back and the brake light was still faulty but no warning light on the dash as well as no dash lights. Returned it. Got it back the 2nd time with functioning dash light and no warning lights coming on. They told me they had fixed the problems as well as my non-functioning brake light. How nice of them. Realized last night that the brake light was still out and no dash warning light.

Fantastic! Was looking up the codes that the ODB system threw during testing in my Bentley manual. Just noticed that there is a sidebar in the ODB codes chart that tells you which codes will cause the Check Engine light to come on. The MAF sensor test should have caused it to light up. It would seem that the dash warning light are not getting the signal.

I'll redo the MAF test to reverify the problem and then it's time for a phone call.

Your problems have nothing to do with anything they did to you car. Coincidental timing is all that is happening. From what you describe, your check engine light is working just as it should. Also, the fact you have a light out and no dash warning light usually indicates that you have a bulb warning relay with a bad circuit. Again, nothing having anything to do with the shop working on your car.
 
bobxyz - I'll pull the bulb and see if the bulb warning light works. It could be a bad relay and you're surely correct that the lack of dash lights was just an oversight by the mechanic when putting it back together. Thanks for the link.

2manyturbos - The fact that the CEL lights up when the key is in the accs. position would seem to corroborate what you're saying and I now understand that the CEL and bulb warning are completely separate systems. What worries me is the Bentley manual indicates that ODB code 1-2-1 should be accompanied by CEL illumination. I unplugged the MAF which caused the ODB system to throw error code 1-2-1 but no CEL. Is unplugging the MAF not enough to cause the CEL to illuminate? If not is there a good way to force the CEL to illuminate? I would love to test the system to verify that the CEL system is working properly and move on to more enjoyable car work.

Thanks for all the help.
 
bobxyz - I'll pull the bulb and see if the bulb warning light works. It could be a bad relay and you're surely correct that the lack of dash lights was just an oversight by the mechanic when putting it back together. Thanks for the link.

2manyturbos - The fact that the CEL lights up when the key is in the accs. position would seem to corroborate what you're saying and I now understand that the CEL and bulb warning are completely separate systems. What worries me is the Bentley manual indicates that ODB code 1-2-1 should be accompanied by CEL illumination. I unplugged the MAF which caused the ODB system to throw error code 1-2-1 but no CEL. Is unplugging the MAF not enough to cause the CEL to illuminate? If not is there a good way to force the CEL to illuminate? I would love to test the system to verify that the CEL system is working properly and move on to more enjoyable car work.

Thanks for all the help.

Your thinking is spot on. If the ECU stores the code, the dash light should display. What you are experiencing is something I have never seen. Meaning, if the check engine light displays when the key is in the run position, engine off, turns off when the engine starts, yet, doesn't illuminate when you purposely cause a fault that the ECU stores, that is a new one for me. I would try to borrow a different ECU and see if everything works as it should with a different ECU in your chassis. What you are describing has to be related to the ECU itself, IMO.
 
I've seen CEL's rewired to oil pressure switches and relays in order to fool the smog techs into passing the car. Just a thought.
 
Yep. Go direct to each box and check if they are indeed putt8ng out a check engine l8ght signal voltage. Can't hurt.
 
Your thinking is spot on. If the ECU stores the code, the dash light should display. What you are experiencing is something I have never seen. Meaning, if the check engine light displays when the key is in the run position, engine off, turns off when the engine starts, yet, doesn't illuminate when you purposely cause a fault that the ECU stores, that is a new one for me. I would try to borrow a different ECU and see if everything works as it should with a different ECU in your chassis. What you are describing has to be related to the ECU itself, IMO.

That's been my thinking lately. This has to be an ECU problem for the CEL and a relay problem for the Bulb warning. The CEL is getting current. It's just not getting any on/off signal. I looked through the wiring diagram and it doesn't appear that the ECU signal runs through anything other than a fuse and a multi-pin socket before it hits the CEL. Looks like I'm going to be testing my ECU and socket.

I may be letting the garage off to easily here but I don't think I can really blame them if the ECU has issues. I don't think they would have done anything but unplug it and plug it back in for this repair job. I might have an argument for a new relay for the Bulb warning but I have better things to do with my time than fight with them for a relay. I'll pull it and look for solder cracks and re-weld it or pick one up at a pick-n-pull. Am I letting them off too easy?
 
TLDR: Try reving up the engine a bit after pulling the MAF plug to see if the CEL turns on. Just starting and staying at idle may not be sufficient (even though diag codes are posted).

Long version:
I have a chopped off 240 engine harness, a couple extra ECU/EZK boxes, and a JimStim (used to test MegaSquirt) so I tried running them on the desktop to see what happens when sensors are unplugged. I saw similar behavior to what you described -- diag codes were posted but sometimes the CEL was off.

Both the EZK and the ECU turn on the CEL at power-up, and turn it off after the engine starts. If the EZK coolant temperature sensor is disconnected, the EZK will keep the CEL on and post a 2-2-4 code. I had mixed results with disconnected ECU CLT, MAF, O2, IAC sensors. Sometimes, I could turn on the system and turn up the RPM to idle (~900rpm) and the CEL would go out, but there were still diag codes posted. If I turned up the RPM more, to ~2500rpm and then went back to idle of ~900rpm, the CEL would turn on.

Unplugging the dual EZK/ECU temperature sensor should keep the CEL on all the time (but it's not so easy to reach under the intake manifold to unplug the connector). Unplugging the MAF should turn on the CEL after reving up the engine a bit then going back to idle. [On the desktop, I can simply turn the knob and go to any RPM I want. I'm assuming that you can rev a real engine, at least a little, with the MAF disconnected, but I may be mistaken.]

It sure seems that the CEL logic is more complex than simply turning on the light when a fault is first detected. The diag codes do seem to be posted immediately when a fault occurs.
 
I don't think a Bently or other general manual is going to show what you want. I have a factory wiring book for the 940 Regina, and I'm pretty sure your wiring will be similar. The Malfunction indicator light - Check Engine - has one side connected to battery. The other side is going to need a Ground or 0v signal provided by the ECU, to turn it on. The rear of the instrument panel would be my first stop, as the lights connect via a flimsy plastic membrane. Make sure there is a good connection there. The instrument panel is easy to remove...the wire you want is probably part of connector C on the instrument panel. That's the one on the left side of the panel (closest to the door). Cracks in the membrane solder traces are possible, so use a glass.

Even with the factory wiring book this part is very difficult to follow, as pieces of the puzzle appear on a variety of different pages. It looks to me like the CE light wire (the switched wire) goes to a large multi-pin connector under the driver side part of the dash. My book labels this connector as "C".

In any case, my guess is that you should separate these multi-pin connectors and look for signs of moisture contamination on the terminals. Spray with contact cleaner before snapping them back together. That would include the multi pin connectors right at the ECU.
It looks to me like the CE light wire runs underneath the dash and goes to the ECU, rather than going through the firewall twice. All this does not rule out a bad ECU. You could ask if one is available at the repair shop and plug it in for a quick checkout.

Good Luck!
-L
 
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