which computer drives the Check Engine light

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ReluctantRed
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Location: St. Louis USA

which computer drives the Check Engine light

Post by ReluctantRed »

Hello Coupers, I have a new-to-me 1987 657Csi which does not run.

Right in the middle of the dash board there is a light called "CHECK":

a) is this the Check Engine light?
b) which computer turns this light on? (ECU? OBC?) I don't see this light on any of the schematic I have.
c) is there a serial data bus between the EUC and the OBC?
d) could the OBC tell the ECU not to start via serial coms? (I know the OBS relay can keep the car from starting but that is working, I have fuel pressure and 10-12v on pin 35.)
e) this check light flashes with key in the "run" position and when cranking. Is that normal?

I need to understand the "check" light.
This car has no spark but all the ECU inputs, flywheel sensor, etc. look right.
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clipper47
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Re: which computer drives the Check Engine light

Post by clipper47 »

The light and a chime comes on whenever the ignition is on. The brake light out warning light on the left side light out panel should also come on until you press the brake. Possibly the brake light switch under the brake pedal is defective or not adjusted if it still doesn't go out when you push the pedal. I believe it is connected to the brake light switch.
Dave
_____________
1985 635CSi US
Cosmos Blue/Pearl Beige
baders
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Re: which computer drives the Check Engine light

Post by baders »

"Looks right" doesn't mean they are. You need to start systematic testing. Do you own a multi-meter ? You have a known fault (no spark), start with that. Are you getting 12v at the 15 pin on the coil when ignition switch is in start/run position ? We can help you through if you test.
1986 635csi LOCUTUS / Arktis Blau Metallic / Pearl Beige / S38B36/ Close ratio dogleg 5sp
2002 E46 M3 / Topaz Blau Metallic / Black Nappa / 6 speed SMG software tuned / Bilstein PSS10 Coilovers / CMP RACP reinforcement
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86_6series
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Re: which computer drives the Check Engine light

Post by 86_6series »

When you first start the engine or turn on the ign that check light is
to check brake lights. If it lights up while driving, then the display
panel on the left should have a light to indicate the problem.
86-635CSi
96-MB C220 SOLD
98-740i
71-Continental MK3
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Brucey
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Re: which computer drives the Check Engine light

Post by Brucey »

if the 'check' light appears in the dash it means that

a) you have not pushed the brake pedal yet (which tests the brake lights) and/or

b) one of the things in the check panel (to the side of the dashboard) has failed its check. There will be an LED to say which one.

This system is entirely passive, i.e. it does not interfere with the actual running of the car in any way.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ReluctantRed
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Re: which computer drives the Check Engine light

Post by ReluctantRed »

Thanks a lot guys.

So the cars does not have a check engine light like new cars.

Does this cars have any means to read codes?

I do have a red LED brake light and it ans the "Check" light go out when I press on the brake.

What do you call the LED display on the left? Is there a computer that drives it?

I'm an electrical engineer with a shop, I have everything, multi-meters, oscilloscopes, loads, lights, even a spectrum analyzer.

This car seems to have a Mototronic Basic system with a 35 pin ECU.
The coil's green wire gets battery voltage in run and crank. The coil's black wire stays the same as the green wire except for occasionally a single pulse during the first revolution of cranking. I never get the train of pulses at the coil primary you expect. The black wire from coil to ECU is good.

My Pin 4 voltage is only 9.98v when cranking
Pin 18 is hot all the time 12v usually, but 10v when cranking
Pin 35 has 12v in run, 10v when cranking
Pins 5,10,16,17,19, are all grounded to body, engine, and battery negative.
Pins 27, 25 sensor common are both ~1ohm to ground
Pin 26 to 25 Cylinder ID sensor is 896 ohms, 0.8Vac when cranking
Pin 8 to pin 27 Engine speed sensor is 900 ohms, 18Vac when cranking
Pin 13 Coolant temp 2.5K ohms to ground
Pin 2 is grounded with throttle closed
Pin 3 is grounded when throttle wide open
Pin 22 Air temp is 1.9Kohm to Pin 6
Pin 9 to Pin 6 air door resistor is 562 ohm
Pin 7 to Pin 6 varies 800-50ohms as air door is moved
Battery negative to Car chassis is good connection ~ 0.001ohms (.01v with 10 amps flowing)
Engine cranks at 253 rpm
CylinderID26tip-25.JPG
CylinderID26tip-25.JPG (439.92 KiB) Viewed 5626 times
CylinderID close up.JPG
CylinderID close up.JPG (415.87 KiB) Viewed 5626 times
Engine Speed8tip-27.JPG
Engine Speed8tip-27.JPG (428.03 KiB) Viewed 5626 times
Notice the Cylinder ID sensor goes 12v negative but only 5v positive. Could this be a problem?

I also tried cranking with a heavy duty battery charger attached to raise the 10v to 10.5v but still no spark

I greatly appreciate any suggestions.
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Brucey
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Re: which computer drives the Check Engine light

Post by Brucey »

it is not (on pre 6/87 cars) strictly a cylinder ID sensor, it is effectively a TDC sensor. In BMW/Bosch speak it is the 'REF sensor'. The injectors fire every turn of the engine (so twice per combustion cycle) and are 'batch fired' i.e. all six at once. The only way the engine identifies a cylinder is via the distributor rotor and camshaft.

On post 6/87 engines (which don't have 35 pin ECUs) there is no CSI, there is only one crank sensor (at the front of the engine, which reads an N-1 wheel) and there is a cylinder sensor that reads one of the spark plug leads. Still not fully sequential injection though; just two batches.

IIRC there are good screen shots of the 'scope traces in the factory manual (available online) and anyway these are nothing to do with the check panel, which is to the side of the instrument cluster.

The function of the check panel is described in the owner's manual and it doesn't ever stop the engine from running.

Also there are no ECU codes in Bosch 35pin ECUs.

cheers
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ReluctantRed
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Re: which computer drives the Check Engine light

Post by ReluctantRed »

Yes the "Cylinder ID" sensor only produces one pulse per revolution on this car.

I found the attached oscilligram in the shop manual for "M6" 1985 and up 633Csi and 635Csi.
'M6' Sensor Osciligrams.pdf
(885.87 KiB) Downloaded 177 times
My speed sensor is correct. Strangely, the manual says the amplitude does not matter.

I can't tell about the "Cylinder ID" sensor. The oscilligram in the manual does not have time on the horz axis, its in %. It doesn't show the positive and negative nature of my snapshot. The book just shows a positive pulse, and again with no amplitude. If I adjust my scope, I can get a waveform that looks like the book.

For kicks I made some adapters and flipped the polarity of my Cylinder ID sensor, but still no spark.

At this point I think the ECU is bad despite the guy in Florida I sent it to saying its good. I need to get a known-good ECU or a running car to try mine in.
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Brucey
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Re: which computer drives the Check Engine light

Post by Brucey »

again, it is called a REF sensor. In the workshop manual the horizontal axis is indicated as being calibrated in ms.

I have asked this before; have you tested the feed to the coil under load?

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ReluctantRed
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Re: which computer drives the Check Engine light

Post by ReluctantRed »

Yes the coil's green wire has power during cranking. See this oscillogram of the green wire to chassis. The "1>" in black on the lower left is the 0v mark and its 2v per division vertically. It begins a 12.5v with key in run position, drops to 8v at starter inrush, and is 10.5V during cranking.
Coil Green Wire to chassis during cranking.JPG
Coil Green Wire to chassis during cranking.JPG (421.77 KiB) Viewed 5593 times
The Black wire to chassis show the same trace (since the ECU is not doing anything). See:
Coil black wire to chassis during cranking.JPG
Coil black wire to chassis during cranking.JPG (411.52 KiB) Viewed 5593 times
Across the primary, (green on tip, black common) there is no voltage, no pulse train. See:
AcrossCoilGreen tip to black during cranking.JPG
AcrossCoilGreen tip to black during cranking.JPG (431.24 KiB) Viewed 5593 times
The speed of cranking is a healthy sounding 250 rpm. If I put the big charger on, it really cranks fast and the 10.5V is 11V or more, but still no spark.

Has anyone cut apart a REF sensor? Are there any electronic components in there?

If Brucey is right, and the big ticks on the oscillogram in the manual are 1ms, the REF trace should be ~3ms across. Mine is only ~1ms wide. Perhaps the proper sensor has some electronics other means to stretch the pulse.

Thanks everyone.
ReluctantRed
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Re: which computer drives the Check Engine light

Post by ReluctantRed »

Yes the coil's green wire has power during cranking.

BUT WAIT there has been a development.

I noticed the Engine Speed and REF sensor had continuity from the yellow wire to ground, but that's not on the schematic.

The guy before me was a parts swapper and he put on new sensors(both) but there we not exactly right, the wires were too short.
AND they are internally connected yellow to the outer shinny cylinder which grounds when inserted.

The old sensors were in the trunk and I put them back on.

I've got spark!

It starts and dies now. Every time. Only runs a few seconds.

I'm getting injector pulses. I see them getting larger as the rpm increases.

The gas in this car is of questionable repute. I'll see about changing it tomorrow and report.

Thanks so far everyone.
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Brucey
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Re: which computer drives the Check Engine light

Post by Brucey »

good result about the crank sensors!

Starting and dying after a few seconds are signs that the CSI is working but the main injectors are perhaps not. Worth just checking that the main injectors are actually opening when they see the signal. If they are clicking when the signal is applied this is a good sign that they are at least trying to open. However the injectors may be blocked; there are filter screens in the tops of the injectors and these can get blocked with crud when you have a car that hasn't run for a long while.

cheers
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ReluctantRed
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Re: which computer drives the Check Engine light

Post by ReluctantRed »

4 of the 6 injectors were completely stopped up. I got them cleared, but they drip, and don't atomize (just shoot a line of fuel).
The car runs now but terrible. Got new injectors coming.

Thanks everyone especially Brucey for all your help.
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