My backup vehicle, a '90 Buick developed a start/stall condition.  The below series of screen pictures from a OTC Solarity shows the 5V Bypass drop, ICM pulls the EST low but both return only to finally quit for good.  Also shown is Ref(Hi) from the ICM to ECM and the 1X output from the crank position sensor.

The same thing happened in '03 but I was getting a DTC 42 but not this time.  I've read that if it happens fast enough, it won't set the code.

Checked voltage drops on grounds at ECM and all are good.  I'm about ready to pull the ECM as I did in '03.  The annoying thing is that there is only 5K miles on this ECM!

Am I missing anything??

Randy H. wrote on Sep 9^th, 2011 at 1:23pm:

Yes, it has a crank sensor, that's what the Ref(Hi) is.  The ICM output from the crank sensor to the ECM and the signal is constant when the Bypass drops to zero.

How do you put it into Bypass mode?

The vehicle is used for family loan outs when their main vehicle is in a shop, etc.

It sat for year without driving on a float charger.  Time for a loan out but battery was dead.  Battery and alternator, out for two weeks, back for two months, out to my sister for a few days.  She past it to her son who started to have a rough idle and stall.  My Bro-in-Law rescued him but got it back to me by using quick start several times.

Two days later, it starts right up!  I looked at the fuel pump, FPR, etc. for a week with it starting up every time.  Then it started to stall and then start/stall now while never leaving my garage.

Retired wrote on Sep 9^th, 2011 at 1:58pm:


All you do is extract the EST wire from the harness with a terminal tool - thats what id O. But im so good/fast at pulling terminals it takes less time for me to do this than to pull out a jumper wire and figure which wire needs 5 volts to do what.
Bottom line is if you are getting a good crank sensor signal, and the reference signal going to the ECM is bad, the ICM is bad. Here it appears you are having failure of which signal? I cannot see very much here on your files. Is EST signal failing?
Also have you scopes the actual crank sensor or just REF hi? REF hi is the signal created from the crank sensor by the ICM.

The ICM is a common failure on GM vehicles, but it sometimes fails because of the coils, other times due to heat, and other times the ECM is not grounded properly. The ICM gets its ground from the ECM.

You should check out Toms creative lab scope technique page on the autonerdz home site. He goes over these failure quite thoroughly.

I would suspect the wiring first - check it for corrosion and do a wiggle test. (especially crank sensor wiring) Then look at the modules.
Maybe you can post some more waveforms that are a but bigger? I can barely see anything there.....

Finally what is the exact year, make/model vin/engine size? I can help you better if i know for sure
Right now I am guessing a bit on exact operation

You should get yourself a picoscope

Randy H. wrote on Sep 9^th, 2011 at 3:31pm:

Hard to follow you.  Extract EST with a terminal tool?  I "think" your saying get a 5V signal from elsewhere and put it on the Bypass wire?  At startup?  Can it start that way?  Break that down simpler for me.  Remember, this is the Non-Technician section, I'm a DIY but a retired machine repairman from Ford Motor.

Yes, the Bypass goes to zero, therfore the ICM pulls the EST zero and/or back to base timing and it stalls.

All I have for info is from AllDataDIY.  The diagrams only show two ground circuits or BLK/WHT wires, yet there are four that color on the ECM.  Voltage drop KOEO, the higest read 0.03V, well under the max allowed.

'90 Buick 3.3L VIN N.

I consdered a Pico years ago but the thought of needing a laptop on a stand with wheels didn't sound too good to me.  A Pico would be worth more than the car!

I'll post some bigger bigger pictures later tonight.  Not sure why you can't see the pictures well, maybe you need a better PC or maybe using a smarty phone?











Below is the EST, Bypass, 3X and 1x signals.

Mick wrote on Sep 9^th, 2011 at 8:17pm:

Mick:

Wait a minute!  Open the EST at the ICM?  Ah, isn't that going to make it stall for sure?  That's the spark timing going to the ICM which is what's happening now.  Don't you need the 5V Bypass signal to keep the EST high to control the ICM so the engine runs controlled by the ECM?

There is a Ref(Hi) and Ref(Low).  Not sure which you one you are refering to?  You want to check whatever Ref signal at the ECM, correct?  That way checking the wire?  All of my connections have been at the ICM so far.

I'm at the ICM seeing the EST, so it should be good.

What do you mean by, does it at this point kill the ECM?  Like fry it?

There was one day in the past, where it started, ran a few minutes and stalled.  I looked at some diagrams for thirty minutes.  It started right up and ran till the fan cycled On.  That was in the past and now, it doesn't start.

Retired wrote on Sep 9^th, 2011 at 8:59pm:


Retired wrote on Sep 9^th, 2011 at 4:19pm:


Please give us the complete serial number

I am having a hard time finding a 3.3L N engine with a distributor.

Retired wrote on Sep 10^th, 2011 at 6:55am:


First off do you have fuel pressure?

So this is the C(3)I DIS system with 3 separate coils mounted onto the module.

No sign of a distributor on this vehicle correct?

If I have it correct go to the ECM and connect your scope to D8 purple/white DREF and C8 white EST and an Amp clamp on C11 and C12 injectors and put time on the screen to see at least 2 engine cycles
Then zoom in on the injectors to verify for shorts.

You do have an Amp clamp right?

If we don’t have a signal on D8 we will work back from there.

Retired wrote on Sep 10^th, 2011 at 10:22am:


With not knowing the time scale on that screen I would have to guess that the engine was idleing slower than the othere screens.

Something you have to understand is that dref is buffered inside the ECM, that will give a delay in EST being produced.

Retired wrote on Sep 10^th, 2011 at 10:22am:


The fuse box is ok but you will need 2 fuse adapters.

Why have your scope leads all over the place when you can get what I asked for right from the front seat.