No spark - weird voltage readings. Any help? - Chevy Impala SS Forum
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post #1 of 7 (permalink) Old 06-17-2019, 07:43 AM Thread Starter
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No spark - weird voltage readings. Any help?

Putting the car back together. Shop blew the car apart and f-ed with the wiring. Good times. It ran fine yesterday, now it won't start. No codes, no spark leaving coil. The tests on Shbox are a bit confusing, but this is what I have:
Car is a 1995
At this point, I am not sure what is going on. I tested KOEO at the opti harness (ICM disconnected):
Pin A: .5 volts
Pin B: .5 volts
Pin C: 0 Ohms to ground (key off)
Pin D: 0 volts / .2 ohms to ground (key off) This goes to B3 at the PCM.


Checked fuse - good. Checked continuity to B3 at the PCM: Good. No power out of PCM at B3

ICM harness:
Pin A: 11.4 volts
C: o ohms to ground key off
D: 11.4 volts

Any ideas, folks? I am stumped.


1995 Caprice ERE 383, CIA long tubes, Dynomax VT's, RAISS, Kore3/Z06 brakes, 3.42 posi, Boss 338 20's, 12 way seats, Impala SS console, etc etc etc
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post #2 of 7 (permalink) Old 06-17-2019, 09:22 AM
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Quote:
Originally Posted by atlantadan View Post
Putting the car back together. Shop blew the car apart and f-ed with the wiring. Good times. It ran fine yesterday, now it won't start. No codes, no spark leaving coil. The tests on Shbox are a bit confusing, but this is what I have:
Car is a 1995
At this point, I am not sure what is going on. I tested KOEO at the opti harness (ICM disconnected):
Pin A: .5 volts
Pin B: .5 volts
Pin C: 0 Ohms to ground (key off)
Pin D: 0 volts / .2 ohms to ground (key off) This goes to B3 at the PCM.


Checked fuse - good. Checked continuity to B3 at the PCM: Good. No power out of PCM at B3

ICM harness:
Pin A: 11.4 volts
C: o ohms to ground key off
D: 11.4 volts

Any ideas, folks? I am stumped.
Iím no help, Iím trying to learn from you guys, but what gets me is how the engine can run fine one day and wonít start the next and there doesnít seem to any commonly found fault to point to, other than the entire electrical and fuel system.
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post #3 of 7 (permalink) Old 06-17-2019, 09:44 AM
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Quote:
No power out of PCM at B3
B3 is distributor ref low signal. This is the isolated ground for the opti. D pink/blk
B14 is distributor ignition feed +12V(?) C red
B20 high res to pcm B ppl/wht
B2 low res to pcm A red/blk


When the engine is cranking: Try a 12V led on ICM B wht. B5 from the pcm grounds this in pulses for the coil.(other led wire to +12V). I am going by memory and it could be pulsing +12V. A digital DC volt meter should show something. Migh t show it as a AC volt? If no pulse check the wire from pcm to icm if it is good chase the opti. If there is a pulse the ICM and coil need a look. Is the ground good to icm and coil?


You should have injectors if the opti is working. No fuel smell opti or wiring.

Z09B4U

Last edited by Z09B4U; 06-17-2019 at 09:54 AM.
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post #4 of 7 (permalink) Old 06-17-2019, 10:12 AM Thread Starter
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Quote:
Originally Posted by Z09B4U View Post
B3 is distributor ref low signal. This is the isolated ground for the opti. D pink/blk
B14 is distributor ignition feed +12V(?) C red
B20 high res to pcm B ppl/wht
B2 low res to pcm A red/blk


When the engine is cranking: Try a 12V led on ICM B wht. B5 from the pcm grounds this in pulses for the coil.(other led wire to +12V). I am going by memory and it could be pulsing +12V. A digital DC volt meter should show something. Migh t show it as a AC volt? If no pulse check the wire from pcm to icm if it is good chase the opti. If there is a pulse the ICM and coil need a look. Is the ground good to icm and coil?


You should have injectors if the opti is working. No fuel smell opti or wiring.
Thanks for your assistance.Figured out something - I was testing the opti harness and not the ICM harness. The Shbox definitions were not that clear, but thanks to you - I figured it out. Using the meter on DC, I was able to see 2.4-ish pulse on B5, so the signal is getting to the ICM to fire. Also tested all of the grounds in the PCM connectors - they're all good.

I checked voltage on the ICM terminal B and got 1.8-2.4v DC pulse when cranking. To me, that looks like the PCM is telling the coil to fire. Injectors have 12v on pink. Don't have a noid light, but the Fluke wasn't able to pick up a pulsewidth reading on the other wire when cranking?

The ICM harness and the opti harness test out good. Coil ohms out good. 12.3 volts on pink wire to coil. No volts on pink/black at coil. Keep the ideas coming - any help I can get is appreciated


1995 Caprice ERE 383, CIA long tubes, Dynomax VT's, RAISS, Kore3/Z06 brakes, 3.42 posi, Boss 338 20's, 12 way seats, Impala SS console, etc etc etc

Last edited by atlantadan; 06-17-2019 at 11:48 AM.
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post #5 of 7 (permalink) Old 06-17-2019, 12:14 PM
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from shbox

ICM Harness B-should be tested with AC voltage

A = ~10vdc-12vdc (or your system voltage)
B = 1vac-4vac while cranking
C = ~ 0 ohms to ground with key OFF
D = ~10vdc-12vdc (or your system voltage-might be slightly less than reading at "A")

hope that helps
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post #6 of 7 (permalink) Old 06-17-2019, 12:35 PM Thread Starter
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SOLVED: It was the coil. Swapped it out with my spare and it fired right up. The fact that it was getting the pulses but not producing anything was a clue. The thing ohmed out ok, but whatever. The car runs now.
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1995 Caprice ERE 383, CIA long tubes, Dynomax VT's, RAISS, Kore3/Z06 brakes, 3.42 posi, Boss 338 20's, 12 way seats, Impala SS console, etc etc etc
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post #7 of 7 (permalink) Old 06-18-2019, 11:21 AM
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Quote:
SOLVED: It was the coil. Swapped it out with my spare and it fired right up. The fact that it was getting the pulses but not producing anything was a clue. The thing ohmed out ok, but whatever. The car runs now.
atlantadan


The ICM and coil connectors take a beating from the heat. You might want to make sure the contacts are clean and put a bit of silicon grease(spark plug grease) in the connector.


One of the worst issues with electrical problems can be that separating and connecting a electrical connector may temporarily clean the contacts and allow the circuit to work.

Z09B4U
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