Still No Spark
#12
CF Veteran
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Sorry for the delay in posting these - the past couple days have been hectic.
All the power and ground feeds for the PCM can be verified with a test light or a meter - my recommendation is to start with a test light (hooked to battery positive or negative respective to the feed you’re testing), then if it’s glowing dimly, use a meter to verify.
If using a test light, remove the connector from the PCM - if using a meter, you can remove the connector or back probe it using a sewing needle or safety pin (if you don’t have probes for the meter).
You should also disable the starter - some of the switch feeds should have power when the key is in the on or crank position, so disabling the starter will prevent unnecessary drain on the battery.
C1 (Blue)
- pin 4, pink wire, from ENG 1 fuse (underhood), hot in run and start
C2 (Red) - no power or ground feeds
C3 (Clear or Gray)
- pin 12, gray wire, 5v reference to engine controls from PCM (check with meter while back probing connector, key on engine off - if it’s less than 4.5 volts and all the other power/ground feeds are good, the PCM is bad)
- pins 17 & 18, black/white wires, grounds to rear of right cylinder head
- pin 21, orange wire, direct battery feed via ECM b fuse
- pin 27, gray wire, 5v reference to engine controls from PCM
C4 (Black)
- pin 13, brown wire, from brake fuse (cabin), hot in run
- pin 18, pink wire, from ECM 1 fuse (underhood), hot in run and start
Note that the underhood fuses ENG 1 and ECM 1 both get power from the same circuit from the ignition switch. If you find that there’s a significant difference between the power feed in C1 vs the power feeds in C4, it could be a problem with the underhood fuse box or with the splices located in the harness. However if you find that all 3 power feeds read the same but are substantially lower than battery voltage, trace the circuit between the underhood fuse box and the ignition switch to isolate the open or high resistance and repair as needed.
All the power and ground feeds for the PCM can be verified with a test light or a meter - my recommendation is to start with a test light (hooked to battery positive or negative respective to the feed you’re testing), then if it’s glowing dimly, use a meter to verify.
If using a test light, remove the connector from the PCM - if using a meter, you can remove the connector or back probe it using a sewing needle or safety pin (if you don’t have probes for the meter).
You should also disable the starter - some of the switch feeds should have power when the key is in the on or crank position, so disabling the starter will prevent unnecessary drain on the battery.
C1 (Blue)
- pin 4, pink wire, from ENG 1 fuse (underhood), hot in run and start
C2 (Red) - no power or ground feeds
C3 (Clear or Gray)
- pin 12, gray wire, 5v reference to engine controls from PCM (check with meter while back probing connector, key on engine off - if it’s less than 4.5 volts and all the other power/ground feeds are good, the PCM is bad)
- pins 17 & 18, black/white wires, grounds to rear of right cylinder head
- pin 21, orange wire, direct battery feed via ECM b fuse
- pin 27, gray wire, 5v reference to engine controls from PCM
C4 (Black)
- pin 13, brown wire, from brake fuse (cabin), hot in run
- pin 18, pink wire, from ECM 1 fuse (underhood), hot in run and start
Note that the underhood fuses ENG 1 and ECM 1 both get power from the same circuit from the ignition switch. If you find that there’s a significant difference between the power feed in C1 vs the power feeds in C4, it could be a problem with the underhood fuse box or with the splices located in the harness. However if you find that all 3 power feeds read the same but are substantially lower than battery voltage, trace the circuit between the underhood fuse box and the ignition switch to isolate the open or high resistance and repair as needed.
Last edited by Gumby22; July 24th, 2020 at 11:10 PM.
#13
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Okay, here's what I found out using a test light and a volt meter. All pins on each connector that has power does. Everything seems to test out right. So I went ahead and ordered a new PCM. Is there any way to test the PCM while it is out just out of curiosity.
#14
CF Veteran
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On connector 3, back probe pin 9, white wire, and watch for the voltage to peak around 4 volts AC while cranking - this is the signal the PCM sends to the ICM to command spark. If it’s there, check for an open or high resistance between pin 9 and pin B at the ICM (or monitor for 4 volts AC at the ICM).
If it’s not there, first verify the crank signal at the PCM by back probing C1, pin 31, yellow wire and looking for about 10 volts AC while cranking. If it’s there, consider the PCM bad. If it’s not there, troubleshooting the crank sensor and related wiring is the next step before PCM replacement.
#15
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Update to an older thread I had posted. 1996 K1500 5.7 TBI. Turns over great just no spark. I have replaced and tested everything except my ECM. DUI Distributor, Coil/Ignition Module, CrankSensor, and Ignition Switch. Have 12 volts to everything. Ohms test out on coil and ignition module. All fuses except my dash illumination have power. Compression is good as well as timming. No spark from anywhere. Did also do a test light on coil while cranking and it not flashing when cranking. But coil does test good even replaced good coil with another acdelco that tests out and still nothing. (Do not have fuel pump hooked up yet but still it should spark) Have no engine codes. Gauge cluster is the only thing that is weird as if I turn on my headlights gauges go crazy, turns on my blinker lights. All grounds on engine are good. I'm at a total loss. Anything is appreciated before I go broke.
#16
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1)You have a DUI distributor? Which part number ?
2) you must have cam and crank signal for spark.
The crank gap reference comes around every TDC (exhaust and compression). The cam sensor gap reference come around only on #1 TDC compression(when its time to spark). The computer requires the starting point from the cam sensor before it knows where the crank is.
i dont know why someone told you the cam sensor isnt needed for spark, and i havnt read in your thread where you've scrutinized the cam sensor.
3)you may have a compatibility conflict with the sensor in your aftermarket distrib, or its non functional, or you have a distributor with no cam sensor, which you must have for the tbi to function.
Sorry for your troubles, hope this gives you some help.
2) you must have cam and crank signal for spark.
The crank gap reference comes around every TDC (exhaust and compression). The cam sensor gap reference come around only on #1 TDC compression(when its time to spark). The computer requires the starting point from the cam sensor before it knows where the crank is.
i dont know why someone told you the cam sensor isnt needed for spark, and i havnt read in your thread where you've scrutinized the cam sensor.
3)you may have a compatibility conflict with the sensor in your aftermarket distrib, or its non functional, or you have a distributor with no cam sensor, which you must have for the tbi to function.
Sorry for your troubles, hope this gives you some help.
#17
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October 14th, 2011 6:55 AM