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Discussion Starter · #1 ·
Well this week was supposed to be a time to celebrate the first start of GTM #357. Unfortunately, my "slightly used" LS3 is showing NO signs of life.

When I turn on the ignition I hear the fuel pump running and fuel going to the fuel rails. When I crank the starter it turns over fine, but no signs of starting beyond that. In fact, I don't seem to be getting any injector pulse or ignition spark. I checked all connections to all sensors, all power connections, and grounds (~3-4 times!). Everything is hooked up fine. I even went so far as to change out the Cam sensor and Crank sensor thinking the ECU wasn't seeing the engine turning over causing starting difficulties. Drilling down further on the diagnostics I'm getting no trouble codes from the ECU but it also doesn't seem to be getting info from all sensors.

I'm using the ISIS system, and in talking to the supplier who sold me the engine, custom harness, and ECU there are some reports out there of people having similar trouble with ISIS in the mix. He also told me there is some sort of security mode that is tripped if the power connections to the ECU aren't right.

I realize this could be a million things, but does anyone have ideas about next steps to diagnose this problem? Is there a known issue with the ISIS powering the ECU adequately for starting conditions?

I'm ready to bring this build to life but I'm totally stumped. Thanks.

Premium Member
13,237 Posts
I've used ISIS on several builds here with no issues at all. You don't have the ISIS "start" output tied directly to the starter solenoid, do you? It needs to be ran thru a relay. Are you guessing that there is no spark and no injection pulse, or have you physically checked them? If you haven't actually verified that you have no spark or injector pulse or fuel pressure......that is where I would start. I would start with spark and go from there.......if you have no spark, you know that the engine won't start no matter what else you do.......and also tells you that either you have something physically wired wrong, or, for whatever reason, the ECU is not seeing the engine crank.....or not sending out the outputs to provide spark. If you do have spark OR fuel injector pulse, then you know for a fact the ECU is seeing the engine crank. Do you have a heavy ground strap from the engine to the chassis and all of your engine harness grounds properly grounded?

23 Posts
for what its worth

I have been down this dusty road. First I dont know everything and own 7 second car or claim to have 76 years in auto troubleshooting.but this may help. there are two connections to your pc x1 and x2. If all is wired right then look at each part thats not working.This is the pin breakout. x1 shold have nothing to do with your problem but its here so you know.There are two connectors on the PCM, X1 and X2. X1 is a 73-way connector while X2 is an 80-way.

X1 pinout (73 way):
pin# wire color description________________________________
1 OG/BK Park/Neutral Switch (MYC)
2-7 N/A N/A
8 L-GN Backup lamp switch signal (M10)
9 BU/WH Brake pedal switch signal
10 PU/WH HO2S High Signal Bank 1 Sensor 2
11 PU/WH HO2S Low Signal Bank 1 Sensor 2
12 OG/BK A/C Refrigerant Pressure Sensor Signal
13 TN Low Reference
14-15 N/A N/A
16 D-BU Fuel Level Sensor Signal
17 D-BU High Speed Cooling Fan Relay Control
18 D-BU Serial Data Wake-Up Signal
19 PK Ignition Voltage
20 RD/WH Battery Positive Voltage
21 OG Brake Booster Vacuum Sensor Signal
22 YE/BK Low Reference (L99)
23 PU Low Reference
24 D-GN Fuel Tank Pressure Signal
25 N/A N/A
26 YE Clutch Pedal Position Sensor (M10)
27 TN Hi Speed GMLAM Serial Data Bus-
28 TN Hi Speed GMLAM Serial Data Bus+
29 D-BU APP Sensor 1 Signal
30 BN Low Reference
31 BN/WH Low Reference
32 L-BU APP Sensor 2 Signal
33 GY 5 volt Reference 1
34 GY 5 volt Reference 1
35 N/A N/A
36 TN 5 volt Reference 1
37 L-BU Intake Air Temp Sensor Signal
38 TN Low Reference
39-40 N/A N/A
41 YE MAF Sensor Signal
42 WH Low Reference (M10)
43-46 N/A N/A
47 PK/BK Ignition Voltage
48 N/A N/A
49 TN HO2S Low Signal Bank 2 Sensor 2
50 D-GN/WH Fuel Pump Relay Control
51 N/A N/A
52 PU/WH Starter Relay Control
53 GY 5 volt Reference 2 (L99)
54 TN 5 volt Reference 2 (M10)
55 N/A N/A
56 WH/BK 5 volt Reference 2
57 N/A N/A
58 D-GN Low Speed Cooling Fan relay Control
59 YE Powertrain Relay Control
60 N/A N/A
61 WH EVAP Canister Vent Solenoid Control
62 N/A N/A
63 D-GN/WH A/C Compressor Clutch Relay Control
64 OG/WH HO2S Heater Low Control Bank 2 Sensor 2
65 GY/WH HO2S Heater Low Control Bank 1 Sensor 2
66 GY Skip/Shift Solenoid Control (M10)
67 N/A N/A
68 BN/WH MIL Control
69 PU HO2S High Signal Bank 2 Sensor 2
70 L-BU Fuel Level Sensor Signal-Secondary
71 GY/BK VSS Signal High (M10)
72 PU/WH VSS Signal Low (M10)
73 BK/WH Ground

X2 pinout (80 way):
pin# wire color description________________________________
1 N/A N/A
2 PU/WH 5 Volt Reference 2
3 GY 5 Volt Reference 2
4 N/A N/A
5 BN TAC Motor Control -2
6 YE TAC Motor Control -1
7 N/A N/A
8 D-GN/WH EVAP Canister Purge Solenoid Control
9 GY Valve Lifter Oil Manifold (VLOM) Assy Cyl 6 Shutoff Solenoid (L99)
10 D-BU Valve Lifter Oil Manifold (VLOM) Assy Cyl 1 Shutoff Solenoid (L99)
11 OG Valve Lifter Oil Manifold (VLOM) Assy Cyl 4 Shutoff Solenoid (L99)
Reverse Lockout Solenoid Control (M10)
12 GY/WH HO2S Heater Low Control Bank 1 Sensor 1
13 L-GN HO2S Heater Low Control Bank 2 Sensor 1
14 L-GN Valve Lifter Oil Manifold (VLOM) Assembly Cylinder 7 Shutoff Solenoid (L99)
15 N/A N/A
16 PU CMP Actuator Solenoid Control-Intake Bank 1 (L99)
17 D-BU/WH Fuel Injector 8 Control
18 TN/WH Fuel Injector 5 Control
19 YE/BK Fuel Injector 6 Control
20 TN Fuel Injector 1 Control
21 YE ECT Sensor Signal
22 TN Low Reference
23 TN/WH Transmission Fluid Temp Sensor Signal (M10)
L-BU Oil Temp Sensor Signal (MYC)
24 OG/BK Low Refernce (M10)
25 N/A N/A
26 D-BU Knock Sensor (KS) 1 Signal
27 GY Knock Sensor (KS) 1 Signal
28 N/A N/A
29 L-BU Knock Sensor (KS) 2 Signal
30 GY Knock Sensor (KS) 2 Signal
31 N/A N/A
32 GY Generator Field Duty Cycle Signal
33 BN Oil Level Switch Signal
34 BK Low Reference
35 TN Low Reference
36 N/A N/A
37 L-GN/BK Fuel Injector 2 Control
38 PK/BK Fuel Injector 3 Control
39 L-BU/BK Fuel Injector 4 Control
40 OG/BK Fuel Injector 7 Control
41 GY 5 Volt Referencve 1
42 N/A N/A
43 GY 5 Volt Referencve 1
44 D-BU 5 Volt Referencve 1
45 N/A N/A
46 N/A N/A
47 N/A N/A
48 N/A N/A
49 N/A N/A
50 TN/Wh Oil Pressure Sensor Signal
51 N/A N/A
52 TN Low Reference (L99)
53 OG/BK Low Reference
54 N/A N/A
55 N/A N/A
56 TN HO2S Low Signal- Bank 1 Sensor 1
57 PU/WH HO2S High Signal- Bank 1 Sensor 1
58 L-GN MAP Sensor Signal
59 PU HO2S High Signal- Bank 2 Sensor 1
60 TN HO2S Low Signal- Bank 2 Sensor 1
61 BN Charge Indicator Control/Charge Indicator Signal
62 N/A N/A
63 PU TP Sensor 2 Signal
64 D-BU/WH Camshaft Position Signal
65 D-GN TP Sensor 1 Signal
66 BN Low Reference
67 N/A N/A
68 WH/BK Crankshaft Position Sensor Signal
69 GY/BK Low Reference
70 PU IC 1 Control
71 PU/WH IC 8 Control
72 OG IC 7 Control
73 OG/WH IC 2 Control
74 BU/WH IC 6 Control
75 D-GN IC 5 Control
76 D-GN/WH IC 4 Control
77 L-BU IC 3 Control
78 BN Low Reference
79 BN/WH Low Reference
80 N/A N/A

This given I would look at x2 close you should first check grounds. Then at pins both sides for good connections on x2 side its common of these to be poor, look close. Like everything on this kindof build a little info can get you out of a tough spot, to much can send your head spinning. Hope this helps.
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