I am a first time forum user so bear with me.

My sister-in-law purchased a 99 Trailblazer, 4.3L, 4X4 with Auto trans.
The seller disclosed that the transfer case had a leak and that the A/C also leaked. The seller had been told that she would need a new A/C compressor at some point. My sister-in-law drove the vehicle for a few months in the spring with no problems other than a small amount of fluid dripping on her driveway. The 1st hot day of the year she decided to use the A/C. After the A/C running for several minutes the engine stalled. She had it towed to her house and I determined that ECM 1 fuse had blown. I installed the spare 15 AMP fuse and the Blazer was back on the road. She drove it for a few more weeks without an incident, never using the A/C. She then had the A/C refrigerant filled up and drove it the same day using the A/C. ECM 1 fuse blew again. She replaced the fuse (we bought extras just in case), turned the A/C off and headed home. The ECM 1 fuse blew 2 more times on her way home. After several days, I told her I would take the Blazer and see if I could determine what was causing the fuse to blow. I ran the engine in my driveway for 15 minutes with the A/C running and the problem never surfaced. I did note that the A/C clutch would engage for several seconds and then disengage and then reengage. It just kept turning on and off the whole time. My sister-in-law told me that the technician said it might do that if it is slightly overfilled. I left the Blazer sit for a few days and decided to start it up again. Now the A/C will not turn on at all. I don't have any gauges to check A/C pressure and figured it was probably unrelated to the ECM 1 fuse issue. I searched this forum and found many similar issues, but none of the solutions applied to my situation. I decided to drive the Blazer to work on a HOT day (100+ degrees) and sure enough, the fuse blew while I was in the middle of one of our city's busiest intersections no A/C running. The good news is, now there is a dead short somewhere that cause the fuse to blow as soon as I put it in. I have disconnected the following connectors to see if the short would go away: ICM, A/C low pressure switch, MAP, MAF, A/C compressor and basically every connector that is visible from the top side of the engine. The short remained there. I am testing from the non energized side of the fuse to ground. So I started to suspect the ECU so when I started unplugging the connectors from it, viola, the short went away when I unplugged C3 (says WHITE on the ECU but the only wiring pinouts I have found are for 2000 S10 and they call it CLR). However, the pinout I do have has the exact same wire colors as this 99 so I am hoping they are the same. The ECU service # is 16263494. When I test the various pin to see what is shorted to ground I found the following: 17,18,19,22,26,27,and 31. From the piout I have, it looks like 17,18, and 19 are supposed to be grounds. What I don't know is if any of the rest of the pins should be connected to ground. Remember this is with the key off! I retested this a few days later and found only 17,18,22,26 and 31 remained shorted to ground. However, when I plug the connector back onto the ECU, the short is detectable from the non-energized side of the ECM 1 fuse. All of the wiring looks perfect and I have crawled under the Blazer and inspected the wiring back to the fuel tank and to the transmission and it all looks great. So now to my questions:

1. Should all of the pins listed above be connected to ground on the C3 wiring connector?
2. Could the ECU have the short?
3. Since some of the pins listed above go to the transmission range sensor, could it be faulty and causing this problem
4. Does anyone think the A/C could be the root cause of this failure and if so, what is the likely fault?

Sorry for the long winded story...it runs in my family

Thanks in advance for your responses

 

First off, the AC issue is a separate problem which we can address later, perhaps in a new thread to avoid confusion.

Does the SES, (Service Engine Soon) light come on and turn off with the bulb check? Are there any DTC's, (Diagnostic Trouble Codes) in memory or pending? The PCM, (Powertrain Control Module) could be shorted internally but, they are darn near bullet proof and rarely fail, so we need to exhaust all other possibilities first. The ECM1 fuse feeds several circuits through a pink wire, all of which are under the hood. It feeds all 6 injectors, the crankshaft position sensor, canister purge solenoid, ignition control module, ignition coil, the PCM, MAF sensor, might be more, but that's all I can remember

It's possible that the wire harness going to the crankshaft position sensor shorted to ground near the right exhaust manifold. Start by reconnecting all connectors. Remove ECM1 fuse, carefully inspect the wire harness from the crankshaft position sensor electrical connector as far as you can follow it back to the main loom. (We're looking for broken or melted wires.) Carefully disconnect the crankshaft position sensor electrical connector. In the harness connector, check resistance from the pink wire to ground. Must show open circuit, (infinite resistance).


EDIT: woops hit the wrong button too soon, wasn't finished

The ECM1 fuse only gets voltage when the ignition is in the START and RUN positions so, if it pops the fuse with the ignition off, it could mean there is a problem with the ignition switch or a gob of wires are melted together somewhere.

 

** Moved to Lighting - Electrical **

 

Captain Hook, Thanks for the quick response.
I just checked the SES. With 5 different key insertions and turn to RUN, the results were:
one cycle the SES flickered
one cycle the SES came on the off within .5 seconds
three cycles the SES light never came on

The check engine light is not on and no DTC's are stored. I have not pulled the skid plate off and checked the Crank Position sensor and it's wiring, I'll do that this morning just to be sure but I was also under the impression that the ECM 1 fuse should only get power in the Run and START positions. My sister-in-law had mentioned to me the very first time I checked out the vehicle that the ignition switch was "tricky" to turn. I told her that the switch could be the problem and then so much time had gone by, that when the ECM 1 fuse blew the second time I was focused on finding a bad sensor or component. I don't have a service manual on this vehicle (yet), how involved is pulling out the ignition switch and replacing it. I have done a few in my days but none on a vehicle this new.

 

If she's had trouble with the ignition in the past, that is something that needs to be looked into. The switch could be a common denominator. The upper and lower column covers must be removed and some of the fasteners for the switch are torx and can be a challenge. Just take your time, it's not "rocket science". There might even be a "how-to" here on the forum.

This link explains how the ignition switch works:
https://blazerforum.com/forum/articl...-switch-33465/

Keep us posted.

 

I have watched a few different how to videos and it does not look that bad. I also see in the link you provided (thanks) that the ECM 1 fuse and A/C relay are both feed by the same large PINK wire:

"The large pink wire is the IGN 1 wire that powers the ECM, ENG 1 fuse, O2 sensors, B/U lamps, DRLs, A/C relay, etc."

If the A/C relay is shorted out, wouldn't that cause the ECM 1 fuse to blow? I tried to remove this relay from the fuse block in the engine compartment but was unable to get it out. Is there a trick to get these out or do you just need to give them a good yank? I tried wiggling it from side to side while pulling up and I also tried to get a small screwdriver under one corner and pry gently but the thing did not want to come out.

 

The IGN1 fuse is a separate circuit from ECM1, both are fed by the ignition switch. ECM1 blowing will not blow the IGN1 fuse. The A/C relay just pulls out, might have to carefully use pliers to wiggle/rock it back and forth to get it out.

 

Try disconnecting the ignition switch and see if it still blows the fuse

 

I disconnected the ignition switch harness under the dash (connector that measures about 1.5" X 3" and has the heavy Brown, Orange and Pink wires in it). The short is still there. I am measuring 0 ohms resistance between ground and the ECM 1 fuse contact that is closest the firewall. So far, the only connection that I can break and have the short go away is the "WHITE" C3 connector on the ECM, So the 3 circuits on this connector that are shorted to ground that I would like to verify should be shorted to ground when the key is off are:
Circuit #1225, DK BLU, Transmission range B input
Circuit #775, WHT, PRNDL D
Circuit #771, BLK/WHT, PRNDL A

If these are supposed to be grounded when the key is off, then the only explanation is a bad ECM right?

 

one last thing then I`ll but out of this post. I was having trouble a few weeks back blowing the ecm 1 fuse. I had wires melted because of the passenger side manifold. One wire was shorting on the manifold blowing the fuse