Reusing a stock fuseblock
02-07-2010, 01:35 PM
#1
Reusing a stock fuseblock This thread is a rough overview of how to incorporate a stock fuseblock into a swap. Truck and LT1 harnesses are notorious for runs but no shut off because the swapper did not utilize the fuseblock correctly. Do not fret as these issues are simple to overcome with basic knowledge of how to identify and use each aspect of a salvaged fuseblock, even it its from a non LSx vehicle. The one pictured in this thread was from some random FWD car I scavenged for connectors. Ill go over LSx truck blocks at the end because just about everything is retained
Ok lets begin. It should look like this with a combination of relays, minifuses and large fuses. Their exact function is irrelevant because these will be reassigned by you for your particular swap
The business end is on the back where most people groan
To put it plainly, wiring is tedious and very confusing, mostly from the gigantic mess of similar wiring each with a specific purpose. Theres no way youll know all of these functions just by grabbing the wad, so separate them into groups starting with the relays. I labeled them to show common color combos. Heavy red and purple is just about always IGN relay, orange and gray is always fuel pump relay, etc. Depending on the fuseblock, the included relays may vary. Just about all of them since the early 90's have IGN and fuel relays which are the critical ones
Lets look at the IGN relay first. Notice the big red wire goes to the large fuse position which is connected to the battery cable lug on the side of the block. This is battery constant 12v. Next, the big puple output goes to two places on the minifuseblock area powering different rows. These rows will be where your IGN fuses will be assigned. Next, move back to your call wires to complete the relay circuit. The Blk/wht is ground, and the pink is IGN. This is where most people miss a step because this IGN is not part of the swap harness. You must connect that to your original harness. For GM carb cars, this is the old coil wire (pink or pnk/blk). Hooking this to battery constant 12v will yield a runs but no shut down scenario and constant battery drain when not running. A easy check for this is a multimeter or test light over the injector connectors with the key out
The next most important relay is for the fuel pump. A 16ga gray wire is your immediate cue of the fuel circuit. This must be connected somewhere to your actual pump wiring. Here is a basic diagram of how that works, ignore the left side, its for 3rd gens
Keep in mind the fuel relay receives fused constant battery 12v, and is often shared with the PCM battery fuse wires to pins B20/B57 (99-04 PCMs)
Now to reassign the minifuses. I cut out the heavy wires to see better. Since the IGN relay is assigned, make a note of the location it powers. Generally it wont be entire rows of fuses at a time. In this block, its the bottom row and leftmost of the middle row. Now assign duties to the fused wires. Common suspects are PCM with MAF, INJ 1, INJ 2, Fans, HO2s with trans and other small features. Please note the heavy red wires feed the battery constant fuses such as the fuel pump relay. These cannot be skipped
Truck LSx guys have it easy as all of the necessary fuses and relays are already in place and wired to suit the harness with the exception of the IGN relay. The 12v switched from the key must be supplied. I cannot remember which pin or body connector it goes to, but it changes between the years. If you cannot locate this wire, then run a new one to the fuseblock IGN relay. Remember it must connect to the original harness on a 12v switched wire or you will run into the runs but no shut down issue
Ok lets begin. It should look like this with a combination of relays, minifuses and large fuses. Their exact function is irrelevant because these will be reassigned by you for your particular swap
The business end is on the back where most people groan
To put it plainly, wiring is tedious and very confusing, mostly from the gigantic mess of similar wiring each with a specific purpose. Theres no way youll know all of these functions just by grabbing the wad, so separate them into groups starting with the relays. I labeled them to show common color combos. Heavy red and purple is just about always IGN relay, orange and gray is always fuel pump relay, etc. Depending on the fuseblock, the included relays may vary. Just about all of them since the early 90's have IGN and fuel relays which are the critical ones
Lets look at the IGN relay first. Notice the big red wire goes to the large fuse position which is connected to the battery cable lug on the side of the block. This is battery constant 12v. Next, the big puple output goes to two places on the minifuseblock area powering different rows. These rows will be where your IGN fuses will be assigned. Next, move back to your call wires to complete the relay circuit. The Blk/wht is ground, and the pink is IGN. This is where most people miss a step because this IGN is not part of the swap harness. You must connect that to your original harness. For GM carb cars, this is the old coil wire (pink or pnk/blk). Hooking this to battery constant 12v will yield a runs but no shut down scenario and constant battery drain when not running. A easy check for this is a multimeter or test light over the injector connectors with the key out
The next most important relay is for the fuel pump. A 16ga gray wire is your immediate cue of the fuel circuit. This must be connected somewhere to your actual pump wiring. Here is a basic diagram of how that works, ignore the left side, its for 3rd gens
Keep in mind the fuel relay receives fused constant battery 12v, and is often shared with the PCM battery fuse wires to pins B20/B57 (99-04 PCMs)
Now to reassign the minifuses. I cut out the heavy wires to see better. Since the IGN relay is assigned, make a note of the location it powers. Generally it wont be entire rows of fuses at a time. In this block, its the bottom row and leftmost of the middle row. Now assign duties to the fused wires. Common suspects are PCM with MAF, INJ 1, INJ 2, Fans, HO2s with trans and other small features. Please note the heavy red wires feed the battery constant fuses such as the fuel pump relay. These cannot be skipped
Truck LSx guys have it easy as all of the necessary fuses and relays are already in place and wired to suit the harness with the exception of the IGN relay. The 12v switched from the key must be supplied. I cannot remember which pin or body connector it goes to, but it changes between the years. If you cannot locate this wire, then run a new one to the fuseblock IGN relay. Remember it must connect to the original harness on a 12v switched wire or you will run into the runs but no shut down issue
