Gary Lewis Posted March 5 Share Posted March 5 What loads do you intend to put on the existing cab harness, that would exceed 70A? That's the key. If the battery-charging load is taken off the shunt by moving the alternator's output to the starter relay then the shunt is only carrying whatever load is in the cab. And if you put a headlight relay harness in that part of the load goes away. Ditto the HVAC fan. But if the battery is still being charged through the shunt it is likely you'll have problems at some point. For instance, if you let the battery get really low and have to jump-start the engine the alternator is going to kick out everything it can to bring that dead battery up to 14.4V. If you have a 100A 1G in and kick the fast idle off quickly then you may get by as that alternator won't give you more than 70A until you get above idle. But Jim's 3G alternator tested at 106A at 1600 alternator RPM, which is about 550 engine RPM assuming you use the standard 460 pulley. Link to comment Share on other sites More sharing options...
viven44 Posted March 5 Author Share Posted March 5 What loads do you intend to put on the existing cab harness, that would exceed 70A? The worst case scenario would be all of the below running - Headlights/Stop light/Turn signals - Electric choke - Electric low-pressure fuel pump - AC/Blower - Maybe a decent speaker/radio - Electric fan - TBD, only if the clutch fan isn't doing its job Link to comment Share on other sites More sharing options...
viven44 Posted March 5 Author Share Posted March 5 What loads do you intend to put on the existing cab harness, that would exceed 70A? That's the key. If the battery-charging load is taken off the shunt by moving the alternator's output to the starter relay then the shunt is only carrying whatever load is in the cab. And if you put a headlight relay harness in that part of the load goes away. Ditto the HVAC fan. But if the battery is still being charged through the shunt it is likely you'll have problems at some point. For instance, if you let the battery get really low and have to jump-start the engine the alternator is going to kick out everything it can to bring that dead battery up to 14.4V. If you have a 100A 1G in and kick the fast idle off quickly then you may get by as that alternator won't give you more than 70A until you get above idle. But Jim's 3G alternator tested at 106A at 1600 alternator RPM, which is about 550 engine RPM assuming you use the standard 460 pulley. I don't believe the shunt is truly bypassed in that case is it ? It will still carry about half the load (2 resistances in parallel). The shunt will always shunt Also I wound't want to bypass the fusible link as it is the safety net if the alternator malfunctioned or shorted internally. Don't ask me how I know Link to comment Share on other sites More sharing options...
viven44 Posted March 5 Author Share Posted March 5 And if you put a headlight relay harness in that part of the load goes away. Ditto the HVAC fan. I thought about relay usage and its effect on alternator load. Even if the current was directly taken off the battery, wouldn't the alternator be in the "loop" and make up for that current loss. This is waay over my ability to predict, Gary, you are definitely better in electrical assessment. I try but always overlook something  Link to comment Share on other sites More sharing options...
Gary Lewis Posted March 5 Share Posted March 5 I don't believe the shunt is truly bypassed in that case is it ? It will still carry about half the load (2 resistances in parallel). The shunt will always shunt Also I wound't want to bypass the fusible link as it is the safety net if the alternator malfunctioned or shorted internally. Don't ask me how I know Wire it this way but don't add the relay nor cut the Y/LG wire on Ckt 654 unless you are installing a Rocketman voltmeter. The output of the alternator goes directly to the battery and not through the shunt. Only the load "in the cab", meaning that going through fuse link L or M which go to the ignition switch, will go through the shunt. Which means the ammeter will always show discharge. And, you should not put a cooling fan on "cab power". The wiring wasn't sized for that much load. Instead put relays out by the fan, powered directly from the battery/starter relay and use key-on power to pull them in. But, having said all that, we've had a long discussion about a better way to wire these trucks and I shipped Jeff/Big Bro 84 a harness to prove out that plan. If you want to read how that is to be done go to this thread, but let's not rehash it here. Link to comment Share on other sites More sharing options...
Gary Lewis Posted March 5 Share Posted March 5 And if you put a headlight relay harness in that part of the load goes away. Ditto the HVAC fan. I thought about relay usage and its effect on alternator load. Even if the current was directly taken off the battery, wouldn't the alternator be in the "loop" and make up for that current loss. This is waay over my ability to predict, Gary, you are definitely better in electrical assessment. I try but always overlook something Assuming you power the relays directly from the battery or the starter relay, which is easier, then using relays to power high-current devices takes the load off of fuse links L & M, meaning off the cab. Yes, the alternator is still in the loop, but the only load on L or M is the miniscule pull-in current. Link to comment Share on other sites More sharing options...
viven44 Posted March 5 Author Share Posted March 5 Wire it this way but don't add the relay nor cut the Y/LG wire on Ckt 654 unless you are installing a Rocketman voltmeter. The output of the alternator goes directly to the battery and not through the shunt. Only the load "in the cab", meaning that going through fuse link L or M which go to the ignition switch, will go through the shunt. Which means the ammeter will always show discharge. And, you should not put a cooling fan on "cab power". The wiring wasn't sized for that much load. Instead put relays out by the fan, powered directly from the battery/starter relay and use key-on power to pull them in. But, having said all that, we've had a long discussion about a better way to wire these trucks and I shipped Jeff/Big Bro 84 a harness to prove out that plan. If you want to read how that is to be done go to this thread, but let's not rehash it here. OK thanks!! Yes, even for the fuel pump I will be using a relay. Horn, Fuel pump always on relay. I figured the fan was a significant current draw so a relay would be needed. The reason I didn't mention relays above is that I was pretty sure even if relays were used the alternator and shunt would see the load. But adding a second charging cable will help definitely. Its almost like halving the resistance of the shunt itself. I'll read that thread fully. Link to comment Share on other sites More sharing options...
Gary Lewis Posted March 5 Share Posted March 5 OK thanks!! Yes, even for the fuel pump I will be using a relay. Horn, Fuel pump always on relay. I figured the fan was a significant current draw so a relay would be needed. The reason I didn't mention relays above is that I was pretty sure even if relays were used the alternator and shunt would see the load. But adding a second charging cable will help definitely. Its almost like halving the resistance of the shunt itself. I'll read that thread fully. You do not want the shunt to see the load if you are adding things like electric cooling fans. And there's no reasonable place for you to get power for the relays after the shunt. So just tie into the starter relay's battery connection and run a fused wire to a fuse/relay unit like Jim and others have. Each individual relay can then have its own fuse and you have everything in a neat unit. But I don't understand what you mean by "But adding a second charging cable will help definitely. Its almost like halving the resistance of the shunt itself." You don't want to add a second charging cable. The big red X means to take the factory charging cable out and replace it with one directly to the starter relay. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted March 5 Share Posted March 5 The worst case scenario would be all of the below running - Headlights/Stop light/Turn signals - Electric choke - Electric low-pressure fuel pump - AC/Blower - Maybe a decent speaker/radio - Electric fan - TBD, only if the clutch fan isn't doing its job The fan & choke are NEVER going through the shunt. The clutch driven fan will always be sufficient (if you have a proper shroud) Gary already mentioned it, so I'm not going to point out the stupidity of electric fans in a remotely stock configuration.. Link to comment Share on other sites More sharing options...
Gary Lewis Posted March 5 Share Posted March 5 The fan & choke are NEVER going through the shunt. The clutch driven fan will always be sufficient (if you have a proper shroud) Gary already mentioned it, so I'm not going to point out the stupidity of electric fans in a remotely stock configuration.. Delco Remy says an alternator is 55% efficient. ScienceDirect says an electric motor is between 70 & 96% efficient. So if we assume the motor is 80% efficient then converting mechanical energy to electric energy & then back to mechanical will be .55 x .80 = 44% efficient. And that, in a nutshell, is Jim's point. Link to comment Share on other sites More sharing options...
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