73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Electrical => Topic started by: Swampyankee74 on June 19, 2023, 04:04:39 PM
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I am converting to a 3 wire alternator to a 1 wire alternator on my 74 gmc c15. In what way do I do this that my ammeter still functions properly? I have big red wire which goes to the lug on the alternator, and then a big white wire that went to the plug at the alternator, it is also tied to the red wire up by the firewall. Then a smaller probably 14 guage brown wire that went to the plug. Any info would be much appreciated.
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Pic(https://uploads.tapatalk-cdn.com/20230619/27dfaa11f463a91994d1a52bca5d082c.jpg)(https://uploads.tapatalk-cdn.com/20230619/6e24a6eea5e55c1931098bbad2599119.jpg)
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What alternator did you get? If it is a quality unit they should have provided you with a tech sheet for rewiring it or call their tech line to ensure your factory ammeter will not become a toasty marshmallow. Sometimes the one wire has the factory connections under the plug so you can run the stock setup.
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The alternator is a cvf 140 Amp. As far as quality level I don't know. I have a wiring schematic, but its kinda vague when it comes to ammeter, just shows running 12 volts to the fuse block. But I'm gonna take your advice and call tech support and make sure I don't end up with a toasty marshmallow. Thank you!
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The factory ammeter is a shunt type that senses the voltage drop across the 10-gauge red wire connecting the B+ stud of the starter solenoid to the firewall junction block. The ammeter is protected by two redundant 4-amp inline fuses, one on either side of the ammeter. As long as you haven't altered the factory wiring between the starter solenoid B+ post and the firewall junction block, AND haven't changed the ammeter connections to the starter solenoid B+ post and firewall junction block, AND haven't eliminated the redundant 4-amp inline fuses, your ammeter is safe and should function as originally designed. However, converting from the original 36-amp(?) alternator to a 140-amp alternator may dramatically increase needle deflection immediately after initial startup when the alternator charging current output is greatest, because the ammeter is located between the alternator and the battery and is calibrated to the original lower capacity alternator output.
The brown (exciter) wire and white (voltage output sensing) wire can be folded back and hidden inside the harness in case you ever wish/need to revert to the original 3-wire style alternator.
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I have not altered any of that, it is still hooked up stock (in that aspect aspect). I did however put a high tourqe starter in and no resistor wire to the starter. But the wiring you are describing is bone stock. Thank you bd
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Well you learn something new every day. I've been staring at this motor for 6 months and sure enough under a rubber cap on top of the alternator is 2 prongs that accept the stock plug. Knowhere in the paperwork does it mention anything about that. Vile you are right again! I'm going to hook it up in the stock manner. Vz and bd thanks for bearing with me. With your help I'm learning my way through this truck.
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Did you check with the manufacturer? Sometimes the two regulator terminals, when hidden by a rubber cap, are bridged and internally connected to power.
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I did not. See, still learning. I will call them. Thank you!
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I should explain, further. My concern is if the regulator terminals under the rubber cap are bridged, connecting the factory two-wire plug to the regulator can back-feed a reduced voltage to the ignition switch, partially powering any Ignition-2 circuits that share the ignition switch terminal, which would result in a constant low-current parasitic draw on the battery. So, you should check with the manufacturer of the alternator before calling the installation done.
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Thank you bd. I'm gonna call the manufacturer today. I have a feeling they are gonna say to follow there schematic. And that's what I'll do. I'll post the answer today.
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Spoke to cvf. They strongly recommend wiring the alternator to their schematic. I guess that is the safest bet.
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Pic(https://uploads.tapatalk-cdn.com/20230706/4b6459a561a8b5d213e3d8a21dd2bb97.jpg)
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The posted image isn't sufficiently clear to read the diagram text. If you want to maintain the factory ammeter function, the alternator B+ (output) lead needs to connect to the vehicle wiring in the OEM location (i.e., the firewall junction block). However, the size of the output lead running between the alternator and the OEM location can be increased (e.g., 4 gauge) without impacting the factory ammeter function.
If you elect to install a high-amperage fuse (rated per the manufacturer's specification or ~120% of the alternator's rated output), it should be inserted into the alternator B+ lead between the alternator and firewall junction block, wherever is most convenient for safe and secure mounting.
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If you upgrade the alternator B+ output lead to 4 gauge, the 12-gauge red wire "2E" that originally spliced into the factory alternator output lead and runs to the firewall bulkhead connector can be rerouted directly to the firewall junction block but will still need to be protected by a 6" long 16-gauge fusible link.
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I hope this pic is better. I labeled a wire "A" I was planning on tying "A" to the red wire that went to the alternator which goes back to the firewall junction box, splits and goes to the bulkhead. Please correct me if that's wrong. (https://uploads.tapatalk-cdn.com/20230707/e77da629a7e37ba785142b774d937755.jpg)
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I just spoke to a friend who mentioned to run the 6 guage wire from the alternator with a 200 amp midi fuse in the line to battery positive. Then go from positive terminal on your battery down to your starter like normal. And coil your 3 old alternator wires up and dont use them. Said everything should function just fine. That sounds right and super simple. That would feed power from the starter up to the junction block on firewall. Does this sound safe and unlike a fire waiting to happen?
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Wiring it that way would work if you had a voltmeter but will defeat a factory ammeter. Sometime this evening or tomorrow I'll create and post a diagram (or modify the existing diagram) that illustrates how you can integrate the alternator into the factory wiring harness so that the original ammeter continues to function.
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That sounds excellent bd! Are you saying it would fry the ammeter or just wouldn't function? Yea, it makes my head is spinning a little.
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The ammeter would indicate zero and battery discharge only. It would never indicate the battery charge current.
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IIRC there are two 4 amp inline glass fuses for the ammeter on each wire. One wire comes off the fusible link at the starter solenoid and the other off the junction block on the firewall. The other stud on the junction block should have a fusible link and go to the main stud on the alternator. As long as these wires are connected the ammeter should function. Let BD do his analysis and wizardry and we can post it in the tech section. I'm working on my 74 as well, trying to get it done! Good luck
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Vz I'm not by the truck now. But that sounds how mine is wired. I found those 2 fuses 4 amp fuse. We'll see what bd drafts up! I get a little nervous, there is a million different ways people wire these things up. Don't want an inferno as an end result! Haha!
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Patients, young Padawan!
8)
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Bd, that is a thing of beauty! Now that I can wrap my head around! That is very much appreciated, thank you for your time!
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Where to begin?
The factory wiring that connected to the original alternator can either be snipped and removed completely or taped up inside the harness (in case you wish to revert to a three-wire alternator sometime later).
During your initial installation, all of the original wiring between the firewall junction block and the starter, including the 16-gauge black wires and dual 4-amp inline fuse holders that connect to the junction block and run to the ammeter can remain unmolested (all of the items contained within the green box in the preceding image, except as noted*). The preexisting 12-gauge red wire that connects the dual stud firewall junction block to the firewall bulkhead connector MUST BE INDEPENDENTLY PROTECTED by a 16-gauge fusible link! To accommodate the significant increase in alternator output, the alternator B+ charge lead size (wire gauge) should be upgraded in accordance with the alternator manufacturer's recommendations. For reference, up to 175 amperes of alternator output can be accommodated safely by a 6-gauge SXL-rated charge wire (which will exhibit a 3% voltage loss at 175 amps of current flow - well within industry-accepted guidelines). Note that the alternator and its charge lead should be isolated from the battery through a suitably rated slow-blow fuse, such as the MIDI, MEGA, or ANL series fuses or an appropriately gauged fusible link. The fuse rating must be at least 120% of the maximum current output of the alternator. For convenience, the charge lead fuse holder can be mounted on the firewall adjacent to the junction block.
Introducing a high-amperage alternator (>100 amps) into a 1973 - 1975 C/K truck with a factory ammeter and original 37 - 55-amp charging system can significantly impact the ammeter's function and sensitivity, resulting in dramatic needle deflection during periods of high charge. The periods of high charge and concomitant extreme needle deflection are generally of short duration as the vehicle battery quickly recovers from cranking the engine during startup and the charge current decreases. Still, needle deflection might appear excessive to the vehicle operator.
The factory ammeter is a shunt type that connects across a specified section of the charge lead, or primary buss, that runs between the battery and the rest of the electrical system in a parallel configuration. Although the gross amount of current moving between the battery and electrical system is carried by the heavy gauge primary buss, a trickle of current bypasses the primary buss through the light gauge parallel configuration routing through the ammeter. Ammeter needle sensitivity to the net flow of charge current is dependent on the proportion of the current passing through the primary buss relative to the current passing through the ammeter. Thus, the wire gauge of the primary buss has a direct effect on ammeter sensitivity. This begs the question, should the factory primary buss be upgraded all the way through from the alternator B+ output stud to the battery cable connection at the starter solenoid B+ stud during the initial alternator installation?
Circuit Analysis:
The factory 10-gauge wire that connects the firewall junction block to the starter solenoid B+ stud will consume (waste as heat) ~7% of the charging voltage while the alternator is generating at its maximum capacity of 140 amperes. This equates to ~1 volt of loss across the 4-foot length of 10-gauge wire. Generally, such a loss isn't particularly significant since the battery will recover fairly rapidly and the charging current will decrease accordingly. As the charging current decreases, the voltage loss across the primary buss will also decrease in an exact relationship according to Ohm's Law (Voltage = Current x Resistance). But a factory ammeter introduces unique circumstances.
If the resulting ammeter needle deflection is wildly excessive or the ammeter's inline 4-amp fuses are overburdened (melt), you can decrease the current flow through and desensitize the ammeter's response by decreasing the voltage loss across the primary buss. This can be accomplished by simply increasing the size (decreasing the gauge) of the original 10-gauge section of the primary buss that connects the junction block to the starter solenoid. Of course, a new appropriately gauged fusible link would need to be installed as well to safely protect the new wire. For example, increasing the size of the primary buss between the firewall junction block and the starter solenoid B+ stud from 10 gauge to 8 gauge would decrease the associated voltage loss during the alternator's maximum output from ~7% to ~3%. Whereas, reconfiguring the primary buss to 6 gauge would decrease the voltage loss to just under 2%.
Based on percentages and the fact that current and voltage are directly proportional, we can estimate the correct wire gauge for the primary buss. The original 10-gauge primary buss, by factory design, normally exhibits <2% voltage loss while carrying the maximum charge current from the original 37 - 55-amp alternator. That means that <2% of the charge current flows through the factory ammeter at maximum charge, which equates to ~1 ampere at a 50-ampere charge rate (2% of 50 amperes). Well, it so happens that only ~2 amperes of current flow through the factory ammeter results in full-scale needle deflection.
The ~7% voltage drop across the primary buss during periods of maximum alternator charging current output after upgrading to a 140-amp alternator will force ~7% of the charge current through the factory ammeter, the ammeter's wiring, and the inline fuse pair — 7% of 140 amps is 9.8 amperes or nearly 2.5 times the current rating of the ammeter's 4-amp inline fuses and ~5 times the current that results in full-scale needle deflection. Therefore, increasing the size (decreasing the gauge) of the primary buss is necessary. Increasing the buss size to 8 gauge produces a maximum ~3% voltage loss with ~3% of the maximum charge current flowing through the factory ammeter — 3% of 140 amperes is ~4 amperes, which is still greater than desired or needed. Increasing the primary buss to 6 gauge produces a maximum voltage loss of <2%, forcing <2% of the maximum charge current through the factory ammeter — 2% of 140 amperes is 2.8 amperes. Hence, by estimation, the factory 10-gauge primary buss connecting the firewall junction block to the starter solenoid B+ battery cable 3/8" stud needs to be upgraded to a 6-gauge SXL wire protected by a 6" long, 10-gauge fusible link installed at the solenoid B+ stud in order for the factory ammeter to function within design parameters.
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Wow BD you are on top of your shoot! Very nicely explained with a nice schematic to match!
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After crunching some numbers I changed my initial thinking regarding modifications to the size of the primary buss and edited my preceeding post and the circuit diagram, accordingly. Assuming my calculations are correct, the 10-gauge primary buss between the firewall junction block and the starter solenoid B+ post needs to be upgraded or the ammeter fuses will blow.
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I will change that wire bd. Thank you again!
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Nicely explained BD 8)
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Thanks, VZ. Hopefully, at least some of it will prove to be true. :)
Swampyankee, keep us apprised so we can incorporate any corrections before it migrates to the Tech Forum... Thanks.
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Sounds good, it's going to be a little bit. Nose is still off the truck, painting core support inner fenders and such. Then to put the nose together. I'm leaving tomorrow thru the weekend for Ohio for Eldora kings royal race. But I will post when I get it up and working. I think you nailed it tho!
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I ordered 1 foot of 10 guage fusible link wire for between the junction block and the starter. Just to verify, even though this is a foot long I still need to cut it down to 6" for it to function properly?
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All fusible links are 6" long. Also, the length of the 6-gauge cable connecting the starter solenoid B+ stud to the firewall junction block should match as close as possible the length of the 10-gauge cable it replaces.
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Sounds good BD, thank you!
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Can I solder the 10 guage fusible link to the 6 guage wire or will that destroy the fusible link?
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First, firmly crimp the wires using an uninsulated, brazed barrel butt splice or parallel splice for a solid mechanical connection. Second, solder the splice using 60/40 rosin core solder. Third, seal the splice using marine grade dual wall, adhesive lined polyolefin heat shrink. Follow similar procedures for the 3/8" and #10 eye terminal ends.
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Thank you bd. Wasn't sure if the heat would ruin it
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A fusible link is just an annealed and tinned, high strand count copper wire jacketed in Hypalon insulation. The insulation provides a temperature tolerance of >300° F. Unless you use an open flame, the insulation will not be damaged by soldering.
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Gotcha, thanks again
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