erskine2017
Member
I feel compelled to address the Deere-Delco 24 Volt System. I am usually nasty and sarcastic, but this time I mean no disrespect to anyone. However, I have taught 24 Volt seminars at Gathering of the Green for 12 years and have published several articles on the topic in "Green Magazine." I have been working on the system for 54 years(not the same tractor). I would be derelict in my responsibility to my hundreds of former students and readers if I didn't clarify some of the misconceptions about the subject. Before I published a word, I confirmed my conclusions with the head of the electrical engineering department at a major university.
Starting and charging are fairly straight forward. The chassis is nor used for starting or charging as the frame may as well been made out of wood or plastic. The starter, generator and regulator are isolated from the frame and each have a dedicated wire back to the positive terminal of one of the batteries. This concept is not unique as the fiberglass body Corvette has dedicated wires for body grounding. For starting, the middle wire connected from chassis to the midpoint of the batteries has absolutely no bearing whether it is hooked up or in the glove compartment of your pickup. Oops, there is that sarcasm creeping through. The generator charges the batteries in series with the regulator set at about 28 volts.
Complexity begins when we add lights, fuel gauge and outlet socket to the system. To save money Deere had Delco devise a way to use 12 volt bulbs, fuel gauge, and sending unit that were the same as used on the conventional gasoline tractors. Also a middle wire(as previously discussed) was added. The importance of the middle wire has been overstressed. The wire is there more as a default mode in case a bulb burns out or someone adds a spotlight or somehow unbalances the circuitry.
Here is how the lights work. For the sake of simplicity let us start with the 720 diesel with one set of headlights. We will be physicists instead of engineers for this exercise and have electricity flow from negative to positive. We will use two 12 volt batteries in series instead of 4. 1) We start at the negative terminal of one battery with a heavy cable to the top terminal of the solenoid. 2) Now we go from the top terminal of the solenoid with a smaller wire to the BAT terminal of the regulator. 3) From there we go to the A terminal of the switch which is a double pole single throw. This simply means two sets of contacts controlled by one lever. 4) We turn the switch and connect terminal A to the terminal that goes by wire to the left headlight. Current goes through the bulb and out the base which is connected to the frame of the tractor. Current now flows through the frame(think of it as a giant wire) and enters the right hand bulb THROUGH THE BASE which is bolted to the frame. 5)Current now flows through the right hand bulb the reverse way and goes down the wire to the switch terminal for the right hand headlight. 6) Remember this is a double contact switch. Since the contacts are closed, headlight 2 terminal is connected to terminal B of the switch. 7) Current now flows from terminal B to the case of the voltage regulator and from there to the isolated terminal on the side of the starter and then to the positive terminal of the other battery and the circuit is complete.
Notice that we don't need the middle wire for this. The bulbs are the same wattage and are 12 volt, but they are in series and the 24 volts split evenly giving each bulb 12 volts. If we hook the positive lead of a voltmeter to the positive terminal of the battery and put the negative lead of the voltmeter on terminal A of the switch it will read 24 Volts. If we put the negative lead of the meter on the frame it will read 12 volts, and if we put the negative lead on terminal B it will read 0.
Much has bee said about John Deere Bulletin 268 and a lot of talk about two 12 volt circuits with one negative and one positive. This was probably written to try and simplify a series parallel circuit and probable by technical writers rather than engineers, It just doesn't work that way. Deere is NOT infallible. The 730 was rated as a 5 plow tractor. It would only pull 5 if you didn't put it in the ground! For example, Let's take the batteries out of the tractor and put them on the floor. We can pull start the tractor and the lights will run totally off the generator. Obviously there are now NO positive and negative 12 volt circuits. 28 Volts are coming out the A1 terminal of the generator to terminal A of the switch. The Left light gets 14 volts, current travels through the frame and enters the right light the reverse way which gets the remaining 14 volts. Current travels through terminal B and then returns to the A2 terminal of the generator to complete the circuit The ONLY Negative is the A1 terminal of the generator and the ONLY positive is the A2 terminal of the generator. There is only one 28 Volt circuit.
As stated earlier the middle wire is mostly a default mode. In a healthy system there is very little current flowing through the middle wire. In fact a healthy system will work fine with the wire removed. However it is useful. In the generator scenario described above, if one of the lights burned out we have neither since it is a series circuit. Even if we put the batteries back in a burned bulb will be lights out. With the middle wire in we now have a series-Parallel circuit and current can go an alternate route, Let us have the right bulb burn out. Current goes through the switch and light from terminal A and through the frame. Since the right bulb is out current goes DOWN the middle wire and through the battery to complete the circuit. Let us take the opposite case where the left bulb burns out. Current Now flows through the battery and UP the middle wire and through the right hand light and through the switch back to the other battery to complete the circuit Notice in the first case current flowed down the middle wire and up the middle wire in the second case. Both cases are 28 Volt circuits. There are no 12 positive and 12 negative circuits in spite of Bulletin 268! For there to be two different circuits of different polarity current would have to be flowing both UP and DOWN the middle wire at the same time and current would have to be backing up against the generator. This concept is both preposterous and impossible.
Conclusions: I have gone on far too long. Bulletin 268 is useful for a good diagram, voltage drop and field resistance data, and some good trouble shooting techniques but has over simplified the explanation of how it works. We have just scratched the surface of the 24Volt system here. We didn't talk about the fuel gauge, the generator light, the remote socket or unbalanced loads except for the burned bulb examples. If you want to learn more about 24 Volts and some great seminars ranging from painting to carburetor diagnosis and over haul come to Gathering of the Green at Davenport, Iowa March 21, 22, 23, an24, My presentation will be in color on power point with lots of diagrams and pictures. Thanks for reading this.
Starting and charging are fairly straight forward. The chassis is nor used for starting or charging as the frame may as well been made out of wood or plastic. The starter, generator and regulator are isolated from the frame and each have a dedicated wire back to the positive terminal of one of the batteries. This concept is not unique as the fiberglass body Corvette has dedicated wires for body grounding. For starting, the middle wire connected from chassis to the midpoint of the batteries has absolutely no bearing whether it is hooked up or in the glove compartment of your pickup. Oops, there is that sarcasm creeping through. The generator charges the batteries in series with the regulator set at about 28 volts.
Complexity begins when we add lights, fuel gauge and outlet socket to the system. To save money Deere had Delco devise a way to use 12 volt bulbs, fuel gauge, and sending unit that were the same as used on the conventional gasoline tractors. Also a middle wire(as previously discussed) was added. The importance of the middle wire has been overstressed. The wire is there more as a default mode in case a bulb burns out or someone adds a spotlight or somehow unbalances the circuitry.
Here is how the lights work. For the sake of simplicity let us start with the 720 diesel with one set of headlights. We will be physicists instead of engineers for this exercise and have electricity flow from negative to positive. We will use two 12 volt batteries in series instead of 4. 1) We start at the negative terminal of one battery with a heavy cable to the top terminal of the solenoid. 2) Now we go from the top terminal of the solenoid with a smaller wire to the BAT terminal of the regulator. 3) From there we go to the A terminal of the switch which is a double pole single throw. This simply means two sets of contacts controlled by one lever. 4) We turn the switch and connect terminal A to the terminal that goes by wire to the left headlight. Current goes through the bulb and out the base which is connected to the frame of the tractor. Current now flows through the frame(think of it as a giant wire) and enters the right hand bulb THROUGH THE BASE which is bolted to the frame. 5)Current now flows through the right hand bulb the reverse way and goes down the wire to the switch terminal for the right hand headlight. 6) Remember this is a double contact switch. Since the contacts are closed, headlight 2 terminal is connected to terminal B of the switch. 7) Current now flows from terminal B to the case of the voltage regulator and from there to the isolated terminal on the side of the starter and then to the positive terminal of the other battery and the circuit is complete.
Notice that we don't need the middle wire for this. The bulbs are the same wattage and are 12 volt, but they are in series and the 24 volts split evenly giving each bulb 12 volts. If we hook the positive lead of a voltmeter to the positive terminal of the battery and put the negative lead of the voltmeter on terminal A of the switch it will read 24 Volts. If we put the negative lead of the meter on the frame it will read 12 volts, and if we put the negative lead on terminal B it will read 0.
Much has bee said about John Deere Bulletin 268 and a lot of talk about two 12 volt circuits with one negative and one positive. This was probably written to try and simplify a series parallel circuit and probable by technical writers rather than engineers, It just doesn't work that way. Deere is NOT infallible. The 730 was rated as a 5 plow tractor. It would only pull 5 if you didn't put it in the ground! For example, Let's take the batteries out of the tractor and put them on the floor. We can pull start the tractor and the lights will run totally off the generator. Obviously there are now NO positive and negative 12 volt circuits. 28 Volts are coming out the A1 terminal of the generator to terminal A of the switch. The Left light gets 14 volts, current travels through the frame and enters the right light the reverse way which gets the remaining 14 volts. Current travels through terminal B and then returns to the A2 terminal of the generator to complete the circuit The ONLY Negative is the A1 terminal of the generator and the ONLY positive is the A2 terminal of the generator. There is only one 28 Volt circuit.
As stated earlier the middle wire is mostly a default mode. In a healthy system there is very little current flowing through the middle wire. In fact a healthy system will work fine with the wire removed. However it is useful. In the generator scenario described above, if one of the lights burned out we have neither since it is a series circuit. Even if we put the batteries back in a burned bulb will be lights out. With the middle wire in we now have a series-Parallel circuit and current can go an alternate route, Let us have the right bulb burn out. Current goes through the switch and light from terminal A and through the frame. Since the right bulb is out current goes DOWN the middle wire and through the battery to complete the circuit. Let us take the opposite case where the left bulb burns out. Current Now flows through the battery and UP the middle wire and through the right hand light and through the switch back to the other battery to complete the circuit Notice in the first case current flowed down the middle wire and up the middle wire in the second case. Both cases are 28 Volt circuits. There are no 12 positive and 12 negative circuits in spite of Bulletin 268! For there to be two different circuits of different polarity current would have to be flowing both UP and DOWN the middle wire at the same time and current would have to be backing up against the generator. This concept is both preposterous and impossible.
Conclusions: I have gone on far too long. Bulletin 268 is useful for a good diagram, voltage drop and field resistance data, and some good trouble shooting techniques but has over simplified the explanation of how it works. We have just scratched the surface of the 24Volt system here. We didn't talk about the fuel gauge, the generator light, the remote socket or unbalanced loads except for the burned bulb examples. If you want to learn more about 24 Volts and some great seminars ranging from painting to carburetor diagnosis and over haul come to Gathering of the Green at Davenport, Iowa March 21, 22, 23, an24, My presentation will be in color on power point with lots of diagrams and pictures. Thanks for reading this.
