richf439 Posted October 22, 2009 Share Posted October 22, 2009 I want to convert a 74 DM 800 from positive to negative ground. I plan on staying with 24 volts and keep the series parallel switch. I started doing it by switching the alt and starter wires, disconnected the gages but I kept getting voltage at the ground terminal at the battery. If anyone has any pointers on the best way of doing this, I'm all ears. Quote Link to comment https://www.bigmacktrucks.com/topic/11134-coverting-dm-800-from-positive-to-negative-ground/ Share on other sites More sharing options...
ThaddeusW Posted October 22, 2009 Share Posted October 22, 2009 I want to convert a 74 DM 800 from positive to negative ground. I plan on staying with 24 volts and keep the series parallel switch. I started doing it by switching the alt and starter wires, disconnected the gages but I kept getting voltage at the ground terminal at the battery. If anyone has any pointers on the best way of doing this, I'm all ears.Well first off reversing the wire to the starter will make it spin the engine backwards. So that needs to be replaced or rebuilt for negative ground. Alternator also needs to be rebuilt, rectifier swapped for negative ground or replaced with one that is negative ground. Any DC motor that is wired backwards will spin backwards. So any pump or wiper motors will also have to be reversed. If a motor is designed with a grounded case then you might have a problem mounting it and getting it to spin the other way.Feel free to keep asking questions. Quote -Thad What America needs is less bull and more Bulldog! Link to comment https://www.bigmacktrucks.com/topic/11134-coverting-dm-800-from-positive-to-negative-ground/#findComment-42242 Share on other sites More sharing options...
HK Trucking Posted October 22, 2009 Share Posted October 22, 2009 I want to convert a 74 DM 800 from positive to negative ground. I plan on staying with 24 volts and keep the series parallel switch. I started doing it by switching the alt and starter wires, disconnected the gages but I kept getting voltage at the ground terminal at the battery. If anyone has any pointers on the best way of doing this, I'm all ears.All the 1974 DM's that I've encountered were a 12 volt positive ground system, are you sure it's 24 volts, or are you just assuming that because it has (4) 6 volt batteries?Never seen a 74 with a series parallel switch.Changing polarity will not make the starter turn backwards, however the heater blower motor will turn backwards unless you reverse the connections to it.If you've got the old type alternator with the floating ground you can just reverse the connections on that too (ground the negative and connect the hot wire to the alt. positive terminal.The voltmeter and temp gauges will have to be replaced with units designed for neg. ground.A 74 would have air operated wipers, so the wipers are not an issue.. Quote "If You Can't Shift It Smoothly, You Shouldn't Be Driving It" Link to comment https://www.bigmacktrucks.com/topic/11134-coverting-dm-800-from-positive-to-negative-ground/#findComment-42246 Share on other sites More sharing options...
richf439 Posted October 24, 2009 Author Share Posted October 24, 2009 Well first off reversing the wire to the starter will make it spin the engine backwards. So that needs to be replaced or rebuilt for negative ground. Alternator also needs to be rebuilt, rectifier swapped for negative ground or replaced with one that is negative ground. Any DC motor that is wired backwards will spin backwards. So any pump or wiper motors will also have to be reversed. If a motor is designed with a grounded case then you might have a problem mounting it and getting it to spin the other way.Feel free to keep asking questions. Quote Link to comment https://www.bigmacktrucks.com/topic/11134-coverting-dm-800-from-positive-to-negative-ground/#findComment-42363 Share on other sites More sharing options...
richf439 Posted October 24, 2009 Author Share Posted October 24, 2009 It has a series parallel switch so the 24 volts is just for starting. You're right on everything else being 12 volts. Quote Link to comment https://www.bigmacktrucks.com/topic/11134-coverting-dm-800-from-positive-to-negative-ground/#findComment-42364 Share on other sites More sharing options...
fastwolff Posted October 28, 2009 Share Posted October 28, 2009 Well first off reversing the wire to the starter will make it spin the engine backwards. So that needs to be replaced or rebuilt for negative ground. Alternator also needs to be rebuilt, rectifier swapped for negative ground or replaced with one that is negative ground. Any DC motor that is wired backwards will spin backwards. So any pump or wiper motors will also have to be reversed. If a motor is designed with a grounded case then you might have a problem mounting it and getting it to spin the other way.Feel free to keep asking questions.Tranks for the info. I to wil be making the ground swap in a 1960 H67LT. I will watch and respond as my project moves forward.fastwolff@q.com Quote Link to comment https://www.bigmacktrucks.com/topic/11134-coverting-dm-800-from-positive-to-negative-ground/#findComment-42549 Share on other sites More sharing options...
MACKS Posted October 28, 2009 Share Posted October 28, 2009 Why not just leave it positive ground,I had two DM's with positive ground and never had any problem with it,just have to be careful when hooking up a radio.Or if you want to pull a trailer,that might be a problem. Quote Link to comment https://www.bigmacktrucks.com/topic/11134-coverting-dm-800-from-positive-to-negative-ground/#findComment-42588 Share on other sites More sharing options...
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