B model voltage

[Licensed to kill]

I am assuming that the B-61's were 12 volt systems since they have a series parallel switch to run a 24V starter in a 12V system (or does it run a 12V starter in a 6V system????, Hmmmmmmm, that just occurred to me.) Am I safe to make that assumption and also, is it safe to assume that they were negative ground??. If this is so (with the trucks that I have) were they 12V neg ground from the beginning or were the first B models 6V, and if so, when did they change?. Both trucks that I have parts from are '59 models. 

[Brian L Blaylock]

I don’t know about the gas powered trucks but the diesel powered trucks were 24 volt starters with the rest of the truck being 12 volts and positive ground 

[doubleclutchinweasel]

All the old stuff I ever saw had 2 6V batteries in series on each side, making for a 12V system.  Many times those got replaced by a single 12V battery on each side.  SOME had the relay to bridge the 2 12V sets in series to run a 24V starter.  Some just had 12V starters.

As far as I know, all the stuff that vintage was originally positive ground.

I never was around any gas powered stuff.  All ours were diesels.

Edited November 6, 2023 by doubleclutchinweasel

[Geoff Weeks]

coupla ways to determine what the truck is now. Some are not 100% fool proof, but will give a good indication.

1) 1st and foremost, look to see if it has an alternator. If so, which polarity is connected to the frame. If it does, it doesn't matter what the truck was originally, but does matter now. Hook the battery up backwards and you'll destroy any diodes.

2) assuming it has a generator, install a battery (it will not hurt a generator if the battery is "backwards) and turn on every load you can think of and look at the ammeter. If it points toward discharge, then you have it correct, if it points toward charge then it is backward. Be sure to disconnect any radio or other complex electrical component before connecting the battery.

3) often times generator voltage regulators are marked as to polarity. Whether the marking are still readable after all these years, is another matter.

4) some electric gauges for temp, fuel etc, will try and read backwards, go below the low stop when hooked to reverse polarity

If it has a generator, and no indication above helps, it doesn't matter other than "looks" for period correctness, it can run either polarity (have to repolarize the generator for the polarity you choose) but neg ground affords much more options in today's world than positive ground.

[Freightrain]

Yup, 12v system, 24 start, POS. Grd.

 

Two 6 v on each side, wired into ser/par switch that mates then either 24 start or 12v running.  You could put one 12 each side and eliminate some cables and work fine.  6v batteries just had better capacity in those days.  As long as the s/p switch was kept in working order all was well.

I spent the first summer trying to get mine reliable.  Gave up and converted to 12v start, alternator and neg. ground.

Of course now I went off the reservation and put air start!!

[mattb73lt]

I have a friend that has an early production B42 that is very original and is 6V Pos ground. It is possible the early gas powered trucks with small displacement gas engines were that way, too. But my ‘63 B42 was 12V Pos ground when I got it.  Most big manufacturers switched to 12 volts around the mid-50’s.

[mechohaulic]

100% in agreement with Matt's post. EARLY  production gas engines didn't have a need for much in the electrical dept. two headlights ONE tail light . no usb  ports, fm radios . 6 volt battery enough to start truck. can't remember tooo many 6 volt units at Mack by late 60's. even series /parallel switches weren't common in many later trucks. did change the switches  on the older trucks . 

[mowerman]

These guys are making stuff confusing is Jeff said  positive ground looks like it sounds positive as grounded to the frame parallel switch for the 24 V starter Bob

[mowerman]

Yes, they were originally four  6 V