F786T Batteries

[att]

Don`t know if you guys can help me.....The truck is a 78` F786T.

Two12V batteries, both pushing 765 amps.

Positive wire goes to the starter motor, on the positive connection.

Earth wire from battery goes to negative/ground on the starter.

I connected the two batteries together, using the patch lead to connect the negative from one battery, to the positive on the second.....Flicked the key to start her up....And the starter didn`t want to do anything, it tried, it was under load, then smoke started coming from the second battery, so we pulled her off the truck real quick!

Any ideas out there?

[Rob]

There is a possibility you have a 24VDC starter motor. There will be a tag affixed to the starter motor body that will be stamped with it's voltage, or if the truck uses a starter solenoid, (magnetic switch) and is mounted on the starter itself, the voltage requirement will be stamped into the unit someplace. This is usually close to where the mounting bolts retain it to the starter motor.

If you will post up the serial number of the truck someone here will probably decode it for voltage requirements. I've seen the symptoms mentioned due to bad/dirty electrical connections, shorted windings in a starter motor or magnetic switch, or a defective battery internally.

It seems like you mentioned the truck's electrical system was 12VDC, and the starting system is 24VDC from another post. If this is the case the alternator for this truck would be of a special variety and I'd probably look to revamp it to a more conventional style replacing the 24VDC starter motor with a 12VDC unit. Battery technology is much better now than when that system was available.

Rob

[att]

Thanks for that info so far....

The serial No. of the truck is 7814, I am not sure this is correct, as it was given to me on the phone tonight.

When I was under the truck, tracing the wires, I noticed there was no solenoid on the outside of the starter.

[David]

Sounds like you wired the batteries in series. That would give you 24 volts. You probably have a 12 volt starter if there's no series parallel switch. The selenoid must be on the starter or you wouldn't be able to hook the batteries directly to it. Are there smaller wires also hooked to the starter?

[David]

The truck may also be a positive ground.

[Rob]

Thanks for that info so far....

The serial No. of the truck is 7814, I am not sure this is correct, as it was given to me on the phone tonight.

When I was under the truck, tracing the wires, I noticed there was no solenoid on the outside of the starter.

In all likelyhood you then have a remote solenoid. If this is defective as in burnt up internally, it can present the problems experienced. Pull the large wires from the magnetic switch, (solenoid) that connect the batteries to the starter motor and have a helper mash the starter button, or turn the key, (depending on application) and measure the resistance from the terminals the battery lead, and starter lead attach to when this thing clunks shut. This reading should be less than one ohm displayed on the meter. If much more, replace, or repair the switch. Don't hold this switch engaged for over 45 seconds as they do generate heat internally and can open internally and you can be screwed.

Your starter uses a "Dyer" drive relying on rapid acceleration of the rotor to throw the drive into the flywheel teeth. and a large spring to retract it.

Rob

[Rob]

Sounds like you wired the batteries in series. That would give you 24 volts. You probably have a 12 volt starter if there's no series parallel switch. The selenoid must be on the starter or you wouldn't be able to hook the batteries directly to it. Are there smaller wires also hooked to the starter?

I think he has that "bastardized" system when they were trying to get away from the series/parallel switch setup but battery technology wasn't quite up to par yet. I've seen the alternator with several ouput posts used in this application. I think they were Leece-Neville/Prestolite, or Motorola. It didn't stay around long.

Rob

[att]

Thanks for your time guys, it is appreciated.

I think I need to get the cab tilted and have a good look around for tags, wire traces and start with 12 volts and do some measurements as suggested with a multi meter.....Always better to start with less and work up, when electrics are concerned.

I have time on my side, so damage will hopefully be eliminated.

Thanks again

att.

[rhasler]

I think Rob may be right. The chassis record shows a 24 volt starter arrangement and a 12 volt alternator.

[b61mack]

I think Rob may be right. The chassis record shows a 24 volt starter arrangement and a 12 volt alternator.

My brothers 78-F Model has a 350 cummins and it is 12 volt negative ground he has 4 batteries. an E6 MACK engine will start with 2 good 12 volt batteries. Ron

[David]

I think Rob may be right. The chassis record shows a 24 volt starter arrangement and a 12 volt alternator.

Can you have this arangement without a series parallel switch?

[Rob]

My brothers 78-F Model has a 350 cummins and it is 12 volt negative ground he has 4 batteries. an E6 MACK engine will start with 2 good 12 volt batteries. Ron

Starting battery technology where a group 31 series cell will put out 1150 CCA was unheard of until the mid 1980's. Most truck manufacturers along that time frame were looking to lose at least one battery for both weight, and cost savings. All of my trucks have a pair of group 31's in them and start fine down to about 15 degrees without heat although they do need to crank a while when that cold.

When this system was brought out they still needed the capacity of a series fed starter circuit but wanted to retain the 12VDC basic electrical system in the truck. As is widely known, series/parallel switches were troublsome but this new system was so against convention at the time it never gained a foothold in the marketplace. Finally battery technology gained ground and obsoleted it.

I seen one on an early 80's something truck and fought it for a couple of days as the truck had caught on fire unrelated to this. Everything had to be rewired and figgering this sucker out wasn't easy at the time.

Rob

[41chevy]

This was in a post I saw awhile back, may help. It also gives a link for wiring schmatics Paul

[att]

Thanks for all the help and advice guys.

Turned out is was crap batteries. Two new batteries and she fired instantly

[Rob]

Thanks for all the help and advice guys.

Turned out is was crap batteries. Two new batteries and she fired instantly

Happy to hear you had success. I had assumed you were using known good batteries.

Rob