R model wire chart.

[hicrop10]

Looking for a wire chart for my 1979 r model.The alternator is not charging the battery.There is just 2 wires coming out of it.One is a ground wire going to the engine block,the other one is another one going to the frame as a ground.There’s no hot wire going back to the starter to charge the batteries.

[Geoff Weeks]

Well, that is a new one on me "both positive and negative ground on the same vehicle!

[mechohaulic]

How is the truck set up with the batteries? ==positive post to ground or negative post?  did  you check wires from the starter looking for a possible burnt wire which may have been power source alt/starter chafed through was thought to be a ground then connected to chassis. alternator have a terminal marked + ? run temp new wire from alt to starter? 

[mechohaulic]

1 hour ago, hicrop10 said:

Looking for a wire chart for my 1979 r model.The alternator is not charging the battery.There is just 2 wires coming out of it.One is a ground wire going to the engine block,the other one is another one going to the frame as a ground.There’s no hot wire going back to the starter to charge the batteries.

when you say "just two wires coming out" both to ground ; are they both on same terminal or on two separate post ? same term ok. missing power wire to alt. two wires on separate terminals - back to following wires.

[Geoff Weeks]

Photo is worth 1000 words

[hicrop10]

12 minutes ago, mechohaulic said:

when you say "just two wires coming out" both to ground ; are they both on same terminal or on two separate post ? same term ok. missing power wire to alt. two wires on separate terminals - back to following wires.

Two separate post.One is a ground to the frame,the other is a ground going to engine block then to frame.I checked the circuit breakers and the 7 in the middle are hot on both sides,with the key off.I throught the power went from the accessory relay to the breakers,mine is the opposite.

[hicrop10]

13 minutes ago, Geoff Weeks said:

Photo is worth 1000 words

I will get them in morning.I will say this the truck has 4 12volt batteries.The 2 on driver side the positive post are on the front side,passenger side they are on the backside.

[mechohaulic]

2 minutes ago, hicrop10 said:

I will get them in morning.I will say this the truck has 4 12volt batteries.The 2 on driver side the positive post are on the front side,passenger side they are on the backside.

far as the post being opposite in box left vs right could be just how they were installed. important issue is they are HOOKED up the SAME ! negative ground all neg post are ground ,   grounds from alt to block to frame should be correct. the other ground wire to frame I go with idea it's wrong. do you know history of this unit terms of who did what and when, ? new alternator ? this just started or been since ??? was worked on .

[hicrop10]

It been in the family since new.My problem is all of our mechanics have passed onto Mack heaven.

[hicrop10]

Here’s the pictures 

[Geoff Weeks]

Ok you have a Delco 25Si or 26SI (they are the same externally). The wire on the top is ground and the one that looks like a side-post battery terminal is output.

If everything is as it should be on the Delco, the post and bolt color should indicate positive or negative ground on the design of the alternator. They were made in both polarity but negative ground is by far the most prevalent.

Blowing up your picture as much as I can, the output bolt looks to be red, indicating a normal negative ground (case of the alternator is neg, output post is positive) system.

confirm by tracing the heavy cables from the neg posts on the batteries and find out where they go.

The output on the alternator should go to battery positive, either at the starter solenoid or somewhere else that can handle full output of the alternator. 

[hicrop10]

Looking at pic 1,the 3 circuit breakers on the right are dead all the time even know the heater does work.There’s no hot next 7 breakers are always hot key on or off.What is making them hot them hot is the third wire to the left,disconnect this wire and they go dead.I thought the power goes from batteries to starter,then key switch.The only thing that should be hot all the time is the breaker for the flashers and maybe the cable dome light.

Picture 2 of the alternator you can see the 2 wires coming off the alternator are both ground wires,I’m thinking the little black plug you see should be the hot wire leading back to the batteries.

[Geoff Weeks]

No, that is the relay terminal and not used in most cases. 

the "hot" output is the one with the blue arrow pointing to it

alt.odg

[hicrop10]

Geoff,the top post that is white in color goes to the engine block then to the frame.The red wire goes directly to the frame.l’m thinking this may be the hot wire?

[Geoff Weeks]

Just now, hicrop10 said:

Geoff,the top post that is white in color goes to the engine block then to the frame.The red wire goes directly to the frame.l’m thinking this may be the hot wire?

Yes, that is the output wire and should go to battery positive.

[hicrop10]

Having trouble downloading your pic,I will run that red wire to the batteries positive side.Thanks for the help.

[Geoff Weeks]

Yeah I tried to edit your picture with a blue arrow pointing to the output but not sure I was successful.

[mechohaulic]

possibly the output wire from alt has a larger eyelet terminal (1/2 dia) on it at other end which should be on the starter battery post and was mistakenly thought to be a ground wire and put on frame. is that wire close enough to reach starter post ?

[Geoff Weeks]

The correct 6 ga lead is Delco # 1971105 which is so long obsolete it doesn't even get any "hits" on the internet anymore.

I used side terminal battery cables and drilled out the center hole for the large (7/16" IIRC) bolt. 

Note: the hex on the output bolt is ISOLATED from the output, and can not be used to check for voltage.

[mechohaulic]

another large gauge (red) wire should also be on the starter post which would go to circuit breakers as shown in picture; explaining why unit has power to breakers just no charge to batteries. number of methods _test light /meter take wire off chassis ground and alt feed using preferred method check see if its the same wire. it is the connect to starter battery post and back on alt

[Mark T]

It would seem the one (big post) on the alternator should go to the starter. Or ???? I guess to the (+) terminal on a battery. NOT be grounded to the truck anywhere. Kinda wonder how that was hooked up like that in the first place, and I wouldn't get my hopes up for that alternator ot be any good if the truck had been running like that. I think that's the first thing needs to be addressed then determine why some posts are hot and some are not on the pillar block.  If you're charging the batteries with a battery charger, anything from the batteries will work as long as they have some charge. Right now ??? looks like the alternator is just a passenger there for an abusive ride .

[Geoff Weeks]

If run with output hooked to ground, it will never produce any voltage and will not be harmed.

[mechohaulic]

6 minutes ago, Mark T said:

It would seem the one (big post) on the alternator should go to the starter. Or ???? I guess to the (+) terminal on a battery. NOT be grounded to the truck anywhere. Kinda wonder how that was hooked up like that in the first place, and I wouldn't get my hopes up for that alternator ot be any good if the truck had been running like that. I think that's the first thing needs to be addressed then determine why some posts are hot and some are not on the pillar block.  If you're charging the batteries with a battery charger, anything from the batteries will work as long as they have some charge. Right now ??? looks like the alternator is just a passenger there for an abusive ride .

not sure if the alt would have damage since there wasn't a power wire to  it. an alt needs power to create power. a generator makes power. two wire to ground might not be total all bad. not best just not bad. some acc go on with the key switch

[Geoff Weeks]

I used the 25SI as the basis for my multi-staged (IUoU) charging systems. I know them intimately down to every last nut and bolt.

 They are extremely robust and hard to damage.

On the ones I modified, they would at times, run full field output for up to 9hrs at a time. 

[Mark T]

That wire that's grounded from the post on the alternator is not supposed to be grounded . The why it is and the path it should make back to the batteries ??? there could be several answers to that part . But it definitely shouldn't be that post on the alternator to ground. Hard one to see from a couple pictures and text, but might make way more sense to see where it got mixed up if able to see it in person. That wire could be correct in a couple different paths (hot to starter or hot to hot side of a solenoid or somewhere else they share a common juncture on some form of isolator) . It's not on one of them now though.