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OT - More Mack truck trailer wiring - longish

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Dave (IL)

06-05-2005 06:34:21

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I posted the following on Big Mack Trucks and then thought, why not here where all the really smart a. er people are :) (just kidding).

One response was flip polarity and said "I had a one wire alternator so it didn't need anything to switch"

Anyway, here's my post:
I have a 70 R700 and am hooking up my trailer for my skid steer. I'm familar with positive ground from English sports cars and old tractors, but I'm wondering how others deal with it. First thing was I disconnected the trailer brake battery and hooked things up, and the tail lights woundn't work. Discovered they were LED lights and don't work with reversed polarity. Replace with lights with bulbs. The battery for the brakes has a diode to prevent discharge to the tow vehicle. I suspect I can flip that, but I'd like to make it switchable, because I still want to pull trailer with my Avalanhe at times. I am trying to verify whether the brake "coils" will work with reversed polarity. Now I'm dealing with the split brake and turn wiring on the truck. I bought a module - Hobby - for that purpose, but don't know if it will work with reversed polarity and hate to burn it out trying. Next, I believe, IL law requires the trailer brakes to be working over 3000 lbs. This means I'm going to have to add a brake controller to the truck and this might have polarity issues also. So I'm just wondering if anyone else has gone through this whole mess before? I'm new to the Mack. This is the first forum I found. Is this a good place for these kinds of questions? Thanks to any and all!

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Hugh MacKay

06-06-2005 02:13:16

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Re: OT - More Mack truck trailer wiring - longish in reply to Dave (IL), 06-05-2005 06:34:21

Dave: I cant understand what this problem could be. Mack were turning out highway tractors as early as most of the others. I didn't do a lot of trucking back in the 70s, however I do know you could back a Mack under most any trailer and off you went. I know of a couple of fleets with one Mack amoung other tractors. They had trailers specific to jobs being performed. Whoever was free hooked on and went.

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Dave (IL)

06-06-2005 18:22:52

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Re: OT - More Mack truck trailer wiring - longish in reply to Hugh MacKay, 06-06-2005 02:13:16

I understand your question, and I think there are three points to the answer.
1. Those trailers didn't have LED tail lights.
2. They did not have electric brakes run with an electric brake controller.
3. They were already wired with separate stop and turn lights - turn signals and stop did not share the same filament.

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Ryan - WI

06-05-2005 11:44:53

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Re: OT - More Mack truck trailer wiring - longish in reply to Dave (IL), 06-05-2005 06:34:21

It might be a bit of a PITA, but if you don;t want to change anything on the trailer you could always wire in a battery that only powers the trailer and then use relays off the wiring on the truck to control the seperate circuits.

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J. Schwiebert

06-05-2005 10:20:28

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Re: OT - More Mack truck trailer wiring - longish in reply to Dave (IL), 06-05-2005 06:34:21

You have me totally confused. What has the positive ground? The truck or the trailer. Also what make and model alternator is on the truck?

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Dave (IL)

06-05-2005 15:26:06

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Re: OT - More Mack truck trailer wiring - longish in reply to J. Schwiebert, 06-05-2005 10:20:28

The 1970 Mack R700 has positive ground just like my MGA and my Farmall H.

The trailer has a small 12v battery to power the safety brakes that is wired to get charged from the tow vehicle. It can't be completely isolated as the grounds on the trailer are tied into the chassis.

I don't have the specifics on the Mack alternator. I just looked at it quickly to see that it was an alternator.

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J. Schwiebert

06-05-2005 16:21:31

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Re: OT - More Mack truck trailer wiring - longish in reply to Dave (IL), 06-05-2005 15:26:06

How do I contact you?

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Dave (IL)

06-05-2005 17:12:20

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Re: OT - More Mack truck trailer wiring - longish in reply to J. Schwiebert, 06-05-2005 16:21:31

Opened e-mail on this.

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Dave (IL)

06-05-2005 17:16:07

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Re: OT - More Mack truck trailer wiring - longish in reply to Dave (IL), 06-05-2005 17:12:20

Guess you can't just "open" up e-mail addy like you used to. Click on my name in the previous post and you can send me an e-mail via this site. If you want to include your e-mail in that I'll respond directly or I'll send you mine.

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Bob

06-05-2005 09:35:12

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Re: OT - More Mack truck trailer wiring - longish in reply to Dave (IL), 06-05-2005 06:34:21

Is the truck straight 12 Volts, or 12/24 Volts, with a series/parallel switch for starting?

If it were mine, and I intended to use it with modern accesories that need 12 Volt negative ground, I would switch the truck over to negative ground. No matter how it's set up, that shouldn't be too tough to do.

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Dave (IL)

06-05-2005 09:47:27

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Re: OT - More Mack truck trailer wiring - longish in reply to Bob, 06-05-2005 09:35:12

I think it's straight 12. My son's been doing most of the work with it.

As much as I hate to, I think I'm starting to agree with the switch. I'll just have to check out that alternator situation.

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Bob

06-05-2005 09:51:04

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Re: OT - More Mack truck trailer wiring - longish in reply to Dave (IL), 06-05-2005 09:47:27

How many batteries, and what is their voltage?

Many of these older trucks have 4- 6-Volt batteries that are charged in pairs at 12 Volts to each pair, and then switched to series for starting.

Hopefully, yours is a simpler, straight 12-Volt system!

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Dave (IL)

06-05-2005 15:31:15

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Re: OT - More Mack truck trailer wiring - longish in reply to Bob, 06-05-2005 09:51:04

I think it's 2 12's in parallel. I haven't checked it out completely yet. We disconnected the trailer battery and used it for a short tow and it didn't burn out any lights, so I'm pretty sure it's running at 12.

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