Alternator PCM controls?

["Mad Max"]

We're installing a '93 6BT/518 combo in our Ramcharger, complete with the '93 engine wiring harness, and in the FSM is indicates the PCM is connected to part of the alternator's wiring, but we cannot figure out why.

There are 4 main terminals coming off the alternator: (A) 6BK/WT - engine ground, (B) A142 18DB auto shutdown relay, (C) - PCM (generator field???), and (D) alternator feed to battery.

Ground and main hot are no sweat, but what does the PCM feed do and while I'm at it what does the auto-shutdown relay do?

Can anyone educate us on what/why the PCM is in the alternator's wiring harness... and does it need to remain in the loop?

We had a 3-wire GM alternator in the RC before and now we're trying to figure out how to make the 6BT alternator play nice with the RC's wiring.

Any input is greatly appreciated.

- Sam

 

[Paychk]

When Dodge went to the computer I guess they figured that was a better way to control charging. The PCM controls the ASD relay, which in turn controls the alternator.

The first pic in this gallery is a basic of the PCM schematic and will show how it all ties together:

https://www.turbodieselregister.com/user_gallery/displayalbum.php?&userid=5777&albumid=678

 

[KOG]

You can just wire it for the previous external regulator and forget the PCM regulator function.

 

[PToombs]

Sam, the ASD relay turns most of the stuff under the hood on and off. The ECM is the voltage regulator on the 91. 5 and up trucks.



The GM had an internal regulator, the Dodge is external.



Run a keyed hot to the Dk BL wire. And put a regulator in circuit in the DkGn wire, then to a keyed hot. Should work perfect.

 

[Moab4x4]

Hi Sam, I would recommend going with the old style Mopar regulator as well and not using the PCM as the regulator. Easy modification as mentioned here. I just pulled down my old Duster manual and wired the regulator that way. I have run this swap and I have run with the factory PCM.



Issue #1, ASD relay. Makes sense on a FI gasser. There are times that you want to shut down that fuel pump and the AutoShutDown relay does that for you. Why that relay is still on the Diesel is beyond me, I can see in no way how it shuts down anything but the alternator.



The alternator is an easier story. The alternator charge current varies as the voltage across the field coil varies. The old school Mopar regulator supplied a variable voltage to one of the field terminals based on system voltage. Need more juice, the regulator increased its output voltage to match the need. The other field terminal was grounded.



The PCM does the same thing, except in reverse. One alternator field terminal is connected to the battery and the PCM field control terminal voltage varies from 12V down toward ground with a lower control voltage producing higher alternator current as the voltage difference across the field windings is increasing. Seems counter-intuitive, but I think the reason is that solid state electronics can sink current easier and cooler than source current. It is easier to pull voltage down than boost it up with SS components. In the original solid state regulators there was lots of space for heat dissipation, but that PCM has pretty dense component layout. I took one apart for fun once.



So why would I go with the old school layout over the more modern PCM?

1. Simplicity: If I have charging issues, a plug and swap of a regulator is cake.

2. Redundancy: Every old Mopar usually has a spare regulator in the glove box. :-laf A spare PCM is more expensive and bigger.

You hear of the regulator component of the PCM dying from time to time. New old school regulator $25, New PCM $250.

If the PCM dies and it is not the regulator you can still drive home.

If you remove the Alternator control from the PCM, all you need the PCM for is Grid heat, Overdrive control, cruise control, and maybe your tach signal. None of those is going to leave you stranded.

My . 02 and maybe worth that.

Ken

 

[Greenleaf]

Run a keyed hot to the Dk BL wire. And put a regulator in circuit in the DkGn wire, then to a keyed hot. Should work perfect.

I think we need some clearification on that, at least I do. :{ Can someone describe where the wires are at and where/how they are cut/spliced etc. to install the ext reg. JUst yank them out of the PCM??



Also, what reg would suite best? Like the one for my 1990 truck? I'd have to have the connector or use a reg that does not require that funky connector.



Somebody, not long ago needed the connector and the only way to obtain one is from a salvage vehicle. Not sure what year/model of vehicle to look for though... ... ... ... Maybe they are all the same regulator though the years??????



WE should get a stick on the installation. THis would really help out alot of folks who have the 91. 5-up trks. Maybe get an article printed in the mag.

 

[CToney]

At our local rebuild shop the tech is putting a voltage regulator right on the alternator for me. That way I don't have to put up with the old external one. My voltage fluctuates with the current setup. The engine speed sensor wires are MIA so that is why this truck had the external regulator. The alternator looks fairly new.

 

[PToombs]

Scott, sorry about the simple explaination. I look at things like this and pick it up quickly, and explain it thinking everyone else gets it as fast as I do.



Grab the wires in the harness near the alternator is the easy place, or near the firewall where you would mount the regulator.



I didn't know there where that many different stock type regulators to choose from. I know you can buy the regulator plug at an auto parts store, I have seen them in the book. I don't believe they have changed much from '70 thru '90.

 

[PToombs]

CToney, the problem with your unit is if it fails, you can't walk into an auto parts and get a new regulator.

 

[Greenleaf]

So, the Dark Blur wire is key hot and then the Dark Green is to and from the regulator then to another key hot?????



That doesn't add up.



Which terminal on the reg is "to" and which is "from"



The regulator is 45. 00 and really don't care to experiment with burning up too may prior to "getting it right"

 

[PToombs]

Both wires from the alternator go to a keyed hot. The dark green has the regulator in it. I don't think it matters which terminal is which on the regulator, but I don't have an older FSM to check for sure.

I understand about the $45 regulators and frying them!



If I was going to guess and do it, I would put the left terminal on the regulator to the alternator, and the right one to power. JMO.

 

[Greenleaf]

I bought four regulators in the few years I have had my 1990 and the fouth came with an instructional sheet/troubleshooting guide/wire harness.



Now I know which terminal does what and where the wires go etc etc. The right term is the field term on the regulator.



I hope the color is the same for the 1992 as I'd like to install one if the need arises.

 

[PToombs]

Scott, I'm sure they are. I was using my 93 manual to help Dale or somebody with their 90 or 91 charging problem.

I'll try to remember that, right is the field... .....

:-laf

 

[CToney]

My alternator with the internal regulator works great. It is supposed to last longer than the one on the firewall and keep a constant voltage instead of fluctuating. Anyways it is just in my work truck which doesn't venture far from home. My truck doesn't need much spark for it to run anyways.

 

["Mad Max"]

... . some re-attack thoughts... .



The PCM - it controls (among other things) the alternator charging system, grid heater (and message center light), overdrive, cruise? Is the OD tied into the speedo?



I have the whole 6BT engine/trans wiring harness, and I'd like to use the PCM to control as many of the 'features' as possible.



So, hook up all the engine/trans wiring, connect it to the PCM, and get 12v 'keyed' power to the PCM???, and that should enable the proper functionality of the accessories, to include the fuel shut off, and the engine should start, correct?



When the ignition key is turned to 'run' (just before start), that is when power is sent to the PCM. All the message center lights light up and the grid heater system cycles, ja? So the PCM operates on 12v keyed, not 12v hot, ja? I'm just putting thoughts onto paper here so bear with me.



If all the sensors are plugged in, the PCM will 'read' them accordingly, and life is grand. Now, my RC has a fully custom wiring harness - no where near what the factory has. So I need to stash the PCM somewhere, plug it into the Cummins engine harness, and that should enable the 6BT to function. The rest of the truck will operate with it's current system. I will have to splice and dice the 6BT harness so only what I need is remaining - the rest will be unnecessary.



Alternator: I have no problem with having the PCM control the alternator - we're using the alternator that came with the 6BT, and I assume the wiring will all hook up and function per usual.



All that jibe with y'all?

 

[Moab4x4]

PCM reads crank position sensor for RPM, TPS, and VSS for speed. I don't know which all inputs are used to trigger the OD. I have read the threads where some use a variable pot to control the OD drop point for towing to override the tps.



The VSS signal ties to both the speedo and the PCM. Speedo seems to work without PCM, but I am hazy on that.



1992 the PCM has both hot and keyed hot inputs, pin 9 IGN pin 3 BAT.



Yup all should work as you have it planned. Quite doable. Nice to have the PCM to run your tach too.



Do you have AC? The PCM also controls the AC clutch so that could be a little sticky and take thought. I would probably run it from the older controls and bypass the AC relay. (IMO clever idea on the old PCM though to disable the AC cluch at WOT when you need the extra power. )



Might need the newer brake switch if you are going to install the cruise.



My pages are worn in section 8? of the 92 FSM from my education.

 

[dpuckett]

My early 91 had no PCM and an electronic speedo.



I dont think the tach signal is controlled by the PCM as much as it just passes through the PCM. Havent tried bypassing the PCM with my current tach set up, though, so I could be wrong. Another option is to get the magnetic H-balancer mounted harness for the tach, esp if you're going to use an old school regulator- that would remove the need for the tach pickup entirely.



Cruise is doable without the PCM- I have seen 89-90s CTDs with cruise, as did the 6. 9 Fords, and they lacked a PCM as well.



OD can be switched with an OD button on the shifter lever (I think you posted the thread, didnt you?) OD is speedo related- my 2 autos have gone into OD at 32-35mph under light throttle. The variable pot is used to bypass the TPS, not override it, isnt it? At least that was my impression.



MM- Keep your 3. 54 gears til I get another 4x4 with 4. 10s, then we can swap. You can never have too many gears with a Cummins engine. Get enough power and stability and you coul dgo 150mph. Oo.



Daniel

 

["Mad Max"]

Alright - Looks like I'll keep/use the PCM, and use it to control the heaters, O/D, and cruise. Everything else will get either a manual switch or not be used, or have an aftermarket version of itself. Not sure how I'll do the A/C - most likely a primitive version on custom ducting.



To recap -



Alternator: Wire in an external voltage regulator. Factory [green] wire splices into one of the regulator wires, [blue] goes to 12v key'd power. Other regulator wire also goes to 12v key'd power. This bypasses the factory PCM as the regulator, thus removing a possible charging system issue with a faulty PCM and adding the cheaper and replacble regulator. I like it.



Grid Heaters: PCM. Could bypass with manual switch.



Overdrive: PCM. Use factory controls.



Cruise: PCM. Use factory controls.



Fuel Shut Off (FSO)(a. k. a. fuel pump solenoid): Key'd 12v power (same curcuit as KSB).



KSB: Key'd 12v power??? (same circuit as FSO???).



Fuel Filter Heater: 12v Key'd power (same circuit as FSO).



Gauges: All have stand-alone feeds.



A/C: Not used yet.



Sound about right?



- S



PS: Dan, I dig your idea... but if the RC does 150 I may not want to swap 'em out!!

 

[Moab4x4]

OK, Sam, I read this one last. I like your approach.

And yes, KSB power comes via short little wire from FSO.

 

["Mad Max"]

Alrighty amigos - the truck is runnin' and the system is chargin'. Had the usual 'issues' but with perserverance and determination we pressed through and we have what appears to be a strong charging system.



Attempted to use the computer to monitor/regulate the alternator output, but alas it did not appear that it was working. Didn't really troubleshoot it much - just went straight to the external regulator approach, and that was the ticket. Alternator's cranking out ~15. 5 volts and the batts both read ~14. 8, regulator and isolator are both doing their jobs well, and the rest is coming along nicely. Good stuff.



Thanks again for all yer help fellas - really appreciate your inputs. Electrical has always been my nemesis, but I can say that I'm a lot less stresses about it than I was a year ago.

Cheers,

- Sam