'nornal' O/D & cruise without the PCM?

["Mad Max"]

Fellas,



I'm nearly complete with the 6BT/518 retrofit and I'm researching a way to have 'normal' operation of the overdrive without using the factory PCM up under the fender.



My question is, is there a way to bypass the PCM and have the overdrive operate per 'factory'? I want the O/D to engage at the normal ~34 mph and hold tight regardless of the load, and have the on-off function from the factory dash-mount switch... which is exactly how my '93 functions. I have heard of a potentiometer, but I confess after unrefined research I still have no idea what it is or how it works.



I'm fine with using the PCM, and if I use it the other related question I'm trying to answer is, what inputs does the PCM use for normal O/D operation? In other words, what factory wiring do I need to keep... and what can I prune. I'm guessing it needs the factory speedo input and TPS input. What else?



The O/D needs a speed sensor so it knows when it's at ~34 so it can engage, and the TPS to determine throttle input, right?



Cruise works just fine in the non-O/D rigs, so do the newer rigs not use the PCM?



While I'm at it, the fuel heater is always on with 12v keyed power, ja?



The list of consecutive miracles gets smaller...



- Sam

 

[Philip]

The cruise uses the PCM to control the vac and vent solenoids. It uses a signal from the stop light switch to turn the crusie off when the brakes are applied. It also recieves a sgnal for set and coast along the same wire from the steering wheel controls.



Can you bypass the PCM??? If you can figure out some way to control the vent and vac solenoids you could. Then you would need to find a way to controll them by the speed also.



It might be easyer to use one of the older cruise systems or something aftermarket.

 

[KOG]

Aftermarket is the only way to go. Rostra has some electric solenoid systems that work well. You cannot use the steering wheel switches with any aftermarket system because Chrysler, for some unknown reason, has built resistors into the switches and nothing other than the stock system will work with resistors in the switch circuits. So you go back to a turn signal mounted switch. The advantage is that the electric solenoid systems work better than vacuum systems as a rule.

For OD you're about left with a manual switch. The stock OD does not just engage at a set speed. OD engagement varies based on throttle position as well as speed. I know of no standalone controller which can replace the throttle position sensor and PCM to perform this task.

 

[Flexiheep]

Sam, I remember there was a transmission company that does Auto OD swaps for old Mopars that had a system to run the 518 and the 47RH with pressure switches in the transmission fluid circuit. I will try and find the place and add a link. I always thought that this would work well and be pretty simple especially to run the lockup on the TC.



Found the link Oo.

transmission control link



BTW if you do this I want to hear how it works out. :-laf

 

[KOG]

Flexiheep, that conversion is for gas engines. Uses manifold vacuum for control. They may also have something for diesel, but it's not mentioned.

I'd love to have a lockup converter on mine too. Then I could turn up the power.

 

[Flexiheep]

Your right, I didn't notice that at first, I wonder if you could find a pressure switch to take it's place... . Install it in the intake manifold so it worked the opposite of the vacuum switch. That way at high boost levels it would kick it out of TC lock and OD... . Just a thought, it wouldn't be as generic of a solution at the manifold vac on the gasser since 0 vacuum is 0 vacuum... On the diesel, some may call 15 psi high boost if your stock and some may need something like 40PSI for the unlock.....



Maybe you could use an on/off switch on the throttle linkage so at 90% throttle it hits the switch (think nitros).



If you were handy with electrons you could do your own simple logic ciruit to take the pressure inputs and do the switching for you... .



Who said custom mods were simple



I think that Sam doesn't have TC lockup anyway so he could just delete the need for the vac switch.....



I think worst case you would have to manually shift out of OD under heavy load, and most autos I have driven I manually shift down anyway since they hold top gear too long on the hills.



Heck I llike the way the autos in the F800s at work run, they hold gears till determined speeds. No shifting on engine load, etc, if your doing X speed they are in X gear. That is how the above link would work without the vac switch since the fluid pressure is pretty proportional to vehicle speed.

 

[cerberusiam]

The PCM uses VSS and TPS signal to determine the OD shift. The good thing is the OD is purely electronic demand, the bad thing is it is purely electronic demand. You have to have some type of gizmo that grounds the OD solenoid circuit.



If one was handy enough with electronics it would not take much to build a unit that would demand OD at a certain speed but without the TPS signal and some bit wise logic controller there is no kickdown.



About the only solution is the transplant package from ATS. It will still require a TPS and VSS signal but you will be able to control OD, lockup, kickdown, etc.



Spendy solution but it works.

 

[Paychk]

Last sentence of the first paragraph:

"This can be made to work with a diesel motor also with or without a vacuum pump and vacuum regulator. "

 

[KOG]

The question is how it can be made to work with a diesel. Without an intake vacuum signal it really doesn't know what the engine is doing.

 

[Paychk]

I would think that Flexiheep has it right, use an adjustable pressure switch to sense engine load.

 

[cerberusiam]

I would think that Flexiheep has it right, use an adjustable pressure switch to sense engine load.

Sense what pressure to determine load?



Boost? Oil pressure? or??



None of the available pressures are really indicative of load. The Hobbs switches are really meant to work with a gasser that has vaccum as the main determinate.



Even using trans gov pressure is not going to give a very good control. It would be fine for everyday driving but worthless for anythng else as it would never shift at WOT.



The lockup control is equally as flaky as it would just unlock and stay that way. Thats about useless under a load on a slow turning TQ motor.



The onlky effective way to control it on a diesel is an electronic control box. Too many different variables to address.

 

[Paychk]

Boost pressure, Hobbs switches are available in both pressure & vacuum styles or another way is a cam on the throttle that can close /open a switch depending on position.

 

[Moab4x4]

The PCM uses VSS and TPS signal to determine the OD shift. The good thing is the OD is purely electronic demand, the bad thing is it is purely electronic demand. You have to have some type of gizmo that grounds the OD solenoid circuit.



If one was handy enough with electronics it would not take much to build a unit that would demand OD at a certain speed but without the TPS signal and some bit wise logic controller there is no kickdown.

I second this opinion. . It is easy to have manual control via switch, but for factory type control, use the PCM.





As to what to prune... Not much. If you are going to keep the PCM, I would let it do it's thing.

Prune:

Maybe AC clutch depending on your setup.

The ASD relay seems to be pretty worthless on a diesel (from what I see it only switches the field on the alternator).



Other than possibly those I think if you were keeping the PCM that you would want:

Alternator control,

Cruise Control,

OD control,

Grid heater control,

Water in Fuel,

Massaged tach signal,





Not much else to it is there?

Ken

 

["Mad Max"]

bump -



Okay, we did Moab last weekend and it was great! Still have lots of fine tuning to do, including getting the O/D to engage.



As I understand, the O/D engagement is based on a very simple 2-pin connector - one to 12v keyed, the other to a switched ground (2nd gen shifter lever or 1st gen dash button). Hit the switch, the circuit grounds, and the O/D triggers on/off (opposite of whatever mode it is currently is in), correct?



My limitations: In short, our RC's custom wiring harness does not use the factory speed sensor (but it does use an electronic Autometer speedo... hmm), so, as it sits it looks like I have to wire up the O/D on a basic switch. Here's my thoughts:



transmission has a 2-pin connector. One goes to 12v keyed power, the other to the shifter lever (the other side of which is grounded). So, in that configuration, the O/D should engage when the shifter button is clicked (which grounds the curcuit), and vice-versa. Correct?



The one question I still have is this: As wired above, the O/D will remain on as you decellerate, and won't automatically disengage at/around 30 mph - I will have to hit the button and disengage it manually, correct?

I'm totally fine with having O/D engagement on a switch - hell in my unmolested '93 I almost always shift into O/D 'manually' anyway, but I'd love to have a way for the O/D to disengage upon decelleration at/around 30 mph, and I don't know how to do that. Maybe somehow tie into the aftermarket speedo harness so that at a specific voltage (i guess... ) a circuit will trip thus disengaging the O/D.



The other idea I have floating around in my mellon is this: Would the factory speedo harness 'work' with the aftermarket Autometer speedo? IOW, can I splice the factory speedo wiring (which tells the PCM what to do with itself) into the aftermarket speedo wiring? Would the signal input be the same for both systems or did the factory have their own 'secret recipe' like the cruise circuits? If they'll splice, and I've got the TPS wired up, then I should be able to use the PCM to do the O/D, ja?



- Sam

 

[cerberusiam]

Yes to everything you said about the OD. Pretty simple system, both in operation and to override.



Not sure what signal the ECU is going to want but using the aftermarket signal generator is worth a try. Is it working on the speedo? If it does it may work fine to tell the ECU what it needs.



The othe roption is spend a little time around some electronic geeks and have them build you a little gate with an adjustable feed input. Probably cost you a whole $10 in parts to get a control that would do the same thing.



BTW, the RC looks just baaaaaadd!!!! NICE job!

 

["Mad Max"]

cool - thanks Alan. Even if I have to make it a full-manual gig that's fine for this rig... ... . everything else is

Me and Keith are gonna try to whip up something simple like yer suggesting. Just havta fiddle with it a bit.

Thanks on the RC man - we really like how it really turned out, and man does it go up and over hills With this much weight it can actually use this big of a tire without sliding all over the place. We broke a r-front hub on Sunday and did the last half of 'Fins' in 2wd, and to may of our surprise it was almost as sure-footed with rear drive only as it was in 4wd. Fortunately going up hill the weight is mostly on the back end... and it takes a lot to break a D70 .



- Sam

 

[chipster]

lock up

If you put a 48 lb. pressure switch in the upper rear 1/8" port on the pass side of the trans the trans will lock up at about 35 mph. it will un lock at about 30mph or a little less. put 12v to the right wire on the 2 wire plug, and run the second wire into the cab ,to a 70 chev p/u headlight dimmer switch on the floor. put the wire on the top center blade, from either of the 2 lower blades run one wire to ground and the other wire to the pressure switch on the trans. in one position the switch will manually ground the o/d , in the other position it will allow the pressure switch to ground the o/d above 35 mph and unground it below 30 mph. if you don`t want o/d at all put a on/off switch on the dash ,to controll the 12v + to the 2 wire plug. DONE!:-laf:-laf

 

["Mad Max"]

ooookay, lemme see if I understand some of the basics here



As I understand, the transmission's 2-prong O/D switch is an on/off switch... not a momentary one, correct? So when wired up, the pressure switch at the governor port will close at 48 psi, thus switching the O/D switch to 'on'... and when pressure drops below 48 psi the pressure switch will open, thus switching the O/D to 'off'. That jibes, right? So then I just need to determine my preffered pressure range, install/wire up both switches, and governor pressure will engage the O/D. Then, to disable the whole system, splice a switch in-line in the cab to disable the 12v feed to the O/D switch and that will prevent the O/D from engaging at any speed, ja?



Please pardon my clarification questions - sometimes I have a hard time precisely understanding how these things work but the more I discuss it the better I learn



- S

 

["Mad Max"]

I just had a real good conversation with Alan ("cerberusiam") and we talked all about this configuration and the details - So here's the plan:



Three switches - the O/D 2-prong, a hobbs pressure switch, and a kill switch. 12v power to the O/D, ground coming out going to one side of the hobbs, the other end going to the kill, the other end of which goes to ground. All connected and 'enabled'... at the specified pressure/speed... the circuit will ground/close thus engaging the O/D. But, flip the kill switch (on-off switch in the cab) and that opens the circuit, disabling the O/D. It couldn't be any simpler, and it completely bypasses the TPS, the PCM, the XYZ, the R2D2 and - and the C3PO... . and all the other maddening factory problem areas there are in a 1st gen wiring harness .



Now, doing it this way guarantees that the O/D will engage regardless of load, temperature, throttle, time of day, day of the week, weeks in a year or alignment of the friggin' planets. So, the driver will need to be gentle on the O/D during warm-up and under big loads... ... and I'm that way anyway so no problem. But it also guarantees that the O/D will not disengage when it's not supposed to... ever... and that is good. If any of you have driven a 1st gen with an auto you know how all-of-a-sudden infurating the O/D can be #@$%! - well this bypass should be the hot ticket, and it'll be easy... . and cheap



And hey Chipster - thanks mucho for the hobbs switch tip amigo - that's about as slick as it gets!



Stand by for the full wiring diagram and testing.



- Sam

 

[BSMiller]

Is the Hobbs pressure switch adjustable? Also can you adjust the cutout pressure? Part number? What happens when your foot is off the throttle going down hill having only a front trans pump without a rear pump to supply 48psi?

Having a 727 and not a 518, what controls the kick down on a normal 518?