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Discussion Starter · #1 ·
So, I've been having trouble with my 92 Ramcharger shifting in and out of overdrive at around 45 mph. I've tried everything I can think of or find through reading (e.g. fluid/filter changes, check VSS wiring, throttle valve adjustment, etc.) and it's still happening.

I'm getting ready to replace the PCM's control of overdrive with the 'hack' that uses two trans oil pressure sensors to drive the overdrive and lockup solenoids in the trans.

I've been working through how to retain the 'Overdrive Off' switch that already exists on the dash, and use it to turn off overdrive/lockup in the new setup. I'm also planning on wiring in some indicator LEDs for 'overdrive' and 'lockup' that turn on when their respective solenoids are energized. In order to do this correctly, I really need to understand the sources/voltages in play for the 3 pin connector on the trans.

From the wiring diagrams in the FSM, Basically AD-12 in the Electrical/Wiring Diagrams section tells the story. However, since these diagrams try to cover all possible engine/trans options in as little space as possible, they show the 3 speed trans 2-wire connector on AD-12 as well as the overdrive trans 3 wire connector. I was unsure if there was a 'Part-throttle Unlock' relay (as shown in AD-11) at all in this truck. I looked in my engine bay, and see 4 relays, and an empty socket for a 5th relay (which I assume might just be there and empty from the factory).

Reason I ask is that the 14 gauge wire feeding the center terminal of the 3 wire trans connector seems to always be hot from the AD-12 diagram (coming from the A21 splice that feeds a bunch of other stuff). If I knew for sure it wasn't fed from the 'Part throttle unlock' solenoid, I'd say that maybe the overdrive trans turns off the overdrive and lockup solenoids by forcing their respective wires 'hot' so there is no voltage difference between the center and outer terminals? That's the only thing that seems to make sense to me as a way for the 'Overdrive Off' switch to work here (inputs from the switch to the ECU/PCM might make the computer force K54 and T60 connections 'hot'/12V?

Anyone know for sure?

Thanks,

Mitch
 

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Mitch - AFAIK the center pin is always hot with the ignition switched to 'run', and the computer/PCM activates/signals the OD and LU to engage by a ground 'trigger' in the PCM which grounds the outer two wire(s).

Here is a write-up I cobbled together from many threads, which has a decent description of the OD/LU systems in my first post - might also help -

https://ramchargercentral.com/diesel-talk/'de-computerize-your-dodge'/
 

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Discussion Starter · #3 ·
Thanks Mad Max. That confirms my suspicions/guesses.

One thing I'm still wondering about... why did the center pin wire on the OD/LU connector need to be 14 AWG, but the outer wires are 20 AWG? If we had current high enough to warrant a 14 AWG center wire (and the 20A fuse recommended when implementing the 'hack' mentioned at https://transmissioncenter.net/shop/patc-727-to-518-46rh-conversion-kit-click-here-for-727-to-46rh-swap-wiring-diagram/ and https://2k8ugo7vg0ythyc326851m1c-wpengine.netdna-ssl.com/wp-content/uploads/2016/05/img3.jpg) I'd think the outer wires would be bigger. Maybe since they each only carry half the current... ?

Edit: As these are solenoids, I'm wondering if the actual current involved is really low, but they use the 14 AWG center wire to make sure they have very little voltage drop across the full run of that wire (sorta long to the back of the trans I guess). If that's true, maybe piggy-backing the 'hot' wire I'll add for my new OD/LU system onto an existing 20A fused circuit would be OK? I had planned to add a new 20A fuse tapped in before the fuse block prior to this discussion.

Mitch
 

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one "HOT" is powering 2 electromagnetic coils , so it has 2x's the load that each coil's ground wire does ...  also know , short the factory wiring to them , it melts .  it did in my '93 518 .... factory .
 

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Discussion Starter · #5 ·
Thanks dodge82273, I am very keen to avoid all potential 'melting of wires' scenarios.  ;D

I'm getting sorta adventurous wiring in indicator LEDs for OD/LU on the dash, so I'm being extra careful to fully understand the existing wiring before I do anything.

Mitch
 

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oh ok , well a "hot" wire passes thru a coil or a bulb or whatever , and is still hot untill it reaches a ground .... its what causes all the feedback thru a ground problems folks contend with and post , lights lighting that aren't powered on because another light is but its ground has failed .. electric requires a full circle .. the draw of an led should be low enough that if you supplied it with power , the pcms ground path to the coils should also handle the leds ground load . You could use the center connector wire to power the led's with a 1 amp fuse in line .
 

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Discussion Starter · #7 ·
Here's what I was thinking for wiring changes:

Red X = original wire cut/capped and left unused
Blue = New component

Notes: The 87a pin is the Normally Closed (N.C.) pin. Using this pin allows me to switch the 'sense' of the existing Overdrive Off switch, that when it's off, the relay is 'on' and delivering power on the 87a pin. When the O.D. Off switch is 'on' it makes the 87a pin go to 'open'.

          I couldn't tell exactly what pin 3 and pin 2 of the O.D. Off switch were doing. I'm assuming pin 2 is constant 12v+ and pin 3 is 12v+ when the switch is closed/illuminated.

          The two switches at the bottom are either open or closed to ground. When closed they complete the circuit from 87a to ground.

 

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seems like alot just to add a coupla leds but ....  the only thing I'll add is this , a solenoid / coil / relay all have a coil in them , when you energize a coil , then de energize it , the + trigger wire gets a voltage spike unless  it protected by a diode . this happens anytime any coil "collapses" . any voltage spikes that lead back to a computer can/will damage the computer . Its why computers switch grounds not power , and why some relays have a diode in them .

example : the glow plug timer on a inter diesel ,( ford) has a very common looking solenoid , like found on some starter systems . If you use a common one , it ruins the engine controller ( computer ) , because it has no "protection" in the form of a diode . The correct one has markings on it "protected" or some such statement . You can SEE this action by placing a test light on the trigger lead of a starter solenoid , and operating the starter . ( the 10 gauge brown wire on a Dodge that goes to the starter ) you will SEE the test light FLAIR up when you let OFF the crank / key ... sometimes it blows a 12 volt bulb ..... I use 24 volt ones in my test lights ...   
 

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On my 90 TBI factory O/D switch, I have circuit U4 (20 OR/WT) that was the O/D sense signal for the SEBC. This was coming from the back of the dash-mounted OD switch.

I believe this is the same as your T6 circuit (also 20 OR/WT).

On mine, this circuit provides ground in its neutral state. It is latched and when 12V is no longer applied to the switch, it resets to neutral state. When I press the O/D switch on the dash, the yellow "off" light illuminates, and the ground path is broken...

So what I did is run a transmission relay and used the U4 circuit as the the ground source for the relay trigger. When the truck first starts up, O/D switch is in neutral state, allowing trans relay to activate, sending 12V+ to the vacuum switch, O/D circuit harness, and pressure switch on the side of the transmission. If I want O/D off, I press the switch, ground path is interrupted, and the relay cannot activate and thus, no 12V+ is sent to the transmission.

It's been working fairly well, though my too tall gears (3.55's on a 34" tire) sometimes drop the engine vacuum below the 5-6" needed to activate the switch under moderate throttle input. I'm hoping once I go to 4.56 gears that it will begin working flawlessly. I had a similar setup on my 440/A518 swap many years ago and was running 32" tires and 4.10 gears and it worked perfectly!

 

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does the dash switch PROVIDE the ground , or does it allow the ground to pass thru from the pcm in its neutral state  ? ...
 

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...not exactly sure how the factory PCM does it, but I can also comment on how I've done it in the past with perfect results.

I prefer to use simple toggle switches to control OD and LU - one of the reasons I also highly value the RH-series transmissions - very simple wiring.  I basically run 12v switched power to the center pin, and run grounds to both outer pics, and I manually 'activate' the OD and LU when I want them, and deactivate them accordingly. 

Also, in trucks with factory wiring, IE a '93 CTD truck I had with the non-LU A-518, I first replaced the shifter lever with one from a second gen truck ('96) and replaced the OD 'trigger' button with a simple ON/OFF switch, and controlled the OD with it and it works flawlessly.  I particularly like having full control of when the OD engages when towing heavy because I always lift a little when engaging the OD - I believe it helps preserve the OD guts by not shifting under such big boosted torque loads.

- Sam
 

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Discussion Starter · #12 ·
Thought I responded to this over a week ago, but my computer crashed as I hit 'Post' so I guess it never got posted.

dodge82273, I'm actually implementing the pressure-switch activated overdrive 'hack' using a kit sold here (https://transmissioncenter.net/shop/patc-727-to-518-46rh-conversion-kit-click-here-for-727-to-46rh-swap-wiring-diagram/). So, I'll be severing all connections from the overdrive switch to the ECU and all connections from the ECU to the trans. I hope this will alleviate any issues with inductive voltage. Good point about switch ground, and not the hot connection.

pc1p, I pulled the console cover today and poked around with the O/D switch connections. Turns out that T6 (OR/WT) is actually +8.7v at all times with the key on, and T61 (BK/OR) is +12v with the switch 'on/illuminated' and +.8v when 'off/dark'. I assume that the ECU senses the difference between T6 and T61 when deciding to turn on/off ground at K54/T60. Very odd about the +8.7v thing though. Also odd that T61 doesn't go all the way to ground when the switch is off/dark.

So, to recap, I plan to...

T61 BK/OR - disconnect from ECU, connect to pin 86 (trigger, normally closed pin) of relay
T6 OR/WT - disconnect from ECU
K54 OR/BK - disconnect from ECU (at trans solenoid connector) and connect to LU pressure switch
T60 OR/LG - disconnect from ECU (at trans solenoid connector) and connect to OD pressure switch

So basically I will just have a dumb toggle controlling LU/OD disable function, but then pressure switches controlling LU and OD when they are enabled.
 

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if you have a fsmanual ... any year w/the od button, all it does is disallow the ground from pcm TO overdrive coil and set the off light "on" , it is just a push button toggle switch , with a light ...
 

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MitchUpton said:
Thought I responded to this over a week ago, but my computer crashed as I hit 'Post' so I guess it never got posted.

dodge82273, I'm actually implementing the pressure-switch activated overdrive 'hack' using a kit sold here (https://transmissioncenter.net/shop/patc-727-to-518-46rh-conversion-kit-click-here-for-727-to-46rh-swap-wiring-diagram/). So, I'll be severing all connections from the overdrive switch to the ECU and all connections from the ECU to the trans. I hope this will alleviate any issues with inductive voltage. Good point about switch ground, and not the hot connection.

pc1p, I pulled the console cover today and poked around with the O/D switch connections. Turns out that T6 (OR/WT) is actually +8.7v at all times with the key on, and T61 (BK/OR) is +12v with the switch 'on/illuminated' and +.8v when 'off/dark'. I assume that the ECU senses the difference between T6 and T61 when deciding to turn on/off ground at K54/T60. Very odd about the +8.7v thing though. Also odd that T61 doesn't go all the way to ground when the switch is off/dark.

So, to recap, I plan to...

T61 BK/OR - disconnect from ECU, connect to pin 86 (trigger, normally closed pin) of relay
T6 OR/WT - disconnect from ECU
K54 OR/BK - disconnect from ECU (at trans solenoid connector) and connect to LU pressure switch
T60 OR/LG - disconnect from ECU (at trans solenoid connector) and connect to OD pressure switch

So basically I will just have a dumb toggle controlling LU/OD disable function, but then pressure switches controlling LU and OD when they are enabled.
Here's what I found on my 90 W150...

The factory TBI harness should have circuit U4 (20 OR/WT) heading to the SBEC to control OD on/off. I poked and prodded and it showed that this is simply a normally closed to ground circuit, meaning the SBEC sends 12v through the switch and it is normally grounded, allowing OD function. When you press the dash OD switch to "off" (when the light illuminates), ground is broken and the circuit is open, preventing the SBEC from allowing OD (and LU) functionality. I'm using this as a transmission relay controller (for now, see below as I'll be changing it).

Now I did recently find something that may be of interest to you: https://ramchargercentral.com/technical-discussion/hgm-compushift-mini-a518-transmission-controller/msg3479392/#msg3479392

I was not happy with the PATC method. I used it previously on my A518/440 cid combo but it worked in a similar fashion. I've been using the Compushift Mini linked above and it's been awesome on everything from street driving, to backroad cruising to highway hauling. I'm actively sourcing a lock-up transmission for rebuild behind the 5.9 stroker I'm building and I expect it to work just as flawlessly as my non-lockup control.
 

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the pcm sends grounds , the key and or the asd sends 12 positive .  pcm does not itself switch positive , it sends the necessary ground TO the trigger side of relays thereby completing a "circut"  when its sensors and programing say to do so . computers do not like voltage spikes , and try to avoid them by switching ground NOT positive . the od switch has a light circut that provides the power for its indicator lamp , which lights when the ground is not going to the coil/solenoid in the trans . the fsm shows this , but not all on 1 page ..... 
if you send power to a bulb , the ground side of the bulb will read as power UNLESS its grounded , power just passes thru the filament , comes out as power .  test a good fuel solenoid for power from key when its wire is attached , you'll find NO power , disco the wire and there will be power thru your meter/test light ,  same happens with a powered bulb ( OD OFF) the pcm reads the switch position , then supplies ground TO the ground side of the bulb . Please note , Z circuts in the FSM are grounds .  ;) 
 

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dodge82273 said:
the pcm sends grounds , the key and or the asd sends 12 positive . pcm does not itself switch positive , it sends the necessary ground TO the trigger side of relays thereby completing a "circut" when its sensors and programing say to do so . computers do not like voltage spikes , and try to avoid them by switching ground NOT positive . the od switch has a light circut that provides the power for its indicator lamp , which lights when the ground is not going to the coil/solenoid in the trans . the fsm shows this , but not all on 1 page .....
if you send power to a bulb , the ground side of the bulb will read as power UNLESS its grounded , power just passes thru the filament , comes out as power . test a good fuel solenoid for power from key when its wire is attached , you'll find NO power , disco the wire and there will be power thru your meter/test light , same happens with a powered bulb ( OD OFF) the pcm reads the switch position , then supplies ground TO the ground side of the bulb . Please note , Z circuts in the FSM are grounds . ;)
This is generally true. When the PCM does send (+) voltage, it is often low amperage and reduced voltage for the reasons described above is particularly true in the case of sensor signals. When the PCM must control a circuit with significant amperage, it does so my manipulating its ground path, as is the case with the ASD relay, OD and lock-up solenoid control, etc. These are known as circuit "drivers", where in instances where the PCM controls a the 12V+ side of a coil or switch, it is known as a "trigger".

However, the PCM will send full 12v+ voltage (aka battery voltage) in some instances. Both the SBEC and JTEC will send a low-amperage 12v+ signal for the OD switch sense (I've checked both).
 

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all circuts require the same path . power from battery  ,to a  load , ground from there back to battery , a full circle if you will , now disconnect your battery negative , place a test lamp or meter between the post and the cable , apply a load ( turn on the lights?) your lamp will light , most meters will blow their internal fuse . You found power + at the negative side of your battery !  :eek: The pcm uses the key for coils/solenoids/relays etc by connecting the power after a load TO ground . The injectors , engine spark coil , are powered by the battery from the asd , the coil driver in the pcm provides the ground path after the coil (load) . same with each of the 8 (or 2 )  injectors .

1 tcs lamp has keyed power , with its ground supplied by the pcm when pcm reads an  "open circut" from the tcs , the other lamp is illumination from headlamp switch , at the same time when the pcm gets open circut from the tcs it stops the ground to the transmission solenoid .  e circuts are lighting z are ground   
 

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Discussion Starter · #18 ·
Thanks guys,

  Well, I've been taking things slowly here and doing only what I think is the least invasive changes for this OD shift modification first. Today I started implementing the 4th gear pressure tap part of the PATC system. Wow, what a PITA. The tap is BEHIND the tcase shift gate bracket mounted to the side of the trans. Absolutely no way to get a fitting on that thing without modifying the shift gate bracket. I ended up drilling a 3/4 in hole in the plate to allow the pressure line to come through it. I'm still wrestling with how to get fittings that will allow me to just pass through the plate and then 90 back along the trans and shift gate bracket and to a body-mounted bracket that holds the two pressure switches of OD/LU. Yuck.

  And then I read about this CompuShift Mini solution and it sounds excellent. My big requirement was to be able to tune the OD and LU RPM points. The CompuShift Mini sounds like just the think for that type of tuning. It still wants a pressure switch, but only in the 3rd gear pressure tap (which is easy to get to thankfully).

  Bummer that I've got a bunch of time (and now money) invested in the PATC kit, wiring, pressure line fittings, etc. If the CompuShift Mini wasn't $349 I'd buy it right now. As it stands, I've already got probably $200 into various parts for the PATC solution. I didn't see the installation/full docs on the Mini on their site. I've sent tech support an email asking if they have an online copy I could look at before buying this thing.

Mitch
 

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dodge82273 said:
all circuts require the same path . power from battery ,to a load , ground from there back to battery , a full circle if you will , now disconnect your battery negative , place a test lamp or meter between the post and the cable , apply a load ( turn on the lights?) your lamp will light , most meters will blow their internal fuse . You found power + at the negative side of your battery ! :eek: The pcm uses the key for coils/solenoids/relays etc by connecting the power after a load TO ground . The injectors , engine spark coil , are powered by the battery from the asd , the coil driver in the pcm provides the ground path after the coil (load) . same with each of the 8 (or 2 ) injectors .

1 tcs lamp has keyed power , with its ground supplied by the pcm when pcm reads an "open circut" from the tcs , the other lamp is illumination from headlamp switch , at the same time when the pcm gets open circut from the tcs it stops the ground to the transmission solenoid . e circuts are lighting z are ground
Haha, perhaps I haven't mentioned it, but I have a significant background in electrical engineering, including years as a R&D chemist in a lab working on semiconductors, arc layers and dielectric etchants :)

When you disconnect a battery and jump it with a voltmeter, you aren't finding "power +", you're simply completing a circuit (with the voltmeter as a conductor in this scenario). Electrons are actually flowing from the negative battery post (which acts as a simply galvanic cell during discharging) through the meter, through the circuit wiring and back to the positive battery post. Electrons will flow whenever there is a difference in potential...

But anyways - I've checked (physically with a voltmeter) both a JTEC and SBEC and the O/D switch sense circuit has 12V+ when connected to the negative battery post, and 0v when connected to the positive, showing that the circuit is is seeking ground, which I've verified that the U4 circuit provide in its neutral state. When pressed, the OD switch provide a diode block, preventing grounding and O/D operation via the ECM. I am using my OD switch as a relay control (via coil ground connected to U4) and it works flawlessly.
 

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MitchUpton said:
And then I read about this CompuShift Mini solution and it sounds excellent. My big requirement was to be able to tune the OD and LU RPM points. The CompuShift Mini sounds like just the think for that type of tuning. It still wants a pressure switch, but only in the 3rd gear pressure tap (which is easy to get to thankfully).

Bummer that I've got a bunch of time (and now money) invested in the PATC kit, wiring, pressure line fittings, etc. If the CompuShift Mini wasn't $349 I'd buy it right now. As it stands, I've already got probably $200 into various parts for the PATC solution. I didn't see the installation/full docs on the Mini on their site. I've sent tech support an email asking if they have an online copy I could look at before buying this thing.

Mitch
I'm sure Mike will respond to you quickly, he's quick like that!

I can scan and send you the PDF if you'd like (heck I may post it up on my Tech page for future reference). The 3rd gear circuit is simply to let the Compushift know you're in 3rd gear (vs 2nd gear, which can have a speed past the MPH you have your lowest OD speed set to).

Let me get this scanned for ya...
 
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