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If you're following my build page - and why wouldn't you, it's totally awesome [/sarcasm font] - you'd know that after replacing my tired and worn out TBI engine with a MPFI Magnum, I was looking for a better solution to control the A518 than the common "PATC method" of using a vacuum and pressure switch combo.

For those unfortunate souls who haven't found the build page yet, please check here-->'s-1990-w1ton50-rcsb-restomod/

If you're not familiar with the setup, let me give you a little background. The A518, later dubbed the 46RH was essentially a TF727 with an electronically controlled overdrive unit bolted onto the tailshaft. It was simple and fairly effective in dropping cruising speed by a good margin thanks to the 0.69:1 OD ratio.

The catch here is that while most of the A518 was hydraulically controlled, the overdrive unit was actually electronically controlled, tied into the SBEC (single board engine controller, aka Dodge's moniker for the engine computer). The SBEC monitored speed, throttle position, RPM and other parameters to gauge engine load. It would then used these inputs to determine when OD can be initiated, and when it should be dropped back into 3rd gear (1:1) to allow acceleration/passing. Later models also incorporated TCC, or a torque converter clutch, used to lock-up the torque converter and give near manual-transmission like transfer (which not only gave better gas mileage, but also helped extend the life of the transmission since it generally produced less heat, especially when towing).

The issue we run into is that if you're not using a SBEC, you have no way to automatically control the overdrive engagement. If you're like me and switch to MPFI, the JTEC and later ECM's won't control the A518 overdrive. Similarly, if you're running a carb you're also not going to be using a SBEC, and you too will be limited on what options were available to you. Many people have simple resorted to using a toggle switch to "flip" in and out of OD manually. It works, but can be inconvenient and certainly not suitable for new drivers or those unfamiliar with the operation.

For many years now, PATC has suggested using a combination vacuum switch and pressure switch to automatically control OD and TCC lock-up (they also sell a nicely packaged kit with all the parts and pieces you'd need). With the vacuum switch, it is simply a normally open vacuum switch that engages when a predetermined vacuum signal is reached. Some kits, including the optional kit from PATC, have an adjustable vacuum switch, which allows the users to fine tune when the switch activates. The pressure switch is a simple threaded pressure switch that is normally open, but closes when the targeted pressure is reached. The entire setup looks something like this:

Photo Source:

How it works: As you increase engine speed, pressure is building up at the governor port, which is where one or two pressure senders are screwed into (1 is used for overdrive only, 2 would be used for overdrive and TCC control). When you hit cruising speed, you are typically generating high engine vacuum, typically +8 in Hg or more. This is enough to activate the vacuum switch, allowing current to flow to the 12v+ side of the OD (and TCC solenoid if equipped) solenoid. The ground-side (12v-) of the OD solenoid (and again, the TCC solenoid if equipped) is attached to their respective pressure switches. Since governor pressure equates to roughly MPH speed (so 55 MPH equals roughly 55 psig on the port side), the switch will close and ground itself to the case when the targeted pressures are met, grounding the OD circuit and allowing the 3-4 shift to occur.

Although it works, and is certainly better than manually flipping a switch back and forth, it isn't perfect. Having used this setup on my recent Magnum swapped W150, as well as my previous '79 D200 with 440 and A518, there were some quirks you'd have to get used to. For example, it was difficult, almost impossible actually, to slowly roll into the throttle while in OD to pass or maintain speed. What happened instead was that the vacuum dropped below the set point, opening the circuit, causing a 4-3 shift, allowing you to accelerate up to the new speed, where you then backed off the throttle and allowed the 3-4 shift to happen again.

I looked around and was even considering modifying a 700R4 lock-up standalone unit to work on controlling my OD based on throttle input and speed. I then happened to run across the "Compushift Mini A518" (while looking at the 700R4 controllers no less!) sold by HGM, the makers of the very popular Compushift Sport and Pro. Some basic research on their website showed that it would be exactly what I was looking for - a standalone TCM with adjustable settings for OD and lock-up, that used engine parameters off the engine sensors for its logic. Unfortunately, when I started doing some research on the various forums and webpages, there was basically nothing of value or substance regarding the Compushift Mini (CSM).

I decided to reach out to the sales team since they had an email address posted on their website ( Much to my surprise, Mike responded back to me within a few hours. After exchanging some emails, I noticed that they were located in Torrance, CA and by pure happenstance, I was going to be spending most of the following week in Torrance at one of my sites. Mike invited me to stop by and discuss what I was doing and see the shop.

After bustling through LAX and getting my rental, I grabbed a quick snack and headed on over where I met the two main guys, Mike and Guy, as well as some of the warehouse workers. I spent the entirety of my lunch break and then some getting a walk-thru of the Compushift Mini, the harnesses and the phone/tablet interface. I also got to see the production and manufacturing area for the entire Compushift lineup.

The shop is modest in size, but let me say - these guys are legit! They do everything they can in-house, even 3D printing their own dust caps for USB connections. They even have a transmission dyno on-site and a few dozen transmissions to test out their new technologies and updates. From my discussions with Guy, nothing ever leaves the shop, not even software updates, without being thoroughly vetted in-house first. Mike walked me through what is included in a typical A518 CSM kit. I was so impressed by their time, professionalism, technology and setup that I immediately ordered a CSM A518 when I got to the hotel later that evening.

When I arrived back home to AZ, I almost couldn't wait to install the CSM and see the results. Installation is straight forward. The harness is well laid out, nicely organized, and everything is labeled (even the obvious stuff like 12V+).

The harness has 3 main sections as you can see below (image #4 in case the Google Pic links aren't working again). On the left is the transmission section, which includes the pressure switch tap (like the PATC setup, the CSM using a pressure switch to verify 3rd gear is engaged, rather than rely on it for control), OD connector, and VSS tap. In the center of the harness, you can see the inline fuse. Included in this section is the ground wire, as well as the software correctable speed signal which can feed a 8000 PPM (pulse per mile) speedometer. On the right of the harness is the TPS lead:

The included instructions are easy to follow and diagrams are helpful for those who aren't so wiring savvy. They even include a picture of an A518 that shows which port you need to install the pressure switch into (image 5):

The CS Mini comes in various forms, even options for Cummins and carb'd engines without TPS sensors. In the case of my MPFI engine, included with the kit is a TPS "tap" (image 6), which allows the CS Mini to tap into the K4 sensor ground circuit, as well as the TPS sensor signal (the one that changes with pedal/throttle position):

Since my setup was already using a late-model 3-wire VSS, all I had to do was clip the VSS signal wire from the harness and splice it into the factory VSS signal circuit (image 7):

The final part to get basic operation is to find a place to mount the CS Mini. I chose on the firewall, next to the brake pedal. This has shown to give good Bluetooth signal and generally be safe from the elements (image #8 ):

Before you fire the CS Mini up, you have to download and install the CS software from the Apple (iOS) Store or Google Play. Once downloaded, you can pair up your phone to the CS Mini device. The first thing you'll need to do is calibrate your TPS signal, which is as simple as pressing "calibrate TPS" and slowly but steadily pressing the gas pedal down completely and letting off. The software will tell you if you screwed up or if the calibration wasn't successful.

From there, you can view the dashboard, which provides a visual table with numbers pulled directly off the attached sensors. Here you can also change parameters, such as max throttle speed, min OD speed, TCC engagement and disengagement speeds, etc (image 9):

You can make changes on the fly if your device is up and connected to the CS Mini (though it's important to note that you do NOT need a device to operate once it's been setup).

The important question - how does it work? Well, flawlessly so far! It is exactly what I was looking for. OD comes on when I want and I can actually pass a car or maintain speed up a hill without having to deal with constant 4-3-4 shifts. The system goes in and out of OD exactly how it did when it was using the SBEC. There are some ancillary features that I'll be hooking up, such as the OD on/off switch (configurable to user needs) as well as the correctable speedo output (allowing you to find tune your speedo without the need to change speedo gears!), so stay tuned and I'll post more info on those extra features as I hook them up and put them to use.


757 Posts
Great write up this company seems to have it figured out well.  Even for the inexperienced wire guy.  Labels and pictures our my best friend. 

Thanks for all the info,

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