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Installation of a MDS ON Indicator Light

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Installation of a MDS "ON" Indicator Light

written by Magnuman

Added 10/23/06: New, updated PDF version by IT Rider can be downloaded HERE.

The following is a "How to" for the addition of a Multi-Displacement System (MDS) "ON" indicator light for the 2005 MDS equipped Hemi engines installed in the Dodge Magnum RT.

This may also work on the Chrysler 300C, but the installation in that vehicle has not been verified. This document is intended for reference only, and is merely a step-by-step of how the author installed this modification on his own vehicle. Anyone using this document as a guide to install this modification is doing so solely at their own risk. This installer should have, as a minimum, an intermediate understanding of automotive electrical principals and systems, electrical circuitry, wire routing and termination, insulation requirements and circuit protection. The author accepts no responsibility for errors, omissions, or any other problems or damage that may be caused by this installation.

The purpose of this modification is to provide a method (lighted switch in "on" position) of monitoring when the MDS is active. Turning the switch off will stop the light from coming on and cease monitoring of the MDS. It will not affect the operation of the MDS. It is accomplished by connecting the lighted switch to the power wire for the No. 4 cylinder MDS activation solenoid. Connection is made to the wire leading to Pin 28 of plug C1 of the Powertrain Control Module (PCM).

Parts Used

All parts were obtained from local auto parts store.

  • 1 ea.Illuminated rocker switch (any color)
  • 1 ea. Plastic rocker switch mount bracket
  • 2 ea. No. 8 X ½ inch, pan head sheet metal screws (to mount switch mount bracket)
  • 10 ft.24 gauge or larger, 2 conductor multi-strand wire
--each wire should be identifiable, i.e. two different colors.
--two color speaker wire works great

  • 6 in. 18 gauge multi-strand wire (Should be this size to tie into 18 gauge MDS power wire on PCM)
  • 5 ea.Solderless fully insulated wire connectors to match wire size(s)
  • 2 ea. Push on style to match connections on back of rocker switch
  • 1 ea. Female push plug barrel style connector (to be used on 6 inch wire)
  • 1 ea. Male push plug barrel style connector (for power wire to switch)
  • 1 ea. Connector with ¼ inch hole or "open" horseshoe type (for switch ground at PCM)
  • 1 ea. 18 gauge saddle type wire splice (the type used for hooking up trailer wiring) --used to tie into MDS power wire on PCM
  • 4 ea. 8-10 inch nylon ties
  • 1 roll Electrical Tape
Tools used

  • --Common screwdriver
  • --No. 2 phillips head screwdriver
  • --8-10 inch long small common screwdriver (used only to feed wire thru firewall)
  • --Set of metric end, box or socket wrenches
  • --Exacto knife (preferred) or single edged razor blade
  • --Wire cutters
  • --Wire strippers
  • --Solderless terminal crimping tool
  • --Standard or channelock pliers (used only to crimp closed saddle splice)
  • --Drill motor
  • --.110 (approximate) drill bit (for drilling mount bracket holes)
  • --Multimeter (handy but not mandatory)
  • --18-24 inch Test lead with alligator clips on each end (handy but not mandatory)
Be sure ignition switch is off. Disconnect the battery, for insurance, if in doubt. Note that if battery is disconnected, resetting of some vehicle functions may be required.

Preparation (under hood)

1. Remove plastic PCM cover held on by two push lock fasteners. Use common screwdriver to pull up on center of fasteners. The PCM is located in the right front fender well near the firewall.

2. Remove the PCM. Held on by one bolt. Let it sit on mount brace.

3. Loosen or remove (depending on type of ¼ inch ground wire connector you have) PCM mount bracket bolt nearest the C1 plug on the PCM.

4. Unplug top (C1) plug on PCM only.

4. Using an exacto knife or single edge razor blade carefully slice the electrical tape covering the wires on plug C1 from the plug to the junction with other wires (about 2 inches), being careful not to damage any of the wires. I used an exacto knife and slid it under the tape and cut away from the wires.

5. Gently peel the tape back, but do not remove it. Wires from plug should now be exposed.

6. Locate and isolate the 18 gauge brown with light blue striped wire. This is the No. 4 cylinder MDS activation solenoid power wire. Power from this wire will be used to turn on illuminated switch.

EDIT (4-8-05):

There is more than one brown/light blue striped wire on plug C1 of the PCM.

The brown/light blue wire we are concerned with is the one that goes to Pin 28, the MDS solenoid for the No. 4 cylinder. (The other brown/light blue striped wire goes Pin 4 and is connected to the injector control for the No. 8 cylinder. There is also a plain brown wire with no stripe going to pin 33, which is the No. 2/2 oxygen sensor signal lead.)

Please refer to the diagram of the C1 plug below and the Plug pinout chart below to ensure you are splicing the correct wire!

C1 Plug
C1 Plug Pinouts

7. From the driver's side under the hood, locate the small cable and grommet that goes through the firewall. It is located about 10-12 inches above and to the outside (right) of the steering linkage. It's the only one there, so shouldn't be too hard to find.

8. Pull the soft center of the grommet away from the firewall (toward front of car). Later we'll feed the wire through this grommet.

Preparation (inside the car on driver's side)

1. With door open remove trim strip (approx. 12X2 inch) on door side of lower dash panel. Gently slide a common screwdriver under the strip and pry the clips loose. I think there is one at each end. This will expose one (1) lower panel mount screw.

2. Remove mount screw and one (1) other located at the bottom of the panel to the left of the steering column.

3. Gently pull the top of the panel from right to left toward the back of the car. You can get hold right under the steering column to start. The panel is held in place by about four snap clips similar to those on trim strip. Just let panel lay on floor.

4. Remove shiny metal panel under the steering column, held on by four metric bolts.

5. Using the switch mount as a template, locate, mark and drill mounting holes. I mounted mine under the ignition key. There is a little recessed shelf separating upper dash from the lower panel that works great. This location is easily visible to the driver, relatively inconspicuous, and does not cause excessive glare at night. Do Not mount switch bracket at this time.
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Installation and Testing

1. Using the saddle splice, connect and securely lock the six inch 18 gauge wire and the previously isolated 18 gauge brown/light blue striped wire on plug C1 of the PCM.

2. Strip and securely crimp the female barrel style push plug to the other end of the six inch 18 gauge wire.

3. Slip the original electrical tape cover back over the wires and, using electrical tape, securely re-tape (leaving just the six inch "pigtail" exposed) to look like the original installation.

4. Strip and securely crimp the male barrel style push plug to one of the speaker wires, making note of wire color or identifying mark. This will be the power wire going to the lighted switch. Do not plug in at this time.

5. Strip and securely crimp the connector with the ¼ inch hole or open horseshoe to the other speaker wire.

This will be the ground wire going to the lighted switch.

6. Connect this wire to the previously loosened or removed mount bracket bolt on the PCM. Tighten mount bolt.

7. Route the wires behind the fire wall support brace (the one the PCM is mounted to) and secure with nylon ties. The nylon ties will slide between the brace and the plastic cover mounted to the top of the brace. The last nylon tie should be place directly over the cable/grommet area. The only thing that should be visible are the nylon ties.

8. Using electrical tape secure the free end of the wire to the small 8-10 inch common screwdriver starting about 2 inches back from screwdriver blade. Note: Leave a little tape pigtail so you can easily remove the tape after feeding through the firewall.

9. Push screwdriver and wire through the firewall grommet. This will require a little force to puncture the grommet. Push in far enough to grasp wire from inside the car.

10. From inside the car under the dash, locate and remove tape from screwdriver. Pull remaining wire into car.

11. From under the hood, remove the screwdriver and push the soft center of the grommet back in place.

12. Route wire to the area where the switch is going to be mounted, being careful not to interfere with any moving parts under the dash. I routed over existing wire looms away from pedal/steering mechanisms.

13. Reinstall shiny metal panel.

14. Reinstall lower dash panel, making sure to hold wire between upper dash and lower panel right behind where the switch mount is going to be mounted. Reinstall trim strip.

15. Mount the switch mount using the previously drilled holes and two No. 8 sheet metal screws.

16. Route the wire through the hole in the switch mount.

17. Strip and securely crimp both wires using the two connectors that fit the back of the switch.

18. Connect both wires to their correct terminals. My lighted switch had 3 terminals: Supply Volts, On Acc., and Earth Ground. The previously identified power lead gets connected to the Supply Volts terminal, and the ground wire gets connected to the Earth Ground terminal. Do not hook anything to the center On Acc. Terminal.

19. Lightly and carefully pull the lower panel outward and push the excess wire back behind the panel. Secure the switch in the mount.

20. If battery was disconnected, reconnect it now. Reset various vehicle functions, as needed/required.

21. If you have a test lead, connect one end to the male barrel style push plug and the other end to the red "positive" terminal on the Forward Control Module (FCM) located in front of where you are working. Go in the car and activate the switch. The light should come on. Turn switch off and the light should go out.

22. Disconnect test lead and connect the male barrel style push plug to the female barrel style push plug on the exposed pigtail at the PCM.

23. Reinstall the PCM and its cover.

24. Go for a ride and try it out.

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mds light

I have a question. Does the pcm provide power to the mds solenoid or provide a ground? Ususally high current output devices (over a few amps) are ground side switched. Is the mds solenoid normally off or on? If this installation actually works and I'm sure you've tested it, the pcm must be providing power to the solenoid to activate mds which seems odd to me, but I've seen chrysler do some wierd things. When they have hot switched circuits in the past there were a lot of pcm failures. They probably got the design bugs out of power switching circuits or at least I hope so.
Well by powering up the solenoid to enable mds operation, this leaves the car running on all 8 if there's a power issue.
While I appreciate the effort that went into figuring out this modification, I have to ask; other than satisfying a passing curiosity, what is the long term use of this?
When I get my HEMI, I plan on doing this but with an LED in the instrument cluster.
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The lead in question does indeed provide power to the MDS solenoid on the No. 4 cylinder. It is not the ground. The MDS is normally in the off position and the solenoid, along with the solenoids for the other 3 MDS cylinders (1, 7, and 6) activates the system. The PCM provides the power, apparently once all preprogrammed parameters are satisfied, and shuts it off, apparently when any of the parameters become out of range. Most small instrument/indicator type lights consume very little power and I have not had any problems with Malfunction Indicator Lights (MIL) or fuses blowing. I've been running the system for about two weeks now, and am still trying to figure out the parameters that have to be met to make the MDS work. It is apparently quite a complex set of parameters, too. So far I think, at least the following are involved: engine coolant temperature, engine oil pressure, engine manifold pressure (vacuum), speed, gear selector position and actual gear the trans. is in, and throttle position.
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I believe you are correct in your statement about power problems would cause the system to revert to the 8 cylinder mode and render the MDS inoperative.
Curiosity is indeed one of the motivators for this mod. I've read somewhere that DCX initially had a light installed but removed it because it was coming on and off too frequently. They apparently thought it would more of a distraction than a help. Since I am starting to figure out what parameters must be met to make the MDS work, I have found that I can coax the MDS to come on when it normally wouldn't, especially below 30 mph and above 50 mph. My last tank of gas, mostly in town (not a lot of stop and go) with some rural driving, was 18.2 mpg. My normal in town mileage runs 15-18, depending on the amount of rural and stop and go traffic that is involved. With the price of fuel now, I'll take any increase in mileage I can get. The reason I used the lighted switch is that if I don't want to monitor it, I can simply turn it off. I do like to tinker and I thought, inquiring minds, like mine, might want to know. Maybe not the best rationale, but that was mine.
Please refer to the addition to this How To in the first post regarding the wire color in the C1 plug. (This refers to the correct wire since there are multiple wires that look alike...we want you splicing the correct one) :)
This is a cool mod, but has anyone figured out a way to manually overide the controls?
It would be cool if you could lock the system into either 4/8 mode when ever you wanted and keep it that way.
This might cause problems though.... the good folks at Dodge know what they are doing I suppose. But it would be nice to have the option if it were safe.
Sort of like the option to select gears with the "Auto-Stick".
This is a cool mod, but has anyone figured out a way to manually overide the controls?
It would be cool if you could lock the system into either 4/8 mode when ever you wanted and keep it that way.
This might cause problems though.... the good folks at Dodge know what they are doing I suppose. But it would be nice to have the option if it were safe.
Sort of like the option to select gears with the "Auto-Stick".
VBOB, I see what your saying, And I sure somebody will figure it out some day.
Although, I woulden't want to keep the car in 4 cyl. mode for to long. Unless say, hypotheticaly there was MDS on all 8 cyls. & the system could change from one bank 1,4,7,6 to the other bank 2,3,5,8.
I'm just a shady tree mechanic, but woulden't this cause uneven wear in the long run ?

just my .02
A few of us are still working on this. Here's the thread:
If you have any ideas, feel free to chime in. Knowledge is power. Thanks for the interest.
Great mode, did it this past weekend. Did not bother to run the light/switch inside for now. Bought a 12v red led at Radio Shack and mounted it just under the hood on the driver's side by the wiper arm.
Bottom line:

How much (in parts) $$

How much time (to do the mod) ?

Maybe related, maybe not:

What's the significance of the boxed number 8 that appears a few spaces to the right of the PRND indicator on the dash (its probably two spaces over from the gear indicator that appears when you are using the autostick)? It only lights up as I turn the key prior to starting the car. When I first saw it, I thought it WAS an MDS indicator but I never see it on while the car is running...
It shows the gear number when you're autosticking, until you hit it to 5th then it goes away.
I don't remember exactly how much the parts were, but it is certainly no more than $25.00. I tried several different light/switch configurations and settled on the illuminated rocker switch, because it served two purposes....the light, and the capability to turn it off if I don't want to monitor (hasn't happened yet though). If you have any more questions, I'd be glad to help.
I am planning on this mod soon. I will use a small led mounted in the dash and a hidden switch somewhere. I can't use the location in the KB, because that is where my radar detector head unit is.

Here is what I wil lprobably use:


Should fit nice where the 300 trunk button is.
Just read your post in the Knowledge Base on the illuminated rocker switch. I really like it. Where do you get them? Have you done the mod. yet? If so, how do you like it?
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