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IdahoRT
Dodge Dakota
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2/19/2004
13:11:40
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 Subject: 4.7 tach signal
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I know some people on here have tried to hook up aftermarket tach's and shift lights to a 4.7.
I have successfully done it and figured I'd share my solution to help anyone else that wanted to do it.
What you need: MSD or Autometer DIS tach signal driver. Some 16 guage wire, insulated crimp on connectors or solder and heatshrink.
Tech info: The MSD and ATM DIS driver works by intercepting the shared (+) feed to the coils and counting pulses and converting that into a tach signal. A true tach signal has 1 pulse per cylinder for every 2 crank rotations, so a true tach signal on the v8 4.7 will have 8 pulses every 2 crank rotations. If you try to hook up the MSD or ATM DIS driver to only 1 coil, you'll only get 1 pulse every 2 crank rotations that is why the instructions show the driver must be installed inline to the (+) feed for ALL the coils.
The problem: The problem with the 4.7 COP (coil on plug) system is is the (+) feed also feeds the injectors. The injectors also create another signal on the (+) feed and since injector pulses vary in width, the signals on the (+) feed wire (DG/OR by the way) are numerous and vary because they are a combination of coil and injector pulses and the MSD or ATM units cannot use the "garbled" signal. [The PCM controls injector and coil firing by closing the ground and that is why they all share the same (+) feed wire.]
The solution: Isolate the (+) feed to the coils and the tach driver works just fine then reading that feed.
What you need to do:
Cut every DG/OR wire to each COP harness connector, leaving at least 2" on the connector, you'll need to carefully remove the tape on the two wires to expose them. Use 16 guage wire and either insulated clamp on connectors or solder/heatshrink and run a new wire to each COP harness connector using the 2" DG/OR wire still attached to the connector. I ran one wire down the main wiring harness on each valve cover and soldered a 6" extension to it every 4.1", which is the spacing between each COP connector. Each extension goes to each COP connector. I then ran a wire in the front to connect the two. Once all 8 splices are made, splice the MSD or ATM DIS signal driver wire (RD/GD) to the new COP (+) feed you just made. Then take the (RD) wire on the MSD or ATM unit and splice it into the (DG/OR) wire that runs in the original factory harness; I spliced into it in the harness conduit between the battery/power distribution center and the left front valve cover. Then ground the MSD or ATM unit, and you will have a proper tach signal on the gray wire coming from the MSD or ATM unit. You can now drive any aftermarket tach, shift light, or other device requiring a tach signal.
FYI,
Jason
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TexasTodd
Dodge Dakota
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2/19/2004
14:35:24
| RE: 4.7 tach signal
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Jason,
What's wrong with the TACH that COMES with a Dak?
Kinda like people who go thru all this effort to put another thermostat is the system of the electric fan, when what's there OEM, works fine without the belt clutch/fan.
Glad you could figure this out. Now, what will you do with your tach that's there?
Things kinda slow in Idaho this time of year?
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IdahoRT
Dodge Dakota
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2/19/2004
15:26:54
| RE: 4.7 tach signal
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LOL, not it's not slow in Idaho this time of year.
Factory tach suits me just fine.
I needed a tach signal for the following devices:
shift light
auxiliary injector controller
WB O2 data logger
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DMSDAK
Dodge Dakota
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2/21/2004
16:55:01
| RE: 4.7 tach signal
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HEy IdahoR/T,
I have a 4.7 L Dakota Im getting ready to install a Venom VCN-1000 Nitrous kit on it, I bought a MSD-89561, which is a CPC (coil per cylinder) RPM Window Switch, how would I go about hooking that up in order for it to get the signal it needs?? Would I have to do the mod your doing and hook its sensing wire up like that so it knows what RPM the motor is at? Cant just tap into the ground side of the coil and let it count the pulses there?
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kj
Dodge Dakota
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2/21/2004
17:13:03
| RE: 4.7 tach signal
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I would be careful messing around with nitrous using stock cast pistons and factory head gaskets.
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IdahoRT
Dodge Dakota
JOIN HERE
2/23/2004
02:03:22
| RE: 4.7 tach signal
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Message:
"""HEy IdahoR/T,
I have a 4.7 L Dakota Im getting ready to install a Venom VCN-1000 Nitrous kit on it, I bought a MSD-89561, which is a CPC (coil per cylinder) RPM Window Switch, how would I go about hooking that up in order for it to get the signal it needs?? Would I have to do the mod your doing and hook its sensing wire up like that so it knows what RPM the motor is at? Cant just tap into the ground side of the coil and let it count the pulses there?"""
I'm not familiar with that device. But, if it expects 1 signal per two crank revolutions, then hooking it up to one coil plug will work. However, if it expects 8 signals per two crank revolutions, then you'll probably need to do the wiring I did.
If in doubt, call the MSD tech line, they'll be able to answer it for you.
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randal33
Dodge Dakota
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4/06/2011
21:59:02
| RE: 4.7 tach signal
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Message:
Hay Idaho R/T...Why couldnt you just run a wire off each coil..and not use the DG and OR wire but use the WHT and RED wire bring each wire that you put on each tach signal wire (red and white) to a connector then hook it up to the after market tachometer ?
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