Customer had me get him a dakota digital dash for his 71 cuda. Have it all wired up but the tach does not work. Been on tech calls with them. Not able to make it work other than testing it. Testing it involved taking the speed sensor that mounts in the speedo cable. Removing that and mounting a drill on the needle for the speedo into the transmission. Then taking the power wire and the input wire from that cable. Putting power to that from the main box for the system and hooking the input wire into the tach port of the box. Running the drill then puts the signal through the cable into the box which then sends the signal into the cluster to move the tach needle. Pretty simple operation to verify the tach and box all are functioning. The way the tach is supposed to be hooked up is one single wire from the negative side of the coil into the main box which then should run the tach in the cluster. Have it set up that way and nothing works. Tried a new coil nothing. Car will start up and run no issues at all so my thought is should not be any reason the thing would not function. Start the car up and all the rest of the gauges work and function the way it should. Any ideas? I have not tried a separate tach yet. Have the original ralley gauges that I might try but not sure if the tach worked before or not. The dakota digital set up does not rely on anything from the original car except that it is wired into the turn signals and the brake light and fuel sending unit. The oil,speedo and water all get new units installed in the factory spots and separate wires provided by them that plug into those spots and are wired into the main control box in designated spots. Everything is clearly labeled. When I was installing everything I labeled the wires for what they were for along with color coded to the way it is labeled on the main unit. The cluster gets 2 supplied wire sets that plug into the back of the cluster and into the main box. They are idiot proof in that you can't change the locations because of the ends. Only designed to go in one spot. Any ideas?

Thanks

corey

Assuming this is electronic ignition and not points or dual points.

Do you have a MSD or other spark controller that issues multiple or non traditional sparks?

Some tachs have issues with those systems.

Shoot yes electronic. Stock ignition no MSD or anything like that. Just good old Ma Mopar electronic ignition.

corey

Stock coil and ballast resistor? The ballast might be lowering the coil primary voltage so the control box can't read it. Have you just checked the tach at idle or have you revved it?

Everything stock. Tried revving it up nothing registers on the tach. Going to try a new ECU box and ballast resister though to see if can figure it out..

Just through a new voltage regulator and ballast resistor on it. Nothing changed. ECU box ordered up for tomorrow to try that. The ballast resistor spring on the back side one side the spring was exposed and the other side the spring was still fully encased in the unit. The one side that was exposed was loose on one end. I was hoping that was the issue but still no luck.

corey

Just put the ECU box on it and not able to get tach to work. Only thing left in my eyes is the distributor. Car started and drove it in the paint booth this am so should not be distributor but then it started just fine before I replaced coil, ECU ballast resistor and the voltage regulator. I have also added a ground from engine to body. Ground from battery to body. Everything else is stock. Just can not get things figured out.

Corey

Most tachs need 12v plus a ground and a sensor wire. How many connections on the tach and how are they marked?

I know when I wired mine with a pertronix ignition and MSD 6a i needed a tach adapter. https://www.summitracing.com/parts/...l6MK3hQMVomtHAR3HNArWEAQYAyABEgLHqPD_BwE

no extra power for the tach. it is all part of the main box for the dakota digital set up. 1 wire from coil into tach port. unit gets a keyed power supply as well as a constant power supply. one main wire feeds the cluster from the unit. one supplys the clock and thats it.thats is what so frustrating. I took a few days break from it to get a new shaker hood painted on it. now is time to get back on it.

corey

Check the voltage on the tach wire...
Apparently there is a resistor in the tach wire, there was a thread in the Corvette forum about the same issue.
They changed the resistor to 4.7K ohms and then the tach worked.
It seems there is pull down voltage from the ECU and if too low the tach doesn't work.

Just my $0.02...

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The wire that I have installed is a brand new wire that goes directly from the neg side of the coil directly into the box for the dakota digital unit. Only one wire and only one install port. so pretty simple. I am going to try https://www.summitracing.com/parts/...hmpKMan0RP1rpz8RPcLVLOElGWgaAiHlEALw_wcB

That looks interesting, but from what I read, tach needs to be set to 1-cylinder mode.
Does Dakota do that?

I initially thought this would be perfect to provide a tach signal to the factory E-body tach with a 3G engine, but it doesn't sound like it.