69-er

I have the VFD3 instruments with the old aluminum control box and I am having what appears to be interference between the tach and speedometer signals. The speedometer and odometer registers anytime the engine is running. Apparently, I didn't allow enough clearance between the two circuits in the harness. I don't know how much separation I need between the two.

What's your opinion on using CAT6 ethernet cable for these two circuits to isolate them from each other and other sensor circuits to eliminate possible interference problems?

Or, I thought of using coax cable, (RG174 is physically small enough), for each tach and speedo wire between the ECM and the instrument cluster.

And, lastly, how about twisting the VSS pair going to the ECM?

Thanks for your thoughts on this!

Larry

blackbyrd

iTrader: (14)

Does it use the OE sending units or does it come with its own?

Are you thinking the interpherance is from the sending unit to the brain box or is it from the box to the gauges?

Is it possible that some of the shielding has come off the signal wires?

mrvedit

Coax is a pain to work with.
Google "shielded wire 2 conductor" for some easier to use choices.
I would first double check all the grounds and/or run an addition ground wire from the gauges to the chassis.

gofastwclass

iTrader: (2)

The VSS input to the PCM is supposed to be a twisted pair, if it isn't that can cause interference. Coax is completely unnecessary for this situation and CAT6 is overkill.

I've had speedometer / tach interference occasionally with Dakota Digital equipment before. The solution is simple at least on the gear I've worked with. There should be an input sensitivity high / low switch on the speedometer interface. Set it to low and see if the problem is resolved.

Pacer Racer

iTrader: (6)

How much shielded twisted pair do you need. I should have some I could send you for the shipping

HwyStarJoe

Also, make sure the wires are not routed next to any other source of interference.... radio, electrical motors, ignition, etc. It doesn't take much in the way of proximity to cause interference.

69-er

This is an old post but I finally got around to troubleshooting this issue again and thought I would give an update.

I wound up disconnecting the VSS input wires to the ECM. I then ran one VSS wire to the D.D. speedo and the other to ground. It worked!

Initially, I thought the ECM needs the VSS signal to allow the reverse lockout to work properly. I realized that the lockout has always been energized regardless of my speed to allow the shifter to go into reverse. Apparently, the company that did the harness and ECM mods made it so it always is energized. Never had a problem so I left it alone.

But, now I'm getting a check engine light. I haven't pulled a code yet but I assume it's because of the lack of VSS signal to the ECM. Can that be programmed out? The car seems to run fine as always without it.