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Tachometer needle jumping (HELP!!)

stmpulr

stmpulr

Member
I purchased a used HD tachometer from a friend of mine. I had to make my own wiring harness and I hooked up the ground and the light/power wire. I coiled up the trigger wire at the ECM and had the dealer make that connection to the #3 terminal at the ECM. The tach seemed to work ok but I never really rolled it on since I had it hooked up. Today I started to put it thru the gears and opened it up. As soon as the tach reached approx 3500 rpm the needle started jumping + - 1000 rpm's. If I were to throttle up at about 1/2 throttle the needle is fairly smooth thru that same range. Also when stopped and revving the motor it seems fine also. I checked all the connections and they seem tight and OK. The only thing that comes to mind now is possibly a bad tach or could it be RF getting into the trigger wire under hard 100% throttle and load. I did use a solid (no strand) wire for the trigger wire. I don't know if I should have used stranded wire for that. Does anyone have any thoughts?? This is on an 07 fxstb. Again normal riding seems to be perfect on the tach.
 
Solid wire will break easier than stranded for openers. Are any of the wires run near the secondary ignition wires?
 
glider said:
Solid wire will break easier than stranded for openers. Are any of the wires run near the secondary ignition wires?
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I realized that after I used the solid wire, but I had all ready made the harness and did not notice the solid wire till I cut the ends. It has only been on there about 1 mo. The wires run along the left upper back bone (behind the fuel outlet on the gas tank) and above the ign.coil a couple of inches. What are your thoughts?? Could it be more subject to RF being solid?
 
I would replace it with stranded wire. The solid wire will eventually break from movement. Also the stranded wire usually has a twist in the strands to help eliminate the magnetic fields that surround any wire. It's possible the solid wire is causing the problem here but I wouldn't bet on it.
 
I probably should have had the dealer do the complete job, but I was trying to save a buck and thought I knew what I was doing. That must have been my problem, I was thinking.
It seems to work perfectly (like cruising) as long as I don't go 100% throttle or under HARD load and above 3500rpm.
The dealer called me (follow up call)the other day and asked how the service was and how the tach was working and at the time I thought it was fine and told him so.
I know it has got nothing to do with them because they only made the one connection to the ecm. Nice neat job.
 
I would highly recommend a piece of Beldon shielded twisted pair and ground the shield on one end to the housing. That should eliminate any chance of RFI
 
also DC runs better through stranded wire. DC electricity runs on the outside of the wire and AC through the inside of the wire. That's why in a house they use solid wire and in vehicle that runs of DC they use stranded.
 
100% correct, and as Glider has already said, you should never use solid anywhere that the connection is subject to vibration. It is not a question of IF it will break, only how soon?
 
I guess the consensus is the solid wire. The only wire that was solid was the tach input wire from the ecm to the tach.
The OEM (tach) harness from HD is not shielded but I was thinking along those lines to change that wire to shielded or would stranded be OK for that one wire? Thanks for the help (info) guys

How I have the wires hooked up = solid wire to the #3 terminal at the ecm, stranded ground to ground strap @ top of back bone that runs to neg on battery, stranded power/light wire to rear light harness (orange/white) with scotch connector thru 3 amp fuse.
 
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