Tachometer Only Reads Correct While Using Power Trim

[spunkymonky]

I'm trying to figure out why my tachometer would only read correctly while using my power trim. It reads correct at idle but after about 1500-1800 rpm it only reads correct if I'm using the power trim. It doesn't bounce either, if I hold the trim switch down it reads perfectly fine verified with an inductive tach on the motor. Any ideas?

 

[GA_Boater]

I would look for a wiring diagram, but I don't know what to look for.

Did the tach ever work properly?

 

[spunkymonky]

I'm not sure whether it was working properly before or not, I've just rebuilt the boat and put it in the water. The faria tachometer was new and the motor is a 1996 Force 75HP. The mercontrol is a commander 2000. I have not tested to see if the same thing happens when using the trim on the motor, only the remote. My instict would be that it's not the tach since it will read correctly when using the trim. I understand that a lot of times the tach will drop due to the voltage drop when using the trim but I'm not sure how a jump to the correct rpm would correlate.
The voltage output to the battery according to the garmin fishfinder is 13.3-14.2 when running. No issues with the outboard at all, at any rpm.

 

[Silvertip]

Check the gauge wiring (all of them). It is very common to daisy chain the +12 volt and ground wires from one gauge to the next. Whoever installed the gauge may have decided to daisy chain the "S" (send) wires in the same fashion which is NOT acceptable. I can think of no other reason why the trim switch would have any bearing on tach operation.

 

[spunkymonky]

The tachometer is the only thing wired in and it get's its signal off the mercontrol harness grey wire and positive purple wire and grounded to the battery.

 

[sam am I]

I was thinking about this issue for a day or two now and I'm thinking it has to do with the gray wire (tech sig) path and some interference......I won't try to explain (not sure I even could) all the gorey details but, thinking about what is happening when the trim is energized and why/how that would make it read correctly? Current is increasing in several area's..... Wires, motor winding's, connections and so on.

Perhaps this higher amount of current when the trim circuits energized, more than the normal idle currents when the tech doesn't work right, there develops a stronger/larger DC field that perhaps is near and perhaps surrounds the gray wire somewhere along its' run(15'-20') from the motor (rec/reg) in such a way that this larger developing field at trim time is somehow/way filtering off (better?) the induced noise that's effecting the tech via the gray wire (antenna) ........A DC field filter if you will.

Can you run a single wire outside the wire loom from where the tech sig originates (the rec/reg) directly to the tech input stud? If the tech straightens out and runs right all the time, it is just noise getting on the gray wire, right? If not, might have to do with grounding still but, it sounds like you've eliminated that so gray wire noise is my best guess at this point.

Also try to replace the rec/reg last case.........Trimming demands more current as stated above and perhaps the additional current demand on the rec/reg at trim time is "fix'ing" a failing internal component such that it straightens out the gray wire sig that is derived internal to the rec/reg