Tach or Rectifier Issue on 98 115 hp Johnson (J115ELECM)

[midsized johnson]

My outboard's tach started acting up a few weeks back. Didn't think much of it, just though the tach was going bad. Fast forward to this past Sunday and the engine has no spark due to low battery. I got it to run hooking it up to my trolling motor battery and everything was working fine. However, later in the trip, the tach started falling off or jumping erratically and then working fine again. It would be working fine then quit if I moved the jack plate up or down. Yesterday, I charged the battery and cleaned all the terminals real good. The battery was reading a bit over twelve volts this morning, then when I started it, it jumped up to 13+ then as high as 14 while it was in high idle but never hitting a number and staying on, sometimes a bit higher, sometimes a bit lower. Once it dropped down to normal idle, it was producing 13.25, but not staying on a consistent number, possibly dropping a hundredth over time. I didn't stay on it, so can't really confirm that, but it was staying right in that range while I had the volt meter on it.
If the rectifier goes bad, does it go all bad, or does it slowly crap out? Maybe both?
Thanks in advance.

 

[emdsapmgr]

If the battery is losing it's charge, likely it is not being replenished adequately by the engine. Your tach signal is driven by the combination rectifier/regulator, which is the device which charges the battery. Once the battery starts going down (low voltage) it is likely something is going on with the rectifier/regulator. First thing to check is to see if the main wires to the battery are tight on the battery end and the engine end. The first time you'd suspect a problem with the tach is when you start to have recharging problems with the battery. They go hand in hand. Likely you need to repl the rectifier/regulator-not a cheap thing on these water cooled versions. A normal functioning rectifier/regulator will start to recharge the battery at 12.5 volts, and eventually get up to a steady 13.5 or so and hold there while running. At best yours sound erratic. When you activate the trim, you place a huge electrical draw on the system, (probably taking voltage away from other devices) -another reason to suspect you have a charging problem.

 

[jakedaawg]

Like emd said. Make sure cables are tightened with a wrench. Make sure they are clean. He is spot on on this. Also load test your battery and make sure its not a cheapo 400 cca bargin buster. Most likely is the regulator/rectifier.

 

[midsized johnson]

I agree with what both of you said and appreciate the responses. I know it's either the rectifier/regulator or the tach, just trying to figure out which one it is without the buying one or the other only to find out I "fixed" the wrong thing. Both cable terminals from the engine were pretty dirty when I pulled them off to clean them, so I'm sure this contributed to the issue, if not caused the whole thing.
This morning, I put the multimeter on it with leads so that I could see the tach and the readout on the multimeter which was right at 12.85 v +/- at idle and slowly climbs as the motor runs and increases if I rev the engine a bit. As I pulled the fast idle lever up and got it running at around 1500 rpms, the tach quit, but the voltage reading increased to over 13. Dropped idle and the tach came back up to 800 rpms and the voltage started dropping, probably would have got back to the 12.85 or so reading I was initially seeing, but I cut the engine off.