1986 v115 tach pegs 7000 rpm

[big foot]

I have twin 115 2 strokes on Mako .
One pegs tach when engine starts . Switched tachs and also changed cdi box and still no difference . When key is on ,not running ,tach is at 0 , When cranking to start , tach shows about 4500 rpm .
Any ideas where to go next ?
thanks,
Bill

[Aquasportfisher]

Since it's a twin setup, swap the leads on the back of the tachs to eliminate a bad tach as the problem. Is the motor in question actually revving that high?

[99yam40]

I thought that the tach signal comes from the charging system not the CDI on those older motors, might need to check to make sure.

Have you checked to make sure the tach is set properly or maybe operate the switches back and forth some to make sure they are making good contact

[big foot]

1986 v115 tach pegs 7000

No ,the motor does not over-rev , but I am getting the alarm and safe mode I assume from tach at 7000rpm . If the signal does come from the charging system , my next step might be to change magneto , I have an extra .
Thanks ,
Bill

[99yam40]

May want to wait for Rodbolt17 to say where it comes from.
I would thing checking the charging system for proper function would be an easy task in service manual, make sure all meets spec

[rodbolt17]

best I recall on that model its an altenator driven system.
look at where the green wires meet at the regulator,if it has a small green wire on one of the posts thats going to be the tach signal.
the alarm is generated by the thermoswitchs or the oil control module.
no alarms for overspeed.

[big foot]

Thanks , but if you had to guess what would make the tach read 7000 rpm at idle , and I tried another tach , did the same thing , a short ? different componet ? This is a new problem worked fine in the past .
Thanks
Bill

[rodbolt17]

my guess? a malfunction in the charging system or wireing harness.

[99yam40]

I am fairly sure those tachs are pulse counters, so something that would cause that to increase or the tach internals are out of wack.
Setting switches not making good connection can cause the settings to be off

Did you try swapping tachs to make sure the one reading too high worked OK on the other motor?

[big foot]

I will try my good engine on the bad tach ,and also look into the charging system next ,this weekend . I will let you know the results and Thank You for your time .
Bill

[big foot]

Well ,whats the chance of your extra tach being bad ? Well thats what is was.
had enough wire on twin engines to plug good motor to bad side and visa versa and came up with problem under dash . When i bought my boat 6 months ago with older motors ,1986 ,I was lucky to find 2 parts motors , complete , for $800. It's more than paid for itself with extra parts , So the tach turned out to be the problem , we will be out setting the crab traps this week in Monterey Bay .
Thanks all again for your input ,
Bill