Thunderbolt Tach issues

[monk-monk]

I have a '78 1400 on a '75 Glastron...the original thunderbolt tach does not work, it acted like it wanted to...it went to about 500 RPM's and then slowly sank to zero...also, from my observation, the previous owner had the wires on the wrong terminals...maybe the tach quit and they just tried swapping wires or something...it is the "square faced" Stewart Warner style just like the early seventies tachs, and it is a 6000 RPM tach and it has a plastic body...NOW, i have another thunderbolt tach, it is just like the previous tach, but it has a metal body, it was built in 1970 and is supposedly NOS...looks brand new too...BUT when i hook it up, it pegs the tach needle past 6000 RPM's....could this newer tach i have, be for a mercruiser with inboard engine with an alternator (i thought all the 6000 RPM tachs were for outboards)...could i be wrong here...AND if i wanted to buy a new tach, what tach do you buy to replace the thunderbolt tachs??...I even hook the tach's straight to one of the yellow wires from the stator at the rectifier...same results all the way around...Got any ideas folks?...

 

[monk-monk]

Re: Thunderbolt Tach issues

Ok, i've narrowed this down some....i went to my storage building and got all the thunderbolt tachs i have (5 of them) i've connected a tach wiring harness to all of them (brown tach wire moved from switchbox to rectifier stator lead terminal)...and when i start the motor, it draws the tach needle in the opposite direction...it goes below "0" RPM's CCW...all my tach do this, and i know they are good...this sounds like a stator problem to me because that is where the tachs are picking up their signal from....OH, and when the tach draws below zero RPM's..the needle is pulsing as fast as hummingbird wings...Stator...???

 

[monk-monk]

Re: Thunderbolt Tach issues

Figured i'd ask an see if anyone else has seen/had this issue with their tach before...no replies yet, but iboats forums is the place to start! SO, i've given this considerable thought...5 different tachs, same type/style tach, all respond the same way when the engine is started...it draws the tach needle CCW below 0 RPM and the needle flutters as fast as a hummingbirds wings...i know it's not the tachs, and im 100 percent positive the tach harness wiring is correct...Fortunately, im very good at troubleshooting, and by asking questions, quite often i discover the answers on my own...i removed the brown tach wire from the switchbox and connected it directly to one of the yellow stator leads at the rectifier....now im straight wired to the stator lead, and where does the tach get its signal from?...the stator...! It has to be a bad stator...tomorrow im going to take one of those 5 tachs and hook it to the MerControl on my "known good" '72 1150 thats on my '72 Glastron...if (and i know it will ) work correctly, then im going to take the stator off my 1150 and put it on my 1400...if it all works perfectly on my 1150, then it should on the 1400...it's just a tach picking up a signal from the stator...i will follow up tomorrow evening to let you know what i find out...

 

[merc850]

Re: Thunderbolt Tach issues

I think those tachs had an orange colored electronic module on the backside, I've seen them on ebay.

 

[monk-monk]

Re: Thunderbolt Tach issues

This is the strangest thing and it's about to drive me crazy...i disconnected all the wires of the internal harness where they connect to the switchbox, i disconnected the coil, the distributor etc...the only wires i have hooked up is the stator wires to the rectifier and the tach, brown signal wire at the rectifier and a ground to the tach...i connected a positive battery wire to the starter solenoid and grounded the motor with a battery negative...when i crank the engine it STILL draws the tach needle below 0 RPM and flutters when cranking the engine...this complete set-up works fine in my "72 1150...stator works great on my 1150, the tach works great too...take them parts and put them on the 1400 and the tach goes in the wrong direction...CRAZY...!!