Where does the Tachometer read off of a Thunderbolt IV ignition

[alamantia]

1987 Wellcraft with a 260 merc I/O

Engine Serial # 0B489983

Last season I had this issue where my tach wouldn't work upon start up and then start working 5-10min into my trip and be fine up until the engine got cold and then it would do it again. I took the tach out of the dash and cleaned the terminals thinking it may be the tach itself but to no avail.

Now I am thinking there may be a lose connection or bad ground in the wire or wires that connect to the distributor? I checked the only wiring diagram I have for the boat but did not happen to see which wire off the Thunderbolt IV runs the tach.

I seem to think that the gray wire that is affixed to the negative side of my coil may run the tach, but I am unsure.

Does anyone know where I should be looking?

 

[mr300z87]

Re: Where does the Tachometer read off of a Thunderbolt IV ignition

It is the gray wire from the negative side of the coil and it goes to pin 2 of the main harness connector. You can find the wiring diagram in the service manual, check the stickies by DonS at the top the this forum there should be a link. Merc Manual #9 should be the one you need.

 

[tazrig]

Re: Where does the Tachometer read off of a Thunderbolt IV ignition

+1^^^ gray wire off (-) coil. Here is a link to a free PDF version of the OEM manual for your engine:

Boatinfo - Mercruiser Service Manual nr. 9 - 1985-1988 - GM V8 Engines

Go to section 4-D that has electrical schematics for your gauges and what wires control what for your gauges.

 

[FreeBeeTony]

Re: Where does the Tachometer read off of a Thunderbolt IV ignition

Maybe your gauge is faulty...........

 

[alamantia]

Re: Where does the Tachometer read off of a Thunderbolt IV ignition

Maybe your gauge is faulty...........

Maybe, do you think I should buy a new gauge or check for a bad connection first?

 

[wrench 3]

Re: Where does the Tachometer read off of a Thunderbolt IV ignition

If you connect a standard 12v test light between a good ground and the signal wire at the tach, and crank the engine. The light should blink very fast. If it does, the wire is OK but you still have to make sure that you have good ground and ignition supply before condemning the gauge.

 

[FreeBeeTony]

Re: Where does the Tachometer read off of a Thunderbolt IV ignition

Maybe, do you think I should buy a new gauge or check for a bad connection first?

I had a gauge that would stick when I first started the engine.......I'd tap it and it would start to read. I replaced it and problem solved.

 

[84EdH]

Re: Where does the Tachometer read off of a Thunderbolt IV ignition

Clean and tighten all connections. On the back of the tach, there should be a screw with an arrow that points at the number of cylinders. yours should be pointed at eight. turn that screw back and forth a few times to help the internal connection on the tach. if still no go, change the tach.

 

[wrench 3]

Re: Where does the Tachometer read off of a Thunderbolt IV ignition

Clean and tighten all connections. On the back of the tach, there should be a screw with an arrow that points at the number of cylinders. yours should be pointed at eight. turn that screw back and forth a few times to help the internal connection on the tach. if still no go, change the tach.

Why does everybody want to use the shotgun aproach instead of geting a $10.OO test light and finding out what's realy wrong?

 

[Fun Times]

Re: Where does the Tachometer read off of a Thunderbolt IV ignition

Why does everybody want to use the shotgun aproach instead of geting a $10.OO test light and finding out what's realy wrong?

Hey there wrench, The way I'm seeing the information being provided by all the members is an approach to the final outcome. All the steps mentioned by a number of iboats members would be just part of the process of elimination to possible a new gauge just it's being done without rewriting what everyone else said including your very informative step of what to do / how to check it all out.

In the end, all we can do is provide the information the best we can and hope the original poster will be able to pick up on the information being provided (weather or not it even helps in some cases) and figurer out their own steps to take as they move along. Then we'll hope that they come back online and say this was the problem and thank you everyone.

 

[wrench 3]

Re: Where does the Tachometer read off of a Thunderbolt IV ignition

Sorry if I came off a little harsh. I've seen to many damaged terminals when they were working fine before some one messed with them. In my hands on work I'm usually the one called in to clean up the mess.

 

[grewvin1]

Re: Where does the Tachometer read off of a Thunderbolt IV ignition

For my tach earlier mentioned turn the switch for 4-6-8 cyl crank it a few times may be some corrosion, then mine worked fine.

 

[NHGuy]

Re: Where does the Tachometer read off of a Thunderbolt IV ignition

So, alamantia, any luck? If you end up looking for a tach google your brand and whatever info you can like size and color. I found a new old stock tach that matched my other gauges for short money a couple of years ago that way.