1997 GT-40 Bouncing Tach

[wetskier2000]

I've had an occasional bouncing tach for quite a while now. Most of the time it registers correctly but can jump to pretty much any RPM all by itself... Today I noticed that it bounced constantly with the radio turned up pretty loud. It was easily reproducable with radio volume. I could also knock on the tach face and get it to bounce... any ideas on where to start on this one? Bad connections? Bad instrument? thanks

[Rednucleus]

1st thing I woould do is check your grounds

[SilentSeven]

That era of boats suffers from an 'underwired dash' symptom. I suspect your problems are related to lack of power or poor grounding. Search the archives....lots of posts on how to resolve.

Sent from my Pixel 7 using Tapatalk

[wetskier2000]

What's the theory? The vibration is rattling an already poor connection?

[bturner]

More likely the boat is 26 years old. Loose wires? Sure, good possibility. Corrosion/deterioration of an already marginal electrical implementation? More likely. Previous owner compromised the electrical system with some "custom wiring". Very possible as well. Point being the boat is 26 years old and anything is possible at this point.

I bought a 70 Mach1 4 years ago and spent 3 of them chasing electrical electrical issues from the head lights to the tail lights. The first year I took the approach of chasing the symptoms and individual issues as they popped up. This lead to a very frustrating exercise of playing "whack a mole", had the dash out of the car 3 times in the first year alone. Switched my strategy to rip and replace and have spent this last season without electrical issues. I'm very lucky with the Mustang as you could probably build an entire car from aftermarket parts. Instead of cleaning or replacing individual wires/connectors, I started replacing harnesses. Lights don't work in the instrument panel? Replace the entire circuit board and all the light bulbs with LEDs. Oil or temp sender works intermittently? Replace the sending units and the complete harness. You'd be surprised how loose, brittle and corroded wire/connectors get after 20 or 30 years. I had wires that looked fine on visual inspection that crumbled in my hands when I moved them.

If this wasn't bad enough, after 26 years on the water you have to deal with previous owners, that in many cases have no clue on doing electrical wiring in a boat or car. Every time I think I've seen it all, someone will surprise me with some new moronic wiring. I've seen everything from people using cheap extension cords, home electrical wire nuts to sealing wiring connection with the infamous tape ball with the worst of those being the tape ball implemented with masking tape. Heck, I bought a house that a previous owner used speaker wire to extend a wiring circuit to a surface mount box.

Winter is coming and I see a winter project for you either going all in on inspecting/cleaning up the wiring or replacing a bunch of it if you want to resolve this issue. That or tune up your skills playing "whack a mole".

[wetskier2000]

No previous owners on this boat... Only wiring change I've made is to get rid of that stupid inline fuse thingy in the battery box. I eliminated it and wired to the CB at the rear of the engine near the ECM. I will pull the dash and do my 5 year cleaning this winter.

[SilentSeven]

As you clean up, consider adding a secondary power and ground. The dash harness plug that everything runs through is limiting. Here's what I ended up doing for my '97 when I started to have odd electrical problems. This pretty much solved everything.

https://www.planetnautique.com/vb5/f...fix-with-photo

[wetskier2000]

what an elegant solution.. thank you! How'd you run from the battery up under the dash. Could you make use of the existing wiring holes/chases exiting the battery box? I presume ENG and IGN are just relays to allow switched power from the 10g battery wire, right?

[SilentSeven]

Going from memory here.....boat's with a new owner now.

- Batt box is under the front seat; I'm pretty sure I ran the wires out of the top of the batt box crevice and then used wire mounts to attach them to bulkhead that separates the footwell as they made their dash run. I don't recall why I didn't pull them into the bilge and follow the follow the factory wiring. Likely it was difficult and less then obvious how to feed them into through bilge and up the wiring hole. I had a heater in my boat which made everything in that area super tight. I never had a problem this this approach as it's all under the seat and really wasn't visible. The main hot and ground block is the upper brass colored block.

- I made the wood bracket from a plywood scrap after using a cardboard template to model the sizing. Without this, you have no place to add all the stuff. It also allows you to build everything on the bench and limit the amount of underdash work.

- Be sure to run an inline fuse into the new hot line. I think mine was like a 15A which was plenty.

- This approach gave me a new hot / ground block in the dash area. The grounds are part of the problem and I ran a new ground line from the main block around the main dash plug which effectively a) gave the factory ground a secondary path and b) works around the limitations of the dash plug. Just add an inline disconnect to the new ground so you can unplug if you need to pull the dash pod. If you inspect the dash ground wiring after the harness plug, you'll see that the factory wiring just is really insufficient.

- For the hot, I ran two standard automotive relays purchased cheap from Amazon - one triggered by the accessory circuit and one trigger by the ignition on circuit. I don't recall where I vampired in the triggers but there's lot of options once you are poking around. For the accessory circuit, I created a secondary switched hot bus (lower silver block) and I used this for audio stuff. My recollection is that used the engine circuit just for the perfect pass.

My hot take on the factory dash wiring was this - it's -just- -barely- -adequate- to run the factory items. The moment you add in a larger head unit and/or a perfect pass, you've exceeded it's capacity and you start to see voltage drops and all sorts of odd behaviors. My perfect pass stargazer was completely flakey until I did this. Notably...and if you have one...the PP has a diagnostic more that will display the unit voltage. As part of my debug process before I built this, I used the PP to check the dash voltage and saw it was low....and how it got worse when I started accessories like the running lights / heater blower / etc. That convinced me that I needed to do something. After cleaning up the ground and getting more switched power to run the accessories, everything was good.

Hope this helps. It was a fun little project that worked well.

[gary s]

The real reason these tach's start jumping is corrosion and it happens to be internal. These tach's are universal, there is a switch on the back to change what engine it is being used with- 4,6,8. It is that connection that is getting corroded and that's why when you tap it's face it changes. An easy temporary fix is to cycle that switch from 4,6,8 a bunch of times which cleans that connection. My 95 has done it, the last time I cycled it it has been ok for a couple of years now.

[Rednucleus]

If that switch is the culprit, spray some Deoxit into it before cycling it