Mercury Thunderbolt tach help

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spamn

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I have a 1978 Merc 500 with the thunderbolt ignition. I also have the Mercontrol with the tach plug on the front of it. Any idea what tach and harness and I need to work with my setup?

Thanks
Sam
 
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knouse94

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Re: Mercury Thunderbolt tach help

I have a 73 Merc 500, with Mercontrol with a tach plug in the front. I have looked for the tach that you are referring and they are very hard to find. If you do happen to find one they are expensive. You would be better off wiring a new tach. There are various forum on here that talk about how to wire a new tach to this motor. I am in the same dilemma you are in right now. Good Luck
 
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68C15

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Re: Mercury Thunderbolt tach help

I have a 71 merc 500 w/ thunderbolt and mercontrol like you have with the tach plug on front. I found a tach at a salvage yard with the correct wiring and plug. it looks cool but does not work.

the front of my motor has a partially destroyed sticker saying something about some part needed to install a tach.

edit: I read this thread and will give it a try.
http://forums.iboats.com/showthread.php?t=307559
 
L

Laddies

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Re: Mercury Thunderbolt tach help

The simple solution for a plug in tach is to remove the tach sender wire from the switch box sender and hook it to 1 of the yellow alternator wires at the rectifier and use the alternator driven tach and it's plug in. The reason Mercury quit using the old thunderbolt tach was if they failed it could short the ign. system and kill the engine.
 
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spamn

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Re: Mercury Thunderbolt tach help

Just noticed the replies. Thanks guys. I am still a bit fuzzy about this though. I understand that the ebay link is the tach and connecter that I need. Hate for it to fail and short the ignition.

The link to the other Merc 500 hooking up the Teleflex is fuzzy to me too. I have the type V ignition system. Does this still apply to me?

Thanks again

Edit: Also just noticed in my Seloc (which of course is very old and outdated) to use the Merc tach that connects to the Mercontrol. BUT, "you can also connect one lead to the pimary (negative) terminal of any one coil. Connect the other lead of the tachometer to a good ground on the powerhead." Does this apply to newer tachs like the Teleflex or just the old Merc tach as well?
 
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Chris1956

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Re: Mercury Thunderbolt tach help

Spam, Your 1978 motor should already be wired for any aftermarket OB tach. Older models are not. Pop off the wrap cowl and look at the yellow stator wires at the rectrifier. You should see the brown wire already connected to one of the yellow wires. If not post back, and we can walk you thru the change.

Tach wiring harness should be avail at a dealer at least.
 
Gomer50

Gomer50

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Re: Mercury Thunderbolt tach help

Here is one like I use on my old Merc,
http://cgi.ebay.com/ebaymotors/TELE...3510908QQptZBoatQ5fPartsQ5fAccessoriesQ5fGear
tach 2.JPGtach1.JPG

What I did is I opened up the Merc. control box and wired the tach directly into the exsisting Tach wires inside the box bypassing the harness Connector(black,white,and brown).Then I went to the engine took off the cowling and looked for the brown wire.Disconnected the brown wire from the switchbox and moved it over to one of the Yellow wires thats coming from the stator to the recitifier.Works like a charm.Also if the recitifier is blown the tach wont work.
 
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knouse94

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Re: Mercury Thunderbolt tach help

Hi Chris, I am at the same point as spam on my quest to put a tach on my motor. I have a 73 Merc 500. I do not have a brown wire hooked in to either of my yellow wires on the rectifier. Where do I go from here. Thanks.

Also will this tach that is being sold on eBay work for this applications. Thanks Again

http://cgi.ebay.com/ebaymotors/NEW-...at_Parts_Accessories_Gear&hash=item255660b511
 
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Chris1956

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Re: Mercury Thunderbolt tach help

According to the sellers description, that tach will not work for your motor. OB tachs work off the stator, so number of cylinders is immaterial.

Remove your wrap cowl. Look on the switchbox for a brown wire, that does not go to the distributor. Disconnect the battery and the bown wire. and put an ohmmeter on the brown wire, to ground. It should be open. Now jumper the horizontal slot of the tach connection to the body of the MerControl, and it should show a ground on the brown wire. This proves it is the correct wire.

Now remove one of the nuts on the rectifier's yellow wire and connect the brown one there.
 
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knouse94

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Re: Mercury Thunderbolt tach help

Ok, that does not sound to complicated I will give it a try when I get a tachometer. I am not sure which one to buy and am on a limited budget. Any suggestions? I was thinking about a digital Tiny Tach. Not sure if that will work though. I am new to the boating world so this is all foreign to me. Thanks again.
 
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Moody Blue

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Re: Mercury Thunderbolt tach help

Spam, this is the tach harness for the older MerControl boxes. The three pin connector plugs into the front of the control box and the connectors on the other end can be cut off and connected to the tach as follows

http://cgi.ebay.com/ebaymotors/NEW-...at_Parts_Accessories_Gear&hash=item45e64d324a

WHT = +12V
BRN = tach signal from motor (see chris1956 post above)
BLK = GND

As for the tach, check ebay listings to get a feel for what's available and costs. I have bought several for under $30 including delivery. Pretty much any marine outboard tach will work with your motor. An automotive tach will not work. No need to get one rated for more than 6000rpm since your motor won't run that fast.
 
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spamn

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Re: Mercury Thunderbolt tach help

Moody Blue said:
Spam, this is the tach harness for the older MerControl boxes. The three pin connector plugs into the front of the control box and the connectors on the other end can be cut off and connected to the tach as follows

http://cgi.ebay.com/ebaymotors/NEW-...at_Parts_Accessories_Gear&hash=item45e64d324a

WHT = +12V
BRN = tach signal from motor (see chris1956 post above)
BLK = GND

As for the tach, check ebay listings to get a feel for what's available and costs. I have bought several for under $30 including delivery. Pretty much any marine outboard tach will work with your motor. An automotive tach will not work. No need to get one rated for more than 6000rpm since your motor won't run that fast.
Click to expand...

Thank you, sir.
 
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knouse94

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Re: Mercury Thunderbolt tach help

Gomer, what year and hp is your Merc?
 
Gomer50

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Re: Mercury Thunderbolt tach help

I have three a 1972 80hp inline 4, a 1974 85hp inline 4, and a 1978 90hp inline 6.
 
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Chris1956

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Re: Mercury Thunderbolt tach help

It will probably work, but I would look up the model number on the Faria website to be sure.
 
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knouse94

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Re: Mercury Thunderbolt tach help

Ok, last question. I am going to purchase the tach that is in the third link above. What Pole position will I set it at for my motor. Thank you everyone for all your help. I appreciate the patients with the new guy. You guys are great.
 
Last edited:
Gomer50

Gomer50

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Re: Mercury Thunderbolt tach help

12 pole and your good to go. Your Welcome let us know how things work out..
 
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