Tach install on Merc 500 Thunderbolt

sc00ter78

Seaman Apprentice
Joined
May 26, 2009
Messages
31
I have a New Merc Tach (PART # 79-883620Q2) to replace an old one for my 1975 Mercury 500, Thunderbolt. 4 Cylinder, 2 Cycle.(Serial # 7052814)

My questions is:

What number in Column #1 (Position) do I turn the dial to? (There is a turn dial to select 2 to 6). I have no alternator/charger.

The tach has connections for 12volts, GND, SEND, and a light.

The istructions:

4 Cycle gas Engine, Outboard engine
Position Pulses/revolution Standard Ignition with alternator with

2 2 4 cyl 4 Pole
3 3 6 cyl 6 pole
4 4 8 cyl 8 pole
5 5 -- 10 pole
6 6 -- 12 pole
 

Attachments

  • Sony 055.jpg
    Sony 055.jpg
    73.4 KB · Views: 2

sschefer

Rear Admiral
Joined
Nov 13, 2008
Messages
4,530
Re: Tach install on Merc 500 Thunderbolt

Position 2 should work but it will probably work on postion 6 also. FYI, you have an alternator its the stator and magenets under the flywheel. I think you mean you don't have a rectifier. How are you connecting the tach to the engine.
 

sc00ter78

Seaman Apprentice
Joined
May 26, 2009
Messages
31
Re: Tach install on Merc 500 Thunderbolt

I already have one on the boat. Reason for changing is for a cleaner looking one. As I recall there is are red, black, and brown wires. Is the idling range 800-1000?

I was told by a nearby mechanic that there was no alternator on it and therfor have to charge the battery after everytime I take the boat out. Do you know roughly how many amps it puts out?
I will look for info on the "stator" you mentioned and more about the rectifier.

Would you happen to know why the Tach needle drops back to zero for a few minutes when running on the water? RPM ranges are various when it happens.

Thanks for all your help. Much appreciated.

Scott
 

Attachments

  • PRE BOAT PIC'S 191-2.jpg
    PRE BOAT PIC'S 191-2.jpg
    39.6 KB · Views: 1

Moody Blue

Captain
Joined
May 24, 2004
Messages
3,136
Re: Tach install on Merc 500 Thunderbolt

Red would be the ignition +12V, black is Gnd (-12V) and brown would be the tach signal from the motor. Chances are with the gauge you have, the tach signal is taken from the brown terminal on the switchbox. With a new style tach you will need to remove the brown wire from the 4 terminal side of switchbox (don't move any wires on the 3 terminal side) and connect it to either of the yellow wires on the rectifier. The rectifier will have three wires connected to it. Two yellow wires from the stator, and a red wire that runs out to the battery. This is the +12V that charges the battery. I think the stator puts out about 9amps at full rpm.

As for the idle RPM range, 800- 1000 is a little high if in the water. On muffs it would be OK. Would shoot for 600-700 in the water.

Would you happen to know why the Tach needle drops back to zero for a few minutes when running on the water? RPM ranges are various when it happens.

Could be bad connection at the switchbox, gauge, tach harness connection at the controls. Maybe a broken wire that intermittently make contact. Perhaps a defective gauge?
 

sc00ter78

Seaman Apprentice
Joined
May 26, 2009
Messages
31
Alternator test/problems?

Alternator test/problems?

Thanks for your help guys "sschefer" and "Mody Blue".

The info really gives me a direction to go in. It's difficult finding info on this motor.

Next will be to install a Uniden Depth Sounder (QT206).

After spending a whole day on the water I've never seen the battery hold a charge or stay topped up. I have always had to recharge it at the end of the day. I am going to get my battery tested (3 yrs old).

Would either of you know if the alternator has problems charging and if so how to test it?
 

sc00ter78

Seaman Apprentice
Joined
May 26, 2009
Messages
31
Re: Tach install on Merc 500 Thunderbolt

sschefer - Looking at your restoration of your boat reminds me of when I did mine. You did a great job. I will post some pic's of mine soon.

I know the feeling of being half way done and thinking - "wow, I didn't expect this much work."
 

Moody Blue

Captain
Joined
May 24, 2004
Messages
3,136
Re: Tach install on Merc 500 Thunderbolt

Either the battery or the rectifier is no good. Do a search here on how to test the rectifier. Many auto supply places will load test the battery for free.
 

sc00ter78

Seaman Apprentice
Joined
May 26, 2009
Messages
31
Re: Tach install on Merc 500 Thunderbolt

Ok, I've been reading some of the rectifier and tach post on here and looked at my boat pic's and saw that I do have a rectifier.

1st
Looking at the wiring drawing posted from earlier, it looks possible to take the "E" Tan wire off the Switch Box and connect it to either yellow wire on the rectifier.

The are a lot of posts saying to run a seperate wire. Shouldn't both ways work? Cause the Tan "E" only connects to the Tach and if disconnected it won't go anware and just hang losely. Why not use it to connect to the rectifier?

2nd
After looking for ways to test the rectifier I only came across testing the voltage on the battery leads from the motor with it off and then on. I thought that because you can pull start the motor without the battery connected and the key "on" you should be able to test the voltage comming off the rectifier or cables going to the battery.

Prior to me I don't know but say the motor was connected backwards (reverse polarity) to the battery, wouldn't that blow the rectifier and possibly the stator if windings were thin and not a heavy guage?

3rd
Looking at the pic I uploaded. It looks like someone has already connected a Green wire to the rectifier and left the Brown wire attached. Does anyone agree?
 

Attachments

  • PRE BOAT PIC'S 11422.jpg
    PRE BOAT PIC'S 11422.jpg
    38.7 KB · Views: 2

sc00ter78

Seaman Apprentice
Joined
May 26, 2009
Messages
31
Re: Tach install on Merc 500 Thunderbolt

Tach
testing tach now, tach is jumping on all positions (2-6) when connected to either yellow wires on rectifier as well as the switch box.
NOTE: new wire ran directly from rectifier and switch box to the tach (no loose connection).

Rectifier
voltage jumps 12.59v for 2 seconds and then drops to 0v for 1 second and then back up to 12.59v (repeating).
 

cell

Petty Officer 1st Class
Joined
Apr 16, 2009
Messages
208
Re: Tach install on Merc 500 Thunderbolt

Rectifier
voltage jumps 12.59v for 2 seconds and then drops to 0v for 1 second and then back up to 12.59v (repeating).

Is this while its connected to the battery?If it isn't don't run it while taking any wires off....you could do some damage.
 

sc00ter78

Seaman Apprentice
Joined
May 26, 2009
Messages
31
Re: Tach install on Merc 500 Thunderbolt

This is while the engine is running and tested at rectifier.

Why does it matter if the battery is connected other than charging it?

What would cause the repeating drop in voltage?

Still looking.....does anyone have an answer for the tach question posted above?
 

sschefer

Rear Admiral
Joined
Nov 13, 2008
Messages
4,530
Re: Tach install on Merc 500 Thunderbolt

Had I not experienced the same problem with mine at one time I would not believe your readings. However, I also experienced a rough idle condition and weak spark. My solution was to go with the hat trick and replace the stator, trigger and rectifier. That solved my problem but I can't tell you for sure which was at fault because they all can cause very similar running conditions. My stator did have a bad spike condition so I just assumed damage to the rectifier and trigger. Expensive way to go but it saves a lot of headaches.
 

jimg984

Chief Petty Officer
Joined
Jul 16, 2007
Messages
403
Re: Tach install on Merc 500 Thunderbolt

dont use the wire to switchbox .... go from either yellow wire on the rectifier to the sender connection on the tach, black from tach to a ground
power/+ should go to a ingnition ON connection if done this way it should work with the proper 1 to 6 settting. if not you got a bad rectifier or stator
most likely would be rectifier than the stator
 

sc00ter78

Seaman Apprentice
Joined
May 26, 2009
Messages
31
Re: Tach install on Merc 500 Thunderbolt

Thanks Jimg984 and sschefer,

I will be looking at that this weekend. Just bought a rect. off Ebay and awaiting the guy to reply and mail it off.

I wil keep you guys posted.
 
Top