1976 thunderbolt 500 - wiring rigged by a previous owner

Hi all, just recently traded a revolver for a 1976 Steury bowrider with a 1976 merc 500 thunderbolt (electric start - 4 coils). Engine runs, pumps water, shifts, etc., mercontrol will control motor (turns), throttle, and shifter linkages all operate correctly - verified all that before That should tell you what I have. I did download the wiring diagram and am looking at it as I write this - I believe this is the correct diagram as it is a 76, electric start and has 4 coils - http://www.maxrules.com/oldmercs/Wiring/1966ona/500_76_78eL.jpg.

Here is the deal, a PO, not sure if it was the one I bought the boat from a prior to him, pulled all the wiring out of the mercontrol box. There is a pushbutton start on the dash (kind of), the pushbutton choke is hanging in front of the mercontrol, and there is no ignition switch.

My future intention (part of my winter project list) is to go through the wiring on this boat and pretty much replace/rewire it, including replace the ignition switch, put the choke switch back in the control, and rewire the tach so the plug in works.

In the mean time, I would like to get the kid on the water before the snow flies adn money is tight....

So, my plan is as follows, and I am writing this in the hope one of you experts will check my sanity and stop me if I am going to burn something up.

1. Starter push button works, verify connections (red wire and white wire) make sure it is solidly mounted in the dash (retaining nut missing)
2. Wire a kill switch toggle in the hole next to the starter push button - orange wire and black wire from harness so I can kill the motor if needed
3. direct connect the brown, white and and black wire to the tach wires so I have a tach. I will have to cut the wires from the tach, but will save the connector so I can solder and reconnect once the mercontrol parts are acquired.
4. Verify choke switch wires - choke switch works, so, pretty sure this is already correct (grey and white wires)
5. Run engine (with muffs of course), verify charging system is working (higher voltage when running, than when stopped at the battery terminals). If not, than I will need to troubleshoot charging system. 2 yellow and red wires to rectifier, brown wire to tach, red wire to starter solenoid. Not sure what voltages to look for on this, but, don't know if it does not work yet.

My other thought, and I know this is kind of redneck, but my family is originally from Ohio, so, is put in a gang board on the dash, wire everything up as above, with the ignition toggle being the kill switch, an aux button for choke, starter button, and light button.

Last question, best source for positive lead for lighting? Would that be the red wire on the push button start switch - should I run a fusable link with a 10 or 15 amp fuse (or smaller?)

Thanks for any help.

Dave

Re: 1976 thunderbolt 500 - wiring rigged by a previous owner

Wire all accessories directly to the battery. It is wise to use a gang of fuses in a panel as well. The motor wiring harness is not designed for heavy current.

When you do go to install an ign key, you want red to B terminal, white to I terminal, yellow to S terminal, Black to M, Orange to other M, and grey to C (if applicable). Right now it appears the red is always conected to the white wire.

Re: 1976 thunderbolt 500 - wiring rigged by a previous owner

Good Catch, the wires must be connected somewhere other than at the starter button though, and Red-white must always be hot for this to work - or he wired teh button to red and yellow, and white was taken out of the loop. The choke switch works when the battery is connected, so it is possible that he wired the white and red together further back in the harness.

Yellow being the starter solenoid wire, would need to be engaged with Red for the starter to work - I think white only needs to be engaged for the choke and tach to work.

Funny, on the drawing, Red is hot, yellow is start, white connects the tach and the choke switch. The white wire runs into the ignition switch side of the wiring harness, but it does not seem to come out on the engine side (per the drawing). Is the only function of white to get power to the choke switch and the tach?

yep - just verified it - he has one side of the choke switch wired hot (white to red), and it looks like only red to yellow on the start button. The tach wires are a question now, as it looks like there are no wires going into the mercontrol so no juice to the tach.

I am thinking that I need an additional "ignition" switch if I put a gang switch in, and from that, run a red wire on one side and a white wire to the choke switch and tach on the other. That red wire can be split off to the start button and connect the yellow on the other side of the start button.

The black wire to the original ignition switch looks like a ground, and the brown goes between the rectifier and the tach.

Am I correct in assuming the battery charges from the rectifier and through the hot lead on the starter solenoid?

Thanks again - I am pretty good mechanically, but still don't trust myself 100% electrically.

d

Re: 1976 thunderbolt 500 - wiring rigged by a previous owner

The ignition system runs off the stator. Grounding the orange wire kills the spark. The white wire probably doesn't enter the engine casing, as it is unnecessary on that motor. Other motors have battery driven ignition. A seperate coil on the stator supplies the rectifier with AC voltage which is rectified into DC voltage and provided to the battery via the red wire. The starter solenoid is likely a convenient place to make the connection. The brown wire carries stator AC voltag pulses to the tachometer. The tach also needs +12VDC and ground to operate.

In your case a single DPDT switch and two pushbottons can substitute for the ign switch. One pushbutton is for starter solenoid, one is for the choke. Switch on connects red to white. Switch off connects orange to black.

Re: 1976 thunderbolt 500 - wiring rigged by a previous owner

Chris, Thanks a bunch.

I spent a little time on the wiring last night, reinstalled original choke button in Mercontrol, it is wired and works (though it is wired hot all the time and I am not sure I like that setup. I temporarily installed the push button in the dash (existing hole), it is wired and works. I am thinking I could run a toggle as you stated, or I could just spend $20 dollars and buy a sierra ignition switch, this one I believe - Part number 11-MP39100, and hook things up in a litte more controlled fashion.

I am still going to need to address the cut lighting wires, wire the tach in, and verify the system is charging, but I feel I am making progress.

I really want to get the boat on the water on Saturday, but, do not want to do so without the ability to kill the motor if needed, and without knowing if the charging system is working or not.

Thanks again.

Dave

Re: 1976 thunderbolt 500 - wiring rigged by a previous owner

Worked a bit more on the boat yesterday, seems the only issue with the main wiring harness was the starter solenoid (yellow) wire. Everything else hooks up, meters out, and seems ok. Except one little issue......

The Tach, wires up, seems to be good, engine idles around 900 on the tach, but when I rev the engine, tach goes to 1500 rpm and sticks there until RPMs drop below then it goes back down.

This is an old factory tach on a 1976 Steury, and I suspect it has not been hooked up in a while, so, it would not surprise me if it has a little corrosion or something inside the tach. just wanted to ask if anyone had seen this before, to rule out a wiring issue.

BTW ignition switch will be at NAPA today, so, wish me luck, I can get her wired right tonight and make the lake in the morning.

I think, if there is time, I am going to try to take a little slack up in the throttle cable tonight also, seems like I have to push the throttle handle to the 10:00 or maybe even a little further before it actually affects RPM.

Thanks

Dave

Re: 1976 thunderbolt 500 - wiring rigged by a previous owner

Ignition switch is in and works like a champ, tach still sticks, but I think I will need to disassemble it and see if it is all messed up. Looked at the throttle cable, there is room to adjust, but it was not intuitively obvious to me on how to disconnect the linkage - also it sets an inch or so below the access cover bottom and I was worried that I would break something or not be able to get it back together before dark and tomorrow would be a wash out. So, I didn't mess with it. I will be ordering a manual, but have not done so yet, so, will probably wait until I have some directions, and a bit more time, before trying to take the linkage off.

saw one other post where someone said they loosed the mounting plate and fixed a tach acting similar to mine, may try that in the morning.

Off to the lake tomorrow to test the boat, and of course I have to take my fly rod

Dave

Re: 1976 thunderbolt 500 - wiring rigged by a previous owner

Boat and motor tested pretty good, but as I thought, the throttle is not hitting the hard stop, it was actually about 3/8 of an inch shy of contact, boat did get on plane and once on plane ran out ok. I disconnected the barrel nut, tightened it until the throttle hit the hard stop at 9:00. also found the linkage left side center at the top had come disconnected, nut missing, so replaced the nut with a nylock, and put the hose on the motor, motor will hit the stop now and runs good, looks like very little upkeep was done on this motor, at least lately so I lubed all the linkages, and she seems to be doing ok. No issues shifting into forward, she does rattles a little going into reverse, and I will tackle that later.

Tach is still buggered up, but, I have not taken it off the dash yet, and I need to replace the reverse stop on the winch, if I can find one, or I will probably just replace the whole winch as the anti-reverse is not working very well and it looks like the lock has a broken tip. I may just build it up with the welder and reform it, but, we will see.

In any case, thanks for the help, pretty please with my new (to me) boat and motor.

Dave

Re: 1976 thunderbolt 500 - wiring rigged by a previous owner

Dave, The throttle cable needs to push the throttle closed. There should be enough travel to open the throttle all the way, when set properly. Start and warm the motor. Close the throttle until it runs at 800RPM or so. Now set the idle stop to keep the throttle at that spot. Now adjust the throttle cable barrel nut to push the throttle closed. Now recheck the max throttle to make sure she opens all the way.

Re: 1976 thunderbolt 500 - wiring rigged by a previous owner

Chris,

Thanks, yep, I kind figured that out when she idled at 1100after the first adjust, I was thinking spring close like a car or bike, but, then saw that I had adjusted too much. I took some of the adjustemnt out, until she set against the idle stop and idled lower (800 rpm), then verified she would hit the max stop and she does at the 9:00 position.

Thanks for the advice. Still gotta get a book, makes these types of things easier instead of trial and error.

Dave