850 Merc Tach Problem

[68 Starcraft SS]

After a mere 43 years my 1974 Merc 850's battery cables were trash. I ordered a replacement harness from CDI and checked that the new and old connectors corresponded to the same wire colors. I opened up the MerControl to see that there wasn't much clearance for all the wires coming into the ignition switch. Everything looked to be in good shape with the exception of a small crack in the insulation of the red hot wire near the ignition switch so I applied a little liquid tape to repair the crack in the insulation and decided to utilize as much of the original harness as I could. The original harness has heavier gauge wiring and the insulation on the individual wires is heavier and seems more like a rubber material than the plastic insulation on the new replacement harness. I spliced the new harness into the old near the rear of the boat using shrink butt connectors and also shrink tubing over each of the individual splices.

After completing the changeover, I fired the engine up to find that the tach is not working. Everything else is working perfectly including the electric choke. I pulled the MerControl apart and tested continuity from the brown wire at the tach plugin to the brown pin at the motor plug and all is good. I did the same thing with the black wire at the tach plugin and again had continuity. I also double checked the red wire to see if maybe there were any other breaks in the insulation and did not find any. I put it all back together and tested it again with the same result. This time I did notice that when cranking I was getting a tach reading although it seems high at 3000-4000 rpm's. Once I let off on the key the tach returns to zero.

If anyone has any ideas on what to check next or what I may have done wrong I'd really appreciate any ideas or suggestions as I'm really stumped at this point.

 

[Chris1956]

The tach runs off the stator pulses. If the rectifier is shorted, the stator maynot have pulses. Disconnect the stator from the rectifer and connect one of the yellow wires to the tach sender wire. The sender wire is brown.

 

[68 Starcraft SS]

Chris 1956 - Thanks for your reply. I went out to do as you suggested. Apparently I have not had the cowl off the engine for awhile. No need as it has been running great - right? A lot of the internal wiring is in bad shape as insulation is falling off and the wire underneath is a lovely shade of green. I'm very surprised it started and ran at all. I pulled the rectifier out and tested it with an ohm meter per the Merc service manual procedure that I found here at iboats. The rectifier tested good. I'm going to order a new internal harness to correct most of the problem wiring. The two wires coming out of the stator are also in bad shape. If possible, I'd like to not go to the expense of replacing the entire stator assembly just to correct the wiring. I'm thinking I'll splice the wires coming out of the stator as close as I can to where the come out of the stator and and tape up the short ends as good as I can. If that is a bad idea or there is a better way to go about this I'm very open to suggestion. I'm hoping that after completing all this the tach will work.

 

[Chris1956]

I would splice the stator wires, if you can. The original harness was marine grade wire, which was pre-tinned. With a heavy duty soldering iron, some flux and a bit of luck, the solder will flow. I would recommend you pull the insulator off some butt joints, flux 'em up and add some shrink tube to the new wire, Now flux the stator and the new wire, crimp and solder them. Now add the shrink tube for insulation.

Those harnesses are available many places. I would look around for a good price. Try surplusunlimited web site.

 

[68 Starcraft SS]

That's a much better idea on the stator wires. I'll go that direction. Once the new harness gets here and I install it I'll give an update on the tach. I'm really glad I started looking at the engine wiring as it appears I potentially had a good chance of testing my new fire extinguisher.

Thanks for the info on surplus unlimited as I was not aware of them.

 

[68 Starcraft SS]

Finally got the time to install the new engine harness. I was hoping the tach problem would go away with the old harness but it still doesn't work. I tried connecting one of the yellow/red wires from the stator to the brown tach feed but nothing changed. The tach still registers between 3-4000 rpm when cranking. What else should I check?

By the way I'm very happy with how the splicing of the stator wires turned out. Thanks for the suggestion that was the right way to do it.

 

[Chris1956]

If the tach doesn't work properly, when connected directly to a yellow stator wire, either the stator is bad, the wiring is bad or the tach is bad. The tach could also be getting a poor +12VDC or ground connection.

If you have a voltmeter, set it to AC voltage and run the motor at idle on the flusher. See if you are getting an AC voltage on each of the stator wires to ground, and then together. I am not sure what the voltage will be, so start with a large voltage scale, and work down.

 

[68 Starcraft SS]

Tested both yellow stator wires and had voltage from both. Also tested both yellows together and had voltage. I had previously tested both yellows for continuity to ground to make sure I had the shrink tubing properly insulating both wires. I checked the brown tach feed wire for continuity from the switch box to the output pole on the MerControl and it is good. Also checked engine ground to MerControl ground output and it is also good. Put a test light on the brown pole on the switch box with motor running and got nothing. Bad switch box?

 

[Chris1956]

The brown post on the switchbox was used for a special Mercury-propriety tachometer. Universal tachs use the brown wire fed by one of the stator wires. On later year Mercs, the brown wire connection was changed from the switchbox to the rectifier. You need to switch it, if it is still connected to the switchbox, and are using a newer tachometer..

 

[68 Starcraft SS]

The tach is an Airguide Sea Speed that has been installed since the motor was new in 1974. The brown wire was connected to the brown terminal on the switchbox and each of the 2 yellow/red stripe stator wires are connected to separate terminals on the rectifier. I have a used switchbox that I acquired years ago for a "rainy day". I installed it today for something to try but the tach problem persists.

Thanks for your help with this.

 

[Chris1956]

Well, it sounds like you have it wired correctly, Maybe the tach has failed?

 

[68 Starcraft SS]

The only thing that makes me think it is OK is because of the reading I get during cranking. I've tested continuity across the 2 wires feeding it and it's good. Other than that, is there any way to actually test the tach for operation?

 

[68 Starcraft SS]

Mods: I apologize for reopening this old thread but I finally resolved this issue and wanted to post what I found on the chance that someone else ever has the same problem.

As it turns out I had 2 problems, both self inflicted. I did an internet search on Airguide Sea Speed tachs to see if I might turn up anything on a way to test the operation. I stumbled on a picture of the same tach that also had the original installation instruction sheet. At the end of the instruction sheet it stated that on a small number of Mercury applications the power and ground cables had to be reversed. I had removed the dash to refinish it with the tach in place and when I reinstalled it and reconnected the black to black and green to green wires as logic dictated to me that had to be how they were supposed to be connected. After seeing the possibility that it required reverse polarity I tried it and the tach sort of worked but it was really wonky.

After proving that the tach itself seemed to work I went about trying to figure out why the readings were so erratic. I had replaced the engine internal wiring harness with a replacement from CDI. They make this harness to fit a broader application than just a '74 Merc 850. As I was staring at the color wiring diagram I pulled off the internet for the umpteenth time, it finally dawned on me that the dashed line that stated 850 and late 800 applied to my engine. Just because the harness wasn't manufactured that way didn't mean it would work without a little modification. It was at this point that I remembered the original factory harness had a big metal one into two wire splitter in it. The new harness required splitting the red positive wire and installing a 2-way connector with one lead then going to the switchbox and the other to the rectifier. Yesterday was finally the first chance to test the fix on the water and I'm happy to report the tach is now working properly. Note to self: Don't throw away old parts until new ones have been installed and are proven to be in working order.

 

[Glennsedlak]

Mods: I apologize for reopening this old thread but I finally resolved this issue and wanted to post what I found on the chance that someone else ever has the same problem.

As it turns out I had 2 problems, both self inflicted. I did an internet search on Airguide Sea Speed tachs to see if I might turn up anything on a way to test the operation. I stumbled on a picture of the same tach that also had the original installation instruction sheet. At the end of the instruction sheet it stated that on a small number of Mercury applications the power and ground cables had to be reversed. I had removed the dash to refinish it with the tach in place and when I reinstalled it and reconnected the black to black and green to green wires as logic dictated to me that had to be how they were supposed to be connected. After seeing the possibility that it required reverse polarity I tried it and the tach sort of worked but it was really wonky.

After proving that the tach itself seemed to work I went about trying to figure out why the readings were so erratic. I had replaced the engine internal wiring harness with a replacement from CDI. They make this harness to fit a broader application than just a '74 Merc 850. As I was staring at the color wiring diagram I pulled off the internet for the umpteenth time, it finally dawned on me that the dashed line that stated 850 and late 800 applied to my engine. Just because the harness wasn't manufactured that way didn't mean it would work without a little modification. It was at this point that I remembered the original factory harness had a big metal one into two wire splitter in it. The new harness required splitting the red positive wire and installing a 2-way connector with one lead then going to the switchbox and the other to the rectifier. Yesterday was finally the first chance to test the fix on the water and I'm happy to report the tach is now working properly. Note to self: Don't throw away old parts until new ones have been installed and are proven to be in working order.

Thanks for the update. I have a 1975 850 merc. I have rewired and basically rebuilt the engine. Now have a fuel issue to deal with. 20 gal built in tank. I should of cleaned it out before filling up.