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i have a 1983 40 hp mercury, at full throttle it only registers 1400, i know it is running higher than that.
i checked compression and spark. i could not check compression on bottom cylinder because i could not get the fitting in the plug hole because the pan is in the way, but the top cylinder came in at 125 psi. could corrosion on wiring or something like that cause me to not get a proper reading? any help on what to check or how to trouble shoot would be much appreciated.
danny

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If it was working properly before and then suddenly started to give back readings then I would trace all of the wiring connections first. If nothing appears obvious then it is probably burned up. When you turn the switch off does it stay on 0 or move up to 15 or 20?

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i'll have to check next time i turn it off, i didn't really pay to close of attention to the tach when i first got it, but i don't recall it ever registering very high. are these the symptoms when a tach goes bad? thanks for the help.
danny

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Also, the lower cowling that's in the way should be removable for you to get the comp tester in there. And take a look on the back of the tach, there should be a little slotted knob or screw head with a pointer that's set to the number of poles for your motor. Take note of the setting, and sometimes clicking that around and returning it to the original setting can clean up the contacts to get things working again if that's the problem.

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it does not go all the way to zero, it goes down between 8 and 900 when it's off, what does this mean?

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Another check...

Where is the tach when the key is ON and not running?

It should be at 0 .....

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i'll check tomorrow and let you know.

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it does not go to zero, it goes down to about 500, and know change when you turn key on. i was checking how many cylinders it was set to and it was set to 4, i set it to 2 where it should be i am gonna start it later today and see if that makes a difference. if it was set to 4 cylinder would that have anything to do with it not going to 0 when it gets shut off?

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Most Tach will fall somewhere around 500 or so when turned off...........If the setting is incorrect there is no doubt that you will get irractic and inaccurate readings, but if it was working properly before and then suddenly began giving bad reading I would suggest that it is either a bad connection to the motor or the tach is bad

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Tom,

When the ignition key is turned "ON" the tach MUST read zero..... or there is something wrong with the tach.

dharmon,
The settings on the back of the tach is NOT for the number of cylinders.
That is a setting for different engines. Example: I have a 6 cylinder engine but my manual says to set the tach at #5 setting....

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Joe, I agree that when the key is ON it should register zero, but don't most go up a few hundred when the key is in the OFF position?

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Yes... they do.. When the key is off, the tach could go up, down, or stay the same.

I misread your post...

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oh wow, i did not know that about the setting, so if i do not have a manual how do i found out which setting to be at?

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Some here will be able to tell you what the setting should be for your Mercury 40.

What brand of tach is it?

You could go online and look at the manufacturer's tach website and possibly find out that information too.

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Depending on the serial number of the engine, the setting it was on (4 poles) was most likely correct:

http://www.faria-instruments.com/site...S0086p.pdf

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Also meant to post this, good article on troubleshooting tach problems. Should help:

http://faria-instruments.com/site_man...meters.pdf

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i will try it this weekend again and see if it is any better, if no i guess i will need a new one. i can not see a brand on the face of the tach, i'll have to get under the console and take a peek at the back.

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well, i tried all the things mentioned and tach is still functioning the same i guess its time for a new tach. any suggestions on a middle of the road tach?

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If the pole setting is correct on the back of the tach, it's most likely a bad rectifier on the engine. Take it in for service before you buy a new tach, or find out how to test the rectifier and voltage regulator.

I would recommend a Teleflex Amega series tach if you need one, like these:
(enlarge photo to full size for clarity). Teleflex has a new domed line called "Eclipse" that is also nice.

http://smg.photobucket.com/albums/v42...an0007.jpg

In the picture, the engine is off, note tach is not "zeroed".

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You may want to consider the stator too. It so happens I read this thread last night. Addresses very similar problems to yours.

http://forums.iboats.com/showthread.p...p?t=259582