My tacho packed up last week and today on start the tacho read a slow reading, about 600rpm when the engine was doing about 1000. It then dropped to zero.
I checked the connections on the alternator and belt tension. Belt tension might be a bit loose but I am wary of over tightening. The connections look unchanged but the tacho connection is a bit suspect (no different than in the past but it could have a hidden problem). The condenser on the alternator looks pretty bad.
I then checked the voltage and it charges at 14.8v most of the time but at lower revs idle and slightly above it every 5-10 seconds the charge would jump to 15.4 for a second or so. When the voltage jumped there was a bit of noise from the engine as if the load increased. At higher revs it wasn't noticeable. I googled it and 14.8v is OK from what I read. 15.4 is high but not critically so.
The alternator has a 3 position switch fitted to either: fully use the regulator, partially bypass it or fully bypass it. The previous owner who had the boat built and fitted out was an electrical engineer, it was his idea and although a nuisance it is useful to put a quick charge in the battery. I went through the three settings and it worked as it always has with the voltage and load increasing as the regulator is bypassed and dropping when reset to normal.
The batteries are well charged and always are from a 40w solar panel.
I ran the fridge for 10 minutes to take some charge out of the batteries but it had no impact on the symptoms I experienced.
What would be the symptoms if the condenser failed?
To summarise:
the tacho doesn't work properly/at all,
the charge is 14.8v but jumps to 15.4 occasionally with a slight change in engine load, only noticeable at lower revs.
I hate electrics and was hoping one of you electricians could identify what the likely issues are with the tacho and with the alternator charging symptoms.
The battery on my car $hit it self a couple of months back. I replaced it and found the alternator was not regulating and putting full charge into the battery. Probably why the original battery $hit it self. I knew something was wrong when I put the new battery in because the charge voltage was getting up to something like 18v (I think it was) even thought the battery was already fully charged. I replaced the regulator (about $80) and all went back to normal.
Maybe your regulator is going to die.
below is some info on your taco signal. (might help you go to sleep)http://www.amplepower.com/primer/tach/
My old boat had a self made charing system as he is and was a scientist and has a excellent machine shop as one of his hobbies including mills and lathes.
Its always been my belief that batteries prefer a slow charge rather than a higher quick charge so its gone and felt it was getting a little old any way
I replaced my alternator with a 85 or 90 amp hour rather than the old 45 amp originally bukh Im happy the motor is happy and the electrical system is happy 
The previous owner of my yacht was an electrical engineer also. If anyone wants a guided tour of how not to wire a vessel contact me!
New condensers are cheap so replace if you want to. The variations in voltage is most likely dirty terminals or dirty connections in fuse holders if you have them. I had exactly the same symptoms with one of my cars recently. I had the alternator bench tested but it was fine. Auto electrician found the ceramic fuse holders were slightly dirty, Could see nothing there with the naked eye. Cleaning solved the problem though. Check your rotating battery switches too, I have had the copper plates come loose.
One at the time,
when you get over 14.5V charge directly on your battery, I would be seriously worry.
/ voltage measured directly on lead outlets of the bat. only..not even on incoming wires /
Cooking lead-acid bat. is matter of weeks and they gone.
Engine revs. ind. is different problem alltogether,
Pulses from alternator or pick up sensor are derived by logic of indicator to smooth analog display.
You obviously getting pulses / there is some readings / but logic getting weaker and weaker. / shows less and less /.
HG. That pic looks like an electricians nightmare, but I guess you love every little wire !!!!!.
Select to expand quotesamsturdy said..
HG. That pic looks like an electricians nightmare, but I guess you love every little wire !!!!!.
There in the bin Ive only have one wire connected to the alternator . I still have to wire in the W wire for the tacho
I cleaned all the earths and connections this morning. No good. Alternator started off charging at 14.8v, after a run around the island it was charging at 15.4v. No tacho on start and it only got to 300rpm for awhile before dropping back to zero. Alternator and tacho are no going to the auto electrician.
Rather than fixing this 40 amp one I am considering getting a higher output one. I will need to find one that fits the same mounts, I hope the mounts are pretty standard.
Select to expand quoteMorningBird said..
I cleaned all the earths and connections this morning. No good. Alternator started off charging at 14.8v, after a run around the island it was charging at 15.4v. No tacho on start and it only got to 300rpm for awhile before dropping back to zero. Alternator and tacho are no going to the auto electrician.
Rather than fixing this 40 amp one I am considering getting a higher output one. I will need to find one that fits the same mounts, I hope the mounts are pretty standard.
what diesel are you running there Morning Bird. My old Bukh was a 35amp hour she now got a 1999 holden 90 amp hour . If you do buy a alternator make sure its got the W wire to plug the tacho to some dont have that option I over looked that when I bought mine . I now have it on my list to pull apart and make my own W wire terminal
So tell them you have a tacho to run off the alternator when you buy one . You proablly have thought of that already just thought youd be pisst if you got to the boat and went to plug the tacho in 
As you know MB I recently replaced my alternator with one I got from Germany at half the price (delivered) of a local one. My mounts are offset so I had to get one the same. Fortunately the mob I bought it from displayed pictures of it so I could see that it was the same, including a view of the back of it so I could see the connections, in my case, three. I don't have a tacho so there is no W connection on mine either. Interestingly the original alternator was 50amp, the replacement was 60amp. I know HG fitted a Holden?? alternator to his Bukh and had to modify the mount slightly, I believe HG's is about 90amps. Good luck MB.
I took the alternator to an auto electrician who will test it tomorrow. Turns out it is a 55amp one which is big enough for me. If it is ok to repair I will.
MB has a 20hp Sole (Mitsubishi) engine. I adjusted the belt this morning. It wasn't too loose but it made a noticeable difference to the smoothness.
I don't want too big an alternator. Mine has a regulator override switch which when set on a higher setting really loads the engine up and strains the belt so you can hear it slipping. It is only a thin belt. A big alternator will probably be too much for it and I doubt I need it. I like to run the engine for an hour or so everyday at sea so 55 amps plus the solar panel is enough for my power load, mainly the fridge and lights at night. The only other load is the tiller pilot which I only use when motoring anyway.
The tacho lead is on the list. He did find a Bosch alternator with the W wire. $325 for anew one, $225 for a rebuilt one. I am just about to look on ebay.
I measured the output on the W wire at 7.43v. The electrician says that is right so the problem with the tacho is inside it.
Select to expand quoteMorningBird said..
I took the alternator to an auto electrician who will test it tomorrow. Turns out it is a 55amp one which is big enough for me. If it is ok to repair I will.
MB has a 20hp Sole (Mitsubishi) engine. I adjusted the belt this morning. It wasn't too loose but it made a noticeable difference to the smoothness.
I don't want too big an alternator. Mine has a regulator override switch which when set on a higher setting really loads the engine up and strains the belt so you can hear it slipping. It is only a thin belt. A big alternator will probably be too much for it and I doubt I need it. I like to run the engine for an hour or so everyday at sea so 55 amps plus the solar panel is enough for my power load, mainly the fridge and lights at night. The only other load is the tiller pilot which I only use when motoring anyway.
The tacho lead is on the list. He did find a Bosch alternator with the W wire. $325 for anew one, $225 for a rebuilt one. I am just about to look on ebay.
I measured the output on the W wire at 7.43v. The electrician says that is right so the problem with the tacho is inside it.
Inghams are usually reasonably priced
$190 to overhaul the alternator. New regulator and new bearings. I had the regulator override switch removed as it probably contributed to the problem and with 50 amps it will be fine. I use about 50 amps a day so with 15 or so from the panel and 50 from an hour a day from the engine I should be right.
Now to check the tachometer. I can get a new one on eBay for $100.
I have the same issue.
Bought a new tacho since replacing the alternator with a new Bosh one, the Yanmar tacho didn't work, well it did but was all over the shop.
This is the tacho:http://www.ebay.com.au/itm/Electronic-Tachometer-RPM-Meter-Hour-Meter-Alternator-For-Trucks-JCB-/131119347377?
Installed with new W wire.
Same issue and for some reason the digital hour meter is way off.
It is obvious, that any tacho wouldn't give you correct reading.
Simply it depend on crankshaft pulley & alternator pulley ratio and how many poles alternator has.
Af course tacho internal calibration.
Posibly 3 different options ...repair tacho
buy tacho / for example VDO / the one whitch can by adjusted /
/simple as.... set the predetermined rev. turn the trim pot to match your revs. ..that's all
buy any tacho and create conversion table.
http://www.ebay.com/itm/Diesel-Engine-Tiny-Tach-HOUR-METER-TACHOMETER-w-6mm-Tansducer-MDS-6-DTL-6-/271251303964?pt=LH_DefaultDomain_0&hash=item3f27d66e1c
Bubble uses one of these impulse off the injection pipe
I wish that would work if the tacho was consistently inaccurate.
At high revs it's too high and at low revs it's too low and in between is all over the shop.
