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BradC 09-13-2019 06:24 PM

Ford Lehman 120 Alternator Tach connection
1 Attachment(s)
I have a single engine FL 120 with a tach on the FB and one on the lower helm. Both worked fine last week. Now neither appears to be working. The hour meter on the lower helm tach is functioning. Not sure if that is any indicator of the tach connection or not.
I am not clear on what exactly is the connection for the tach on the back of the alternator, but it appears there is a single wire that leads to a connection in a silver box mounted on the engine room wall. I do not see any connection issues but really not sure what I need to check at the source. It would seem to me that with both tachs suddenly not functioning, the issue would be at the alternator but not sure where to go from there to figure out a next step.
The attached picture may or may not be any help.
Any assistance is greatly appreciated.
Attachment 93927

High Wire 09-13-2019 06:34 PM

So from the capacitor thread, did you verify the alternator is charging with the battery charger off?
That black square in the pic is a waterproof fuse holder. Is that fuse good?
Other wire connections clean and tight?
Also the oil pressure switch is not connected to anything. Did that used to feed the alternator field? Hopefully the low oil pressure alarm still works!
Other comments... Ther is a thin curled wire down low to the left of the oil pressure switch. Can't tell where that goes.

Also the black heavy wire from the alternator to the block should be the same gauge as the red output cable. The connections need to be cleaned up at minimum. And add a red insulating boot over the red output wire terminal.

BradC 09-13-2019 06:37 PM

I'll be back at the boat tomorrow and will do a better job of wire tracing. I'll check the connections and clean things up as suggested, as well as add the insulating boot.

Alternator is charging with the battery charger off. I did check the fuse in the picture (and another attached to the silver box in the engine room not in the picture) and fuses are good. Connections are tight.

BradC 09-15-2019 09:37 AM

Just a quick update. I still don't have working tachs, but after more research I discovered the PO replaced the original alternator with a Balmar 110 amp and the ARS-4 (the "mystery" silver box on the ER wall) smart regulator, so the oil pressure switch shown as disconnected in the photo is no longer needed, which is why it's not hooked up. The oil pressure sender and alarm switch are hooked and functional.
I also discovered my water temp alarm sender is broken off at the block, and so I will order a new one tomorrow. The water temp sending unit is attached and my panel gauge works fine.

Then I got distracted by another issue, which tends to happen to me easily anytime on a boat, but that's another thread, perhaps. Not to go off on a side tangent on this thread, but there is also some curious wiring decisions made from the alternator to the DC electrical system/house batteries that involves two more battery switches in the engine compartment area, and some routing that on first looks seems odd, so I need to figure that out.

I will return today and my plan is to follow the tach wire off the alternator, to the ARS-4, and on to the tachs so I can clean the connections, and see if anything appear out of place. Not sure where/how it splits between the upper and lower helm but will continue to post progress in the hopes that it may help another in the future.

Pete Meisinger 09-15-2019 09:52 AM

Your "feed" wire will not "Y" off anywhere. There will be a feed wire from the lower station tach to the upper.

Your alternator will be different being that it is a replacement but... there should be a wire that does not look "factory" coming off the alt. it may even feed into one of the vent holes on the back of the unit. This will be your tach feed. I don't see it in the pic. but it could be the white wire which crosses over the fuse box.

There should only be three wires on the back of the lower helm tach. A) POS. B) NEG. and C) "S" (signal). Although unlikely, it is possible that the nut on the "S" terminal is loose or vibrated off. Check it out.

Don't leave us hanging. Keep posting about the problem until you solve it. More and better pics will help everyone.

Welcome to the dark side..


BradC 09-17-2019 06:25 PM

Problem Solved
After running all the tachometer connections and finding no apparent issues, I returned to the alternator and regulator to start looking at them again. I wondered if my initial check of the alternator was correctly accomplished and did I make certain inverter was off and the trickle charger I have on the start batteries was also off.

So I started the engine again to recheck and noticed that the LED display on the Balmar MC-612 was blank (the MC-612 turns out to be what I thought was a ARS-4, but still a Balmar charger/regulator).

I found a simple troubleshooting guide online and determined that there was no power from the field wire coming off the MC-612. I called Balmar and spoke with a tech who walked me through some more troubleshooting steps and we determined that there was no power to the Positive Sense Wire from the battery bank, a necessary source of power for the MC-612 to come on.

Following that wire I found out that when I installed two new batteries last week, I inadvertently disconnected the Positive Sense Wire and never hooked it back up (I honestly don't remember taking it off, but looking back, it was shortly after that I realized the Tachs stopped working).

Reattached the wire. Started the engine. 30 seconds later the MC-612 display came to life, batteries started charging and as I revved up the engine, the lower helm tach sprang to life. Went up the FB and viola, that tach was working also! Success.

I hate when I create my own problems. Boats can do that plenty without my help. that the tachs are working again, the lower helm and the FB tachs are about 300rpm apart. Any way to calibrate them?

Thanks to all who offered help. I've cleaned up my connections, increased the size of my ground wire from the alternator, and installed rubber boots over the connections that lacked them so hopefully as I go through I'm at least preventing myself from creating more issues in the future, but I'm not holding by breath. It's kinda what I do.

I highly recommend the Balmar line of products based on the excellent customer service considering I acquired the equipment and wasn't the original purchaser; and the fact that both the alternator and the MC-612 were installed more than 14 years ago and still working great (the tech told me the MC-612 was taken out of production in 2011 for an improved model).

Comodave 09-17-2019 06:54 PM

Glad you found the problem. Hate it when I shoot myself in the foot, but I do it frequently...

Island fish lifter 10-20-2019 06:37 PM that the tachs are working again, the lower helm and the FB tachs are about 300rpm apart. Any way to calibrate them?

there should be "dip switches" on the back of each tach. youll need to verify the actual engine rpm with a photo tack, and see which tach is giving you a proper reading, then set the dip switches on the other one to match

SeaMoose 10-20-2019 06:57 PM

Ford Lehman 120 Alternator Tach connection
Some tachs have dip switches, some have a little screw. That being said unless the two tachs are from the same manufacturer and - and this is a big if on an old boat - the wiring to both has zero resistance and a perfect ground... you wonít get them to match exactly.

High Wire 10-20-2019 07:25 PM

You are going to need to start with a table:

Actual RPM from a photo tach or some other +/- 1 RPM measurement;
Lower Tach;
FB Tach

Find out which one is closest to true then adjust the other one.
After adjustment you will need to redo the table to see how you did.
It can be very frustrating!
Good news is you can do it at the dock.

Comodave 10-20-2019 07:30 PM

Or you just get them close to each other and live with it. We arenít talking about a super critical issue here.

Island fish lifter 10-20-2019 08:43 PM


Originally Posted by SeaMoose (Post 813156)
Some tachs have dip switches, some have a little screw. That being said unless the two tachs are from the same manufacturer and - and this is a big if on an old boat - the wiring to both has zero resistance and a perfect ground... you wonít get them to match exactly.

you are right, but it would be the first thing I would check as its fast, easy and free.

jleonard 10-21-2019 07:42 AM

I got my flybridge tach pretty close at 1500 rpm. (within 25 rpm of the handheld reading). That's using the adjustment screw via the back of the tach.

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