93 omc cobra 5.0 alternator gets extremely hot

[mobile_guy]

customer bought the boat a couple years ago with an alternator that wasnt charging properly and was occasionally throwing belts. someone else replaced the alternator with the incorrect model and still wasnt charging correctly and threw the belt more often.
i got to it and installed to correct OEM alternator and it throws the belt after 10 mins. i noticed a slight alignment issue thats due to the bracket flexing when the belt is tightened. after correcting that problem i believe the alternator is getting hot enough to melt the belt and the alignment was never really an issue.
the boat has dual batteries and ive always switched them on both while doing my testing and running an engine. im pretty sure both batteries are lead acid and test good.
is there an issue with ford alternators not being able to charge both batteries at once?
is something else going on that im unaware of or missed?
boat is driving me crazy.

 

[gm280]

:welcome: aboard mobile_guy. Nice having you join us...

If you have a standard current draw on the boat, there should be no reason for the alternator to get that hot...period. And since it is, I was wondering what current draw you have that you don't know about. I would suggest removing things to see if the alternator would stay cool. The fact that this isn't the original alternator removes any issue with the alternator itself. So you have something drawing a huge amount of current. Fine out what that is by illuminating circuits until you locate it! JMHO!

 

[mobile_guy]

heres my list of things to do when i get back to the boat:
retest batts and verify both are wet cell
put switch on batt 1 or 2 and see if still over heats
verify current at both batts and output on alt
check for shorts in other circuits
am i forgetting anything?

until now, ive been convinced it was an alignment problem
i know the first time i was at the boat i did all the standard service checks but it was a while ago.

 

[bruceb58]

What is the voltage output of the alternator at when the engine is running at 2000 RPM or higher? You have the sense line hooked up properly? If you had an open sense, you could be causing the alternator to be outputting at full output all the time.

 

[mobile_guy]

output is 14.5ish. i have not checked current yet
its only got 3 leads, as far as i remember, field, B+, and ground
i have pulled up a diagram and plan on double checking all of this next time im there
like i said, ive been leaning towards an alignment issue until after i left the boat the last time. now im convinced its an electrical issue and im mining all of you for tips so i can go there and get it done the next time.

 

[bruceb58]

Take a picture of the back of the alternator and post it. The OEM alternator should have a sense terminal.

 

[Bondo]

after correcting that problem i believe the alternator is getting hot enough to melt the belt and the alignment was never really an issue.

Ayuh,...... Welcome Aboard,..... I ain't buyin' that theory,.... I'd go back to the belt, or belts,....

For that matter, All of the belt routing,...

I donno this motor, so I can't comment on where/ what to look for,.....
But I ain't seein' load, throwin' the belt/ belts,....
You oughta be able to pull full power, without throwin' the belts,...

Have ya got a load tester, to test the batteries,..??

 

[mobile_guy]

Ayuh,...... Welcome Aboard,..... I ain't buyin' that theory,.... I'd go back to the belt, or belts,....

For that matter, All of the belt routing,...

I donno this motor, so I can't comment on where/ what to look for,.....
But I ain't seein' load, throwin' the belt/ belts,....
You oughta be able to pull full power, without throwin' the belts,...

Have ya got a load tester, to test the batteries,..??

ive been doing this 20+ years
the belt doesnt throw. it melts.
everything works fine. engine runs great. then belt starts smoking. alt body is hot enough to burn my callused finger tips instantly.
ive seen pulleys come in wobbling so bad its a wonder how the engine was even still running.
this is not an alignment issue
and yes, i tested the batts on my first visit there. i didnt sell him new ones so im assuming they tested fine but i do not remember the results and i also dont think i ever checked to make sure they are both wet cells, though im sure i checked the water level or to make sure they were sealed. i have seen sealed batts freeze and crack.

the pulley was clearly out of align when i got to it, not to mention the rust. now that ive put a shorter belt on it and the alt is bottomed out on the shortest adjustment, the alignment looks fine but alt is still getting extremely hot.
ill be checking batts and current draws next time im there.
im also adding engine grounds and battery connections to my list of things to check

 

[Lou C]

That alt must be putting out its max out put (65 amps?) all the time for it to get that hot.
On the back on most of the OMC alternators they usually have 3 connections:
B+ output, "S" for sense and "E" for excite.
The way some OMCs were wired they used remote sensing running back to the ignition wiring under the dashboard. What can happen is because of resistence in the wiring the sense wire will read low (sometimes as much as 2 volts) and that can cause the alt to overcharge.
Try this:
Take a reading of your batt volts .Then disconnect the sense wire at the back of the alt, hook up a volt meter between the sense wire and a ground. If it's more than 1 volt lower you have higher resistance than you should in the sense circuit. I got a new alt from Arco and they recommended disconnecting and taping up the sense wire and making up a jumper wire from the B+ to the sense terminal. Doing this dropped my charging volts from 15.5 volts to. 14.2 volts...

 

[mobile_guy]

That alt must be putting out its max out put (65 amps?) all the time for it to get that hot.
On the back on most of the OMC alternators they usually have 3 connections:
B+ output, "S" for sense and "E" for excite.
The way some OMCs were wired they used remote sensing running back to the ignition wiring under the dashboard. What can happen is because of resistence in the wiring the sense wire will read low (sometimes as much as 2 volts) and that can cause the alt to overcharge.
Try this:
Take a reading of your batt volts .Then disconnect the sense wire at the back of the alt, hook up a volt meter between the sense wire and a ground. If it's more than 1 volt lower you have higher resistance than you should in the sense circuit. I got a new alt from Arco and they recommended disconnecting and taping up the sense wire and making up a jumper wire from the B+ to the sense terminal. Doing this dropped my charging volts from 15.5 volts to. 14.2 volts...

now that theory i like
i know the boat has a few electrical gremlins. its pretty common around here with the summer heat and high humidity for there to be excessive corrosion and gauges/circuits that work intermittently.
ill just do that fix you mentioned and see if it works.
if it does, i owe you a beer and a burger.

 

[Lou C]

Let us know what happens. I had a problem with my batts not lasting as long as they should have and I think this may have been the cause. With the way they wired these, the resistance in the dash wiring will cause the volt meter to read approximately normal (13-14) when running; but if you go and check the actual battery voltsge it can be too high like 15.5 v in my case. You can re-wire the sense terminal a few ways; one quick way is to make up a jumper wire from the B+ to the sense terminal; the other probably slightly better is to run a longer wire from the sense terminal to the battery switch common terminal to read battery voltsge....
The moral of the story is don't trust dash gauges 100%....

 

[mobile_guy]

That alt must be putting out its max out put (65 amps?) all the time for it to get that hot.
On the back on most of the OMC alternators they usually have 3 connections:
B+ output, "S" for sense and "E" for excite.
The way some OMCs were wired they used remote sensing running back to the ignition wiring under the dashboard. What can happen is because of resistence in the wiring the sense wire will read low (sometimes as much as 2 volts) and that can cause the alt to overcharge.
Try this:
Take a reading of your batt volts .Then disconnect the sense wire at the back of the alt, hook up a volt meter between the sense wire and a ground. If it's more than 1 volt lower you have higher resistance than you should in the sense circuit. I got a new alt from Arco and they recommended disconnecting and taping up the sense wire and making up a jumper wire from the B+ to the sense terminal. Doing this dropped my charging volts from 15.5 volts to. 14.2 volts...

it was the sense circuit.
there was no wire at the harness for sense.
all i had was excite, B+ and a ground. i tested that ground from every angle to make sure there was no bat voltage, shorts, or opens and its definitely a solid ground connection.
i made a jumper from B+ to S and it dropped my charge voltage to 13.8 at high idle from 14.6

apparently i was the third tech involved in this problem. the customer was overly excited to finally have it fixed and running properly.
thanks for the tip.

 

[Lou C]

Great feed back, is the alt running cooler now?
You never know what wiring issues will turn up in older boats...

PS
Found a current drain on my boat: the auto on features of my Garmin GPS; it was turning itself on after I shut the engine off. I used an ammeter between the neg batt post and the ground cable to check for current draws.

 

[bruceb58]

I asked the question back on July 9th in post #4 if the sense was hooked up properly.

 

[Lou C]

On the Cobras with the dog clutch system the sense wire was in the harness by the valve cover on the port side of the engine, I wonder if it was disconnected by a PO because they installed a non stock (ie not 3 wire like OE) alt...while I followed Arcos advice when I installed the new alt (jumper from S to B+), I think it might even be better to run it from the S terminal on the alt to the common stud on the batt switch...

 

[bruceb58]

On the Cobras with the dog clutch system the sense wire was in the harness by the valve cover on the port side of the engine, I wonder if it was disconnected by a PO because they installed a non stock (ie not 3 wire like OE) alt...while I followed Arcos advice when I installed the new alt (jumper from S to B+), I think it might even be better to run it from the S terminal on the alt to the common stud on the batt switch...

You are correct about the sense line on early Cobras but I believe on later ones in the early 90's they had jumper wires on the alternator itself..