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Low alt voltage when warm

B

bumblebeeman

New member
Joined
Sep 26, 2012
Location
Petaluma
Hey everyone,
Ive got an 86 245 N/A and few years back I swapped in a 100a Denso. worked great for me for the past 3 years daily driving.. But recently ive been having an issue where it makes a solid 14.4 V upon initial start up, but once warm the voltage drops, and with accessories on can even get below 13v. Ive read that heat can cause the Densos to go bad??

Im wondering two things, firstly about swapping my alternator to the intake side, and also if its true that the heat can cause issues??

Thanks!
bbman
 
Redwood Chair said:
Yes swap it out now before it puts you on the side of the road with a dead battery.
Click to expand...

I had a local rebuilder rebuild a dead dense I had. but RWC have you herd anything about them failing with heat?? I think what I read was that the center dowel somehow loosens once it warms up and screws things up... any truth to this??
 
I'd say do some diagnostics with a multimeter first.
Make sure that your battery cables aren't dropping voltage:
Measure DC voltage from B+ on alt to battery + terminal while warm & running. You'll have to stop the engine to safely get down to the alt and attach the wire to the post. Tape to sidewall to avoid wire getting caught in belts.
Measure DC voltage across battery neg terminal to body ground at various points (intake manifold, screw by battery tray, etc).
Measure DC voltage across alt body to engine body. Careful of belts. This will check the little grounding wire going from the alternator to engine body.
Measure AC voltage across battery terminals. Should have very minimal AC volts, if not none. If anything bigger than... idk 0.5V? then you probably have a problem with the voltage regulator diode.
If you have a loop ammeter, you can see what kind of amps the alt is cranking out during warm idle. excessive amps could point to a faulty circuit of some sort.

Or just slap a new one in and pray that was the issue
 
And remember, the alternator's regulator is designed to reduce voltage output as a function of temperature, and in a 240, that regulator is under the exhaust manifold.
 
cleanflametrap said:
And remember, the alternator's regulator is designed to reduce voltage output as a function of temperature, and in a 240, that regulator is under the exhaust manifold.
Click to expand...

Good idea guys. Mu multimeter broke in the move but will try to get some testing done.

Has anyone had any success moving the alternator to the intake side? I tried a search but only found one other thread where it had not been worked out. seems like having it the 740 set up could be best. PS down below exhaust and alt by intake?

thanks!
 
bumblebeeman said:
Good idea guys. Mu multimeter broke in the move but will try to get some testing done.

Has anyone had any success moving the alternator to the intake side? I tried a search but only found one other thread where it had not been worked out. seems like having it the 740 set up could be best. PS down below exhaust and alt by intake?

thanks!
Click to expand...

I tend to fine that when I try to go stirring things up, the design intent and reasoning behind the volvo engineers' decisions becomes apparent. Thee design stuff with purpose. Not much coin flipping going on in sweden. If you're trying to relocate the alternator to save it, when it has been prividing a nice 13.5V to 14V warm to every other volvo 240 on the road, I'd reconsider.

If you want to relocate the alternator to achieve new abilities that aren't available im the stock location, like bigger alt or higher up to allow you to drive through high waters, this makes sense.

But if you're just trying to save a $100 alternator, you'll definitely spend more than $100 in time and money relocating it. Alt has two belts that drive it. PS has one. Unless its an older engine
 
Where are you getting 13v measured at? The densos definitely don't like heat, I had low voltage when temps reached near triple digits. I moved mine to the intake side (no ac or ps) and ran 1/0 cables from the alternator to the battery, then from the battery to the starter. I'll get 14v at the dash volt gauge, but as soon as I start putting a load on it will drop near 13.
 
How's the belt tension? Also, does your temperature gauge seem to read higher than usual when the voltmeter drops? Could be that your belts might be a bit on the loose side, but not enough to cause belt squeal.
 
philski o'flood said:
there are literally 100s of millions of Denso alts getting baked in cars every single day, since like the 1970s, and they work fine

check the brushes, check the belt tension
Click to expand...

tension is good. tiny bit of give.

I think this one is just wearing out.. seems to bounce around, 14.2 at start up, then dropped to 11.9, then bounced and held at 13.5 for a while. AAll on a 65 degree day, no accessories on. this engine does tend to run but warm, only stays in the middle on the temp gauge with a 180 degree thermostat in it.. Going to swap it out and will keep you updated.
 
Update: Ok swapped out the old unit that was finally starting to make the battery and dash lights flicker, for a Denso I had Rebuilt by a local guy, worked great, perfectly for the first 20 miles. then started dropping, first to 13.8, then to 12.5, now hovering around 11.8. Is it possibly to Overtension the belts?? I guess some further diagnostic is in order... This is reading from a digital and stock Volvo gauge in the dash. I'm so confused
 
Hahaha the nut holding the + alternator wire had come loose, tightened it up and working much better, will keep you informed.
 
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