Overheating alternator?

[Avery4]

As the title says, I'm wondering at what point it's generally reasonable to worry about an alternator getting too hot.

The reason I'm asking is I recently had my 1990 Acura Legend's OEM Denso alternator rebuilt by a reputable starter and alternator shop and I'm wondering if I should take it back to have it checked because it seems to me like it's getting ridiculously hot, although I don't have a baseline since I bought the car with a dead alternator.

According to my infrared thermometer (which I realize isn't very accurate), the alternator case is reaching 240+ degrees under moderate loads with a fully charged battery and the hood open in ~60 degree weather. Drops of water instantly boil off of the alternator case and the air coming out of the vents is so hot that it was burning my hands while I was doing voltage drop testing. It gets hot enough at times to drop the internal regulator's setpoint to 13.3-13.4 volts.

Other than that, the alternator seems to work reasonably alright. It doesn't take much load to make the voltage drop at lower RPMs, but again, I don't have a baseline and I haven't put my amp clamp on it yet to verify its output, so that may be normal.

What would you do? The alternator is at the top of the engine and is easy to change if I need to take it back to be fixed, but I don't want to waste their time or my time if that's just normal operation for these.

Thanks in advance, any information or suggestions are greatly appreciated!

 

[ripcord]

First thing I would do is to charge your battery fully with a plug in charger and then have it tested. If it's got a problem and is pulling lots of charging current, then that would work the alternator really hard and cause it to be hotter than normal. If the battery is fine, I would call up the alternator shop and tell them your concerns.

 

[Avery4]

First thing I would do is to charge your battery fully with a plug in charger and then have it tested. If it's got a problem and is pulling lots of charging current, then that would work the alternator really hard and cause it to be hotter than normal. If the battery is fine, I would call up the alternator shop and tell them your concerns.

Great advice. The battery is healthy and fully charged, it sits on a maintainer most of the time. I haven't called the alternator shop yet, but I plan to once I check its amperage output. I know alternators get hot, but it just doesn't seem normal for an alternator to get THAT hot, especially on a cool day.

As hot as it's getting in ~60 degree weather with the hood open under normal loads, I can only imagine how hot it would get sitting in traffic in 100+ degree weather with the AC on max, cooling fans running at full speed, radio, brake lights, headlights, possibly wipers, etc.

 

[Jetronic]

check the ground and battery connection. We've had a hundai do this, but I don't remember what solved it, if anything. Rebuilding the alternator never did, and they didn't last more than about a year.

I think the owner bough an ioniq sometime after we changed the lead to the fusebox (which goes to the battery from there). But I didn't do that, don't know if it ran cooler

 

[berniedd]

Also get the car engine slightly warm before the alt gets too hot. What's the battery voltage the battery voltage at idle and at 1500 RPM? Alternator might be putting out too much voltage, (and hence current) that's causing your high temps. Voltage should not exceed 14.3 to 4.7 volts at any time.

I agree, water droplets boiling off the alt housing immediately on contact suggests too hot a condiion.

 

[CorvairGeek]

Can you check and see how much AC voltage it is outputting? A shorted diode will output AC and they get really hot.

 

[Marcozi]

Can you check and see how much AC voltage it is outputting? A shorted diode will output AC and they get really hot.

^^^THIS^^^. Use a DVM, set it to ac and see what it reads. I have a friend who has a 300z from the 1990's. He was have weird electrical issues. I checked the voltage at the battery. The DC voltage was all over the place. Checked the ac voltage and it was running from 15vac to 60vac. He had the shop install a new alternator.

 

[oil pan 4]

^^^THIS^^^. Use a DVM, set it to ac and see what it reads. I have a friend who has a 300z from the 1990's. He was have weird electrical issues. I checked the voltage at the battery. The DC voltage was all over the place. Checked the ac voltage and it was running from 15vac to 60vac. He had the shop install a new alternator.

Yep that's why it may have quit.
Most shops will take a dead alt and just toss in a new voltage regulator and call it good without testing.

 

[Avery4]

Thanks for the suggestions! Just to clarify again, the battery is healthy and fully charged, it's not drawing any measurable current. At a high idle under no load, it's charging at around 14.2-14.5 volts cold and 13.3-13.6 volts hot.

No significant voltage drops. Under full load, the highest voltage drop I measured was from the alternator positive to the battery positive at about 0.18 volts. The engine to body, engine to battery negative, and battery negative to body were all under 0.05 volts since I added a few extra grounds.

Here's the temperature of the alternator's stator windings after idling for about 15 minutes with just the AC on max and my LED headlights on. 70 degree ambient, so cooling fans on low speed. Hood open the entire time. During that test, the current was around 39 amps at about 12.5 volts.

 

[berniedd]

Can you try see if your thermometer is accurate? Like, try it on water that just started to boil (212 deg F)?

 

[Avery4]

Can you try see if your thermometer is accurate? Like, try it on water that just started to boil (212 deg F)?

At 32 degrees and 212 degrees, that thermometer is accurate to within 1 degree.

 

[paulo57509]

I have nothing to add, other than it makes me happy knowing there's another 1st Gen Legend still on the road.

 

[eljefino]

Something's wrong. Go see the alternator shop, they'll make it right. +2 on the AC ripple idea.

 

[GSCJR]

@Avery4, don't remove the alternator, take the entire vehicle to the rebuilder, and have the rebuilder connect their test equipment directly to your charging system.

Further, during your testing, what were the Amp and Volt outputs at idle, with all accessories switched off?

 

[ripcord]

I looked it up and the max AC voltage you should see is .5V

 

[Avery4]

I have nothing to add, other than it makes me happy knowing there's another 1st Gen Legend still on the road.

Thanks! Mine is a 5 speed L coupe too and it's been an awesome car. Hopefully it also makes you happy to know that the plan is to restore it and keep it forever. It has plenty of cosmetic issues I'm working on correcting (mostly paint and interior), but it's rust free, the body is in awesome shape, and it's mechanically very solid.

 

[Avery4]

@Avery4, don't remove the alternator, take the entire vehicle to the rebuilder, and have the rebuilder connect their test equipment directly to your charging system.

Further, during your testing, what were the Amp and Volt outputs at idle, with all accessories switched off?

Good idea, that way they can't try to blame something on the car. I was just thinking of taking the alternator in to save a trip since the shop is almost 100 miles away, but another trip to drop off the alternator if they find a problem is no big deal.

After initial startup before turning on any loads, the voltage was around 14.3. The current started out at around 12-13 amps and settled at around 8-9 amps after a couple minutes.

Once the alternator was hot, the no load idle voltage dropped to around 13.6 volts. The current was up to around 20 amps only because the battery was no longer fully charged after idling with the AC and headlights on for about 15 minutes.

 

[Avery4]

I looked it up and the max AC voltage you should see is .5V

Maybe on a scope, but for whatever reason, a multimeter doesn't seem useful for testing AC ripple from an alternator. I've seen AC voltage readings of 28+ volts from healthy alternators while I've seen alternators with bad enough ripple to cause speedometer problems barely read anything.

For what it's worth, my expensive Klein meter shows about 0.2 volts AC while my cheap Harbor Freight meter shows 26 volts AC, but both meters show the same 119 volts AC at a receptacle and the same 16 volts AC from a random doorbell transformer I had laying around

 

[RedSpider]

Went through this with two cars. Alternators would get so hot you could smell them in the car, any water on them would immediately boil. Both cases ended up being bad battery cables. The lugs looked fine, but when I took a razor and peeled back the insulation by the crimps, they were full of corrosion.

 

[GSCJR]

Once the alternator was hot, the no load idle voltage dropped to around 13.6 volts. The current was up to around 20 amps only because the battery was no longer fully charged after idling with the AC and headlights on for about 15 minutes.

This is the only part that has me concerned . All the other results seem to be okay, IME. I'm curious as to what the rebuilder finds; keep us posted.