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Alternator pulley heats up with key turned (edit: confirmed by others)

VolvoLatAm

Active member
Joined
Oct 4, 2019
Location
Colombia
In the key-turned ignition-on mode my denso 740 alternators (both) get warm at the pulley after sitting for about 10-15 minutes in the second key turned position when you can see the dash lights on with the seatbelt sign clicking. One alternator is old and worked well in the past, the other is remanufactured.

I never noticed any of this because I had the alternators down below and now I have them mounted on the top left on my 240.

Again, the heat felt in the pulley is independent of the alternator running and is in the mode just before starting the car.

The alternator has a good positive and ground connections with the right gauge wire. The diodes in the alternator are new. There are no odd signals on the warning lights for the alternator in any key position in the dash.

Anybody with a 740 or 100amp denso alternator can verify that the alternator pulley heats up by just sitting in the 2nd key position?
 
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found this on another forum:

and this:
Does a Denso 100amp alternator from a 740 on a 240 require different connection than the old hot connection for the previous Bosch alternator?
 
Your ignition coil is probably getting warm too. Sometimes they even explode when the key is left on for too long.
 
When key on, current from the exciter circuit thru the cluster bulb runs thru the rotor to try to excite the alternator. Not sure how much current as the bulb limits it, but i bet someone could calculate. Gonna guess half an amp so 6 watts of heat sounds close? The alternator isn't spinning so no forced air flow from the fan so it is just going to sit there and heat up.
 
found this on another forum:

and this:
Does a Denso 100amp alternator from a 740 on a 240 require different connection than the old hot connection for the previous Bosch alternator?
Denso has a larger positive stud than the original alternator so you need to crimp a larger terminal on the cable otherwise the rest is the same
 
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When key on, current from the exciter circuit thru the cluster bulb runs thru the rotor to try to excite the alternator. Not sure how much current as the bulb limits it, but i bet someone could calculate. Gonna guess half an amp so 6 watts of heat sounds close? The alternator isn't spinning so no forced air flow from the fan so it is just going to sit there and heat up.
BTU conversion formula
 
Denso has a larger positive stud than the original alternator so you need to crimp a larger terminal on the cable otherwise the rest is the same
This is true unless you find the less common 80 amp Denso which has the smaller 6mm stud and fits the older car harness eye connector. But over all most Denso alternators even the 80 amp ones have the 8mm stud on them.
 
Went to an alternator shop. Both alternators are fine and are generating correctly.

They were able to replicate the heat of having the key turned and these Densos do heat up...
 
I don't blame you for questioning this. The current passed to light the warning lamps for lamp test, and the key-on/engine stalled voltage at the D+ terminal in a Bosch alternator only amounts to maybe 0.8 Watts. For that heat to migrate from the field winding to the pulley enough to be felt would surprise me, but you don't say how long you leave the key on. Next time you're looking at this, put your voltmeter on that D+ terminal while the four lamps are on in lamp test. I've measured 1.6V on a Bosch. Can't fathom why a Denso'd be much different.

alt956.jpg


Edit: You do say how long you had the key on...
 
This is true unless you find the less common 80 amp Denso which has the smaller 6mm stud and fits the older car harness eye connector. But over all most Denso alternators even the 80 amp ones have the 8mm stud on them.
you can get the 100A rated rectifier with the smaller stud... which then also turns an 80 into a 100
 
I don't blame you for questioning this. The current passed to light the warning lamps for lamp test, and the key-on/engine stalled voltage at the D+ terminal in a Bosch alternator only amounts to maybe 0.8 Watts. For that heat to migrate from the field winding to the pulley enough to be felt would surprise me, but you don't say how long you leave the key on. Next time you're looking at this, put your voltmeter on that D+ terminal while the four lamps are on in lamp test. I've measured 1.6V on a Bosch. Can't fathom why a Denso'd be much different.

alt956.jpg


Edit: You do say how long you had the key on...

At around 10 minutes in the key on position it starts happening.

rear connections of the alternators look like this:
https://www.ebay.com/itm/1544779901...rentrq:9bbafda81860a49988798a53fffcb572|iid:1
 
Ah, 10 minutes. I'll have to give that a try with the Bosch.

I've forgotten the key in KP-II several times, mostly after checking OBD. After kicking myself "you'll burn the points" echoing in my ears from 50 years ago, I'm happy nothing bad happened. I've felt the ignition coil (cold), but never put my hand on the alternator.
 
ok I left the key in position II for 2 hours and YES the pulley was warm. Used the Snap On infrared and can confirm that the Denso casing was 66F and the pulley, and JUST the pulley, was 92F. The belts and the rest of the alt and surrounding area were unaffected... I didn't check anywhere else and she started fine and drove home equally as fine. What this means I don't know :-)
 
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