frying out the voltage regulator

[2000StreetRod]

Engine ground?

The alternator output and regulator are dependent on a low resistance electrical path to the battery negative terminal. The alternator has no ground terminal. Internal components are grounded to the case that bolts to the mount that bolts to the engine. The engine has straps that connect it to the chassis and the battery negative terminal has a cable that connects it to the chassis. I suggest that you measure the resistance between the battery negative terminal and the alternator case to eliminate that a possible problem.

 

[2000StreetRod]

Charging system operation

The following note is in the 2000 Explorer wiring diagrams:

"With voltage applied, regulator is activated, allowing current to flow from sense A circuit to generator field coil. Generates an AC output which is converted to a DC output by a rectifier assembly internal to generator, and is supplied to vehicle through the B+ terminal. S (stator) circuit is used to feed back a voltage signal from generator to regulator, this voltage (typically half battery voltage), is used by regulator to turn off indicator."

What is surprising to me is that according to the wiring diagram voltage is always present from the battery to the alternator A and B+ terminals. The reverse polarity resistance of the rectifier assembly prevents a drain on the battery. The transistor in the regulator is also reverse biased preventing a battery drain. But that doesn't explain why in my case the alternator did not charge the battery when the instrument cluster was removed.

In your case if the voltage regulator transistor is burned out (open) then when the ignition is On battery voltage should be present at the "I" terminal since there is no path to ground for current to flow and thus no voltage drop across the 470 ohm resistor/bulb. No current flow results in no bulb illumination. With a functional regulator and the engine running I guess there should be about 6.5 volts at the "I" terminal allowing only 6.5 volts to drop across the resistor/bulb which is not enough for the bulb to illuminate assuming a 12 volt bulb.

 

[SoNic67]

I think that, if the grounding path from alternator to the battery is not good (ground straps missing), the current will try to "return" via the internal regulator connection.
Usually that makes the bulb in dash light.

But if that bulb is actually defective with a short-circuit, it might allow for a higher current than normal to return trough regulator's printed board and probably that fries it.

 

[2000StreetRod]

shorted bulb?

. . . But if that bulb is actually defective with a short-circuit, it might allow for a higher current than normal to return trough regulator's printed board and probably that fries it.

The element of a typical bulb is a high resistance filament. The voltage drop across the filament and current flow heats the filament resulting in light. I'm not aware of a filament type bulb short circuit failure mode.

 

[2000StreetRod]

Measurements

Because of my M90 installation access to the alternator connector is very limited with the upper radiator hose and main intake in place.

However, I was able to obtain some measurements without removing anything. The alternator regulator cable was still connected to the regulator.

Battery - terminal to - terminal = 1.0 ohms (resistance of meter probe wires)
Battery - terminal to alternator case = 1.1 ohms

................... Ignition Off ....... On
..... Battery + to - = 12.32 v 11.71 v
Alt B+ to Battery - = 12.01 v 11.73 v
. Alt A to Battery - = 12.27 v 11.69 v
.. Alt I to Battery - = 0.18 v 0.12 v

The above may not be accurate since my probe connection to the regulator connector was somewhat marginal. However, the measurements seem reasonable. I did not make any measurements with the engine running since I don't recommend you trying that until you confirm all connections and associated readings are correct. You don't want to fry another alternator.

With the ignition on, engine not running and the alternator not charging the "I" resistance to ground must be fairly low compared to the 470 ohm resistor in series with the circuit since 98% of the voltage is dropped across the resistor/bulb.

 

[SoNic67]

The element of a typical bulb is a high resistance filament. The voltage drop across the filament and current flow heats the filament resulting in light. I'm not aware of a filament type bulb short circuit failure mode.

The bulb not, but the circuit to it might... Just throwing darts here.

PS: Your belt routing over the alternator pulley seems... not right.

 

[bruceahlgren]

The alternator output and regulator are dependent on a low resistance electrical path to the battery negative terminal. The alternator has no ground terminal. Internal components are grounded to the case that bolts to the mount that bolts to the engine. The engine has straps that connect it to the chassis and the battery negative terminal has a cable that connects it to the chassis. I suggest that you measure the resistance between the battery negative terminal and the alternator case to eliminate that a possible problem.

OK I ohm battery negative to alt body got a reading of 31.2. I also have a resistor selection on my multi meter and on that I got a reading of .05. And with key on not running from batt negative to alt body I got 8.5 v and with key on not running I got 8.4v from batt negative to the I terminal and from same I terminal to ground on engine I got the same reading 8.4v

 

[bruceahlgren]

The bulb not, but the circuit to it might... Just throwing darts here.

PS: Your belt routing over the alternator pulley seems... not right.

Belt routing is correct rotating clockwise I put it back on exactly how it came off and checked the routing sticker just to make sure

 

[2000StreetRod]

Don't understand

OK I ohm battery negative to alt body got a reading of 31.2.

Was the above with the meter set to read volts, millivolts or resistance?

I also have a resistor selection on my multi meter and on that I got a reading of .05.

What was the resistance range selected (i.e. 200, 2000, 20K, 200K, etc.)?

And with key on not running from batt negative to alt body I got 8.5 v

Highly unlikely. You probably had millivolts selected.

and with key on not running I got 8.4v from batt negative to the I terminal and from same I terminal to ground on engine I got the same reading 8.4v

The above voltage reading should be the same but not 8.4 volts. Even 8.4 mv would be greater than I expect.

 

[bruceahlgren]

Was the above with the meter set to read volts, millivolts or resistance?



What was the resistance range selected (i.e. 200, 2000, 20K, 200K, etc.)?



Highly unlikely. You probably had millivolts selected.



The above voltage reading should be the same but not 8.4 volts. Even 8.4 mv would be greater than I expect.

On voltage I put it to 12v DC I just have one setting for ohms and the resistance is the same just a resistor symbol no other selection

 

[SoNic67]

Belt routing is correct rotating clockwise I put it back on exactly how it came off and checked the routing sticker just to make sure

I was commenting on the above picture, it was different from what I remembered.
Now I see that the routing is different for a 4.0 engine, my 5.0 has an extra idler that helps the alternator pulley to have higher contact angle with the belt.

 

[2000StreetRod]

Suspicious readings

I need accurate meter readings to diagnose the problem.

Measure the voltage between the positive and negative posts on the battery. If it is about 12 volts then without changing modes/selection on the meter measure the voltage between the battery positive post and chassis ground. It should be about the same as between the battery posts. Then measure the voltage between the battery negative post and the same chassis ground. The voltage between the battery posts should equal the sum of the two measured voltages to chassis ground. In a previous post you reported 8.5 volts between the battery negative terminal and the alternator body. If that measurement was accurate then there is either a very poor engine to chassis ground connection, or a bad battery to terminal connection, or the battery to ground cable is corroded internally, or there is a bad battery cable to ground connection.

 

[bruceahlgren]

I need accurate meter readings to diagnose the problem.

Measure the voltage between the positive and negative posts on the battery. If it is about 12 volts then without changing modes/selection on the meter measure the voltage between the battery positive post and chassis ground. It should be about the same as between the battery posts. Then measure the voltage between the battery negative post and the same chassis ground. The voltage between the battery posts should equal the sum of the two measured voltages to chassis ground. In a previous post you reported 8.5 volts between the battery negative terminal and the alternator body. If that measurement was accurate then there is either a very poor engine to chassis ground connection, or a bad battery to terminal connection, or the battery to ground cable is corroded internally, or there is a bad battery cable to ground connection.

I will check those and check battery cables. Won't be home till sometime Sunday afternoon I will try and check ASAP when I return.

 

[abes.shop]

I know that this is an old link but was there ever an out come to find the problem of the voltage regulator frying? I have a 98 x xlt 5.0 and just recently I replaced the battery, alternator, then the starter due to a slow cranking before engine starts. It has just gone through 2 reman alt. and 2 new alt withing a few days of replacing them and I am trying to find what can be causing the voltage regulator to fry so quickly. Please help going nuts..