voltmeter dip when cooling fans are active
03-18-2003, 02:03 PM
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voltmeter dip when cooling fans are active
Greetings all,
I would like a little advice regarding a troubling issue. I have a 93 GTS with 45,200 miles. I have noticed that when the cooling fans come on the car experiences a momentary stutter with rpm drop and vibration associated with a registered drop in the voltmeter reading. The voltmeter normally runs a just a tick below 14 volts. When the cooling fans activate, the meter drops down to around 12 volts and slowly rises again to the normal setting. BTW, I have a new factory battery. Forgive me if this has been covered elsewhere.
I would like a little advice regarding a troubling issue. I have a 93 GTS with 45,200 miles. I have noticed that when the cooling fans come on the car experiences a momentary stutter with rpm drop and vibration associated with a registered drop in the voltmeter reading. The voltmeter normally runs a just a tick below 14 volts. When the cooling fans activate, the meter drops down to around 12 volts and slowly rises again to the normal setting. BTW, I have a new factory battery. Forgive me if this has been covered elsewhere.
03-18-2003, 04:54 PM
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Why was the battery changed? Maybe you have an alternator on it's way out, bad ground, bad final output stage for the fans, or bad fan motor itself. Are you running the A/C when you see this voltage drop?, or just normal fan operation from the engine cooling process?
03-18-2003, 05:04 PM
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I changed the battery because the PO had installed a $50 K-Mart special (not the correct size or the correct amps). I discovered this when I was trying to determine if the ground was faulty. The alternator seems to be charging the battery after restarts. There is no noticeable noise or other vibrations from the fans. Only the monetary drop in rpms and engine stutter. I have not checked with the AC on yet. I am thinking it is a bad voltage regulator.
03-18-2003, 06:11 PM
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With the car off and cool, spin each fan by hand and see if there is any significant difference in the resistance of each. Then observe the
fans when running......any speed difference that
you can tell? If everything seems to be fine, then your probably back to looking for a bad ground or something with the alt.
fans when running......any speed difference that
you can tell? If everything seems to be fine, then your probably back to looking for a bad ground or something with the alt.
03-18-2003, 06:40 PM
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If one of the fans are vibrating there is likely a bad bearing in the motor. Will draw more current and even take time to "spool up" to a point where the rpm's reduce the added resistance. a bad motor bearing, that is my bet.
03-18-2003, 07:38 PM
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I checked the voltmeter reading with the lights on and the two ac units running at full settings and the voltmeter dipped and remained at approximately. 12 volts while at idle and would push back up near 13.5 under acceleration. I do not feel any sustained vibration or bearing type noise from the fan units. I my have not described the sensation accurately before. I usually notice and rpm drop associated with a momentary shudder from the engine. Hope this helps. I spun the fans by hand and did not notice anything unusual. The fans rotated freely and did not emit any bearing grinding noises.
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03-19-2003, 11:34 AM
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The volts sound reasonable for the load you are putting on it. The next thing.....is this only happening when the A/C system is turned on? If you turn all A/C functions off and the fans cycle normally in response to engine temps, does the problem occur? Here's where I'm going....
As the A/C cycles the engine is affected both electrically and mechanically. The compressor clutch draws current to operate, and the compressor also imposes a load on the engine mechanically via the drive belt. Also during A/C operation the fans should be on all the time if I'm not mistaken. So as the compressor cycles on and off there will be an associated
on and off of drag imposed. I believe there is an idle stabilizer that should keep the engine
stable as this load is turned off and on. If the
stabilizer is going bad it may not be able to
react fast enough or at all to the additional load placed on the engine. The stabilizer should
keep the idle steady by making small corrections with different engine loads. Check out this link to help troubleshoot....
<a href="http://www.nichols.nu/tip443.htm" target="_blank">http://www.nichols.nu/tip443.htm</a>
As the A/C cycles the engine is affected both electrically and mechanically. The compressor clutch draws current to operate, and the compressor also imposes a load on the engine mechanically via the drive belt. Also during A/C operation the fans should be on all the time if I'm not mistaken. So as the compressor cycles on and off there will be an associated
on and off of drag imposed. I believe there is an idle stabilizer that should keep the engine
stable as this load is turned off and on. If the
stabilizer is going bad it may not be able to
react fast enough or at all to the additional load placed on the engine. The stabilizer should
keep the idle steady by making small corrections with different engine loads. Check out this link to help troubleshoot....
<a href="http://www.nichols.nu/tip443.htm" target="_blank">http://www.nichols.nu/tip443.htm</a>
03-20-2003, 02:13 AM
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On the way home I tested the A/C functions. I noticed that when at idle and when I press the A/C switch I do not notice a drop in the dash voltmeter. Although, I hear the cooling fans running. Now when I turn on the interior A/C blower fans front/rear the voltmeter drops to below 12 volts and remains there. The voltmeter moves up slightly under increased engine RPM. I have noted on other cars that with all the accessories switched on, including the A/C blowers on full, I did not see a decrease in voltmeter readings. What is the normal voltmeter reading for the 928 when all accessories are on full, including the headlights and A/C blower units on? I thought the voltmeter would register a momentary bump slightly below 14 volts, but it would rebound almost instantaneously back to near the 14 volt reading, not at the sub 12 volt reading I am getting presently. BTW, the engine idle seems to be spot on and responds well to demand.
03-20-2003, 05:44 AM
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James,
My voltmeter also drops when I start a heavy el-consumer - E.g. when I stop waiting for green light and the turning signal is on, the volt-needle jumps up and down accordantly.
My voltmeter also drops when I start a heavy el-consumer - E.g. when I stop waiting for green light and the turning signal is on, the volt-needle jumps up and down accordantly.
03-20-2003, 12:24 PM
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The cooling fans draw power from leads that attach directly to the battery on the S4 and later cars. The likely problem is a combination of a wimpy battery and the fact that the alternator has lower capabilities at idle than it does at higher RPM's.
For other loads that feed from the fuse panel connections at the front of the car, the alternator is feeding the loads through a shorter wire and the lead resistance is less of a concern. With the fans, power flows from the alternator to the battery connection at the back, then forward again through the dedicated power feeds to the fans. This doesn't explain the drop at the feeds closer to the alternator, just talking through how all this stuff plays together.
So start by measuring the output voltage at the jumpstart terminal with a real voltmeter, at idle with the fans running and without, and also at higher engine speeds (maybe 2000 engine RPM) with and without fans running. At idle you'll see both the loaded and unloaded voltage, bolstered by the battery. If the battery is big enough the drop will me less than if the battery is weak. By 200o RPM, the difference should be almost nothing. This is a test of the alternator capacity, where the battery is pretty much just along for the ride.
To turn the fans on and off, look at the top of the intake manifold where the temp switch is located. With the hood switch depressed or disconnected, connect the two leads at that switch to get the fans to run-- just pull the two wires off and connect them together. If the hood switch is working, you can use that as the on-off for the fans once the two temp switch wires are connected. Keep Your Fingers Away From The Fan Blades, and recognize that the fans will run this way whether of not the key is on.
For other loads that feed from the fuse panel connections at the front of the car, the alternator is feeding the loads through a shorter wire and the lead resistance is less of a concern. With the fans, power flows from the alternator to the battery connection at the back, then forward again through the dedicated power feeds to the fans. This doesn't explain the drop at the feeds closer to the alternator, just talking through how all this stuff plays together.
So start by measuring the output voltage at the jumpstart terminal with a real voltmeter, at idle with the fans running and without, and also at higher engine speeds (maybe 2000 engine RPM) with and without fans running. At idle you'll see both the loaded and unloaded voltage, bolstered by the battery. If the battery is big enough the drop will me less than if the battery is weak. By 200o RPM, the difference should be almost nothing. This is a test of the alternator capacity, where the battery is pretty much just along for the ride.
To turn the fans on and off, look at the top of the intake manifold where the temp switch is located. With the hood switch depressed or disconnected, connect the two leads at that switch to get the fans to run-- just pull the two wires off and connect them together. If the hood switch is working, you can use that as the on-off for the fans once the two temp switch wires are connected. Keep Your Fingers Away From The Fan Blades, and recognize that the fans will run this way whether of not the key is on.
03-20-2003, 12:46 PM
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A fully-charged battery should be 12.2 - 12.4 volts.
The alternator should be putting 14.2 - 14.6 volts on the system.
If you have a heavy drain - say the electric cooling fans - the voltage should NOT drop a significant amount. I'm sure that many 928 owners will immediately respond , "But mine does!".
Yep, yours may well drop. Look at it this way - Say that you have 14 volts showing on the dash voltmeter, and you turn the A/C on so that the fans start. You immediately see a voltage drop to 12 vdc on the dash voltmeter. Turn the A/C off - the voltage immediately jumps back to 14 dc.
Do you really think that you pulled the system voltage down? Try hooking a voltmeter to the battery and turn the A/C on - there will be very little voltage drop.
The voltage drops that you are seeing are primarily due to the cumulative effect of voltage drops at each of several connections between the battery and the voltmeter. If you cleaned every connection on the car, you would see the same thing that you see on other cars - very little voltage drop.
In other words, with little current draw, there will be little voltage drop accross a dirty connection. If you start pulling high amperage thru that connection, you will see a voltage drop.
If you use a sensitive voltmeter, you can measure the voltage drop accross each of those connections. For example, use a sensitive voltmeter with one lead in good connection with the negative battery post, and the other lead in good connection with the chassis, and have someone operate the starter. If the meter shows more than 0.05 volts drop, the ground cable or its connection need attention.
The alternator should be putting 14.2 - 14.6 volts on the system.
If you have a heavy drain - say the electric cooling fans - the voltage should NOT drop a significant amount. I'm sure that many 928 owners will immediately respond , "But mine does!".
Yep, yours may well drop. Look at it this way - Say that you have 14 volts showing on the dash voltmeter, and you turn the A/C on so that the fans start. You immediately see a voltage drop to 12 vdc on the dash voltmeter. Turn the A/C off - the voltage immediately jumps back to 14 dc.
Do you really think that you pulled the system voltage down? Try hooking a voltmeter to the battery and turn the A/C on - there will be very little voltage drop.
The voltage drops that you are seeing are primarily due to the cumulative effect of voltage drops at each of several connections between the battery and the voltmeter. If you cleaned every connection on the car, you would see the same thing that you see on other cars - very little voltage drop.
In other words, with little current draw, there will be little voltage drop accross a dirty connection. If you start pulling high amperage thru that connection, you will see a voltage drop.
If you use a sensitive voltmeter, you can measure the voltage drop accross each of those connections. For example, use a sensitive voltmeter with one lead in good connection with the negative battery post, and the other lead in good connection with the chassis, and have someone operate the starter. If the meter shows more than 0.05 volts drop, the ground cable or its connection need attention.
