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My other ride is a C-130J
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Any electrical savants here?
I’ve got an electrical question for the brain trust. . .
Right to the point: What is the best way to test for an internal short in a resistor? Here is a little background:I have a 48 V dc golf cart that I used to putter around our second home. Like my 911 there are always maintenance/upgrades to do on it. I was in the process of replacing battery cables when I noticed the resistor that goes across the top poles of a recently installed (new) 400 amp solenoid was very hot to the touch. Normally is is cool or maybe a little warm after a ride. The resistor was new and installed 2 months ago along with the beefy 400 amp solenoid. I tried several things to attempt to diagnose the problem, disconnecting and replacing cables, verifying I did not short anything out etc. . . Finally I replaced the hot resistor with a used extra one I had in the toolbox. After installing the old resistor the problem appeared to go away. The old resistor remained cool and after a three-quarter mile drive did not warm up appreciably. My golf cart forum has a number of gurus who suggested several possibilities. One of the possibilities was a bad capacitor in the controller can cause the resistor to heat up. Regardless of the cause, it does appear that the new resistor that was installed two months ago may have an internal short. The correct thing to do now is to monitor the resistor that is currently installed and should it begin to heat up and fail then likely the problem is a bad controller causing the resistor to fail. If the resistor that was installed and is currently working does not heat up then it is fair to assume the new resistor that was installed two months ago was faulty. Do you guys have any suggestions the best way to test the failed resistor? Here is some additional info. I checked the voltage across the top of the solenoid and got these readings: Cart Battery Pack: 51 volts Voltage across the top of the solenoid without a resistor installed: 50.0 volts  Voltage across the top of the solenoid with the new resistor: 31.9 volts  Voltage across the top of the solenoid with the old resistor: 49.9 volts  Thanks for any insight you may have.
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1975 911 Targa S 3.0 2000 911 Carrera Cab 2005 Cayenne Titanium Metallic 2022 Mercedes-Benz E450 Coupé 2020 Mercedes-Benz E350 2006 ACG Hummer Previously Owned Art from Stuttgart 2000 Boxster -1983 911 SC Cab -1984 944 N/A |
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08-08-2021, 08:00 PM
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Band.
Join Date: Dec 2003
Location: Denver, CO
Posts: 13,320
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To start please measure the resistance of each resistor and report back. Unhook at least one side of its in the circuit.
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1983 SC Coupe 1963 BMW R60/2 1972 Triumph Tiger 1995 Triumph Daytona SuperIII |
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08-08-2021, 08:19 PM
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My other ride is a C-130J
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The new (suspected internally shorted) resistor measured 467 ohms (need to decipher ohm rating)
 The old resistor measured 220 ohms (one end was removed from the circuit). This resistor is stamped 10w 220 RJ. I am assuming it is rated at 220 ohms. So this one is good. 
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1975 911 Targa S 3.0 2000 911 Carrera Cab 2005 Cayenne Titanium Metallic 2022 Mercedes-Benz E450 Coupé 2020 Mercedes-Benz E350 2006 ACG Hummer Previously Owned Art from Stuttgart 2000 Boxster -1983 911 SC Cab -1984 944 N/A Last edited by RNajarian; 08-08-2021 at 09:06 PM.. |
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08-08-2021, 08:43 PM
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Registered
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Location: North of You
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They both look to be good.
What's the voltage on the coil? 48v? The 220 ohm will draw 10 Watts, the 467 ohm will draw 5 watts. That might feel warm and is normal.
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"A machine you build yourself is a vote for a different way of life. There are things you have to earn with your hands." Last edited by 1990C4S; 08-09-2021 at 07:20 AM.. |
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08-09-2021, 07:16 AM
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Typically a 48 V dc golf cart 400 amp solenoid uses about a 470 ohm resister and a 36 volt uses around 220 ohms.
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74 Targa 3.0, 89 Carrera, 04 Cayenne Turbo http://www.pelicanparts.com/gallery/fintstone/ "The problem with socialism is that you eventually run out of other people's money" Some are born free. Some have freedom thrust upon them. Others simply surrender |
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08-09-2021, 07:48 AM
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My other ride is a C-130J
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The 467 resistor was hot, it came with my 400a solenoid. The 220 resistor is cool, it came with a 200a solenoid.
Voltage over the top of the solenoid is 50.9 volts
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1975 911 Targa S 3.0 2000 911 Carrera Cab 2005 Cayenne Titanium Metallic 2022 Mercedes-Benz E450 Coupé 2020 Mercedes-Benz E350 2006 ACG Hummer Previously Owned Art from Stuttgart 2000 Boxster -1983 911 SC Cab -1984 944 N/A |
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08-09-2021, 07:48 AM
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I think you're wasting your time...but what's the rating on the green one? Is it 5W or greater?
It's very unlikely to be failing in use, but you could measure the current to be sure. My guess is you will see 0.1 Amps... If you're worried about it, but a 467 10 Watt or 20 Watt resistor, or make a heat sink for it.
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"A machine you build yourself is a vote for a different way of life. There are things you have to earn with your hands." |
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08-09-2021, 08:14 AM
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My other ride is a C-130J
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Why is the green resistor hot to the touch and the white one is cool?
I need to decipher the markings on the green one to see what it is rated at. Thanks guys Rgn
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1975 911 Targa S 3.0 2000 911 Carrera Cab 2005 Cayenne Titanium Metallic 2022 Mercedes-Benz E450 Coupé 2020 Mercedes-Benz E350 2006 ACG Hummer Previously Owned Art from Stuttgart 2000 Boxster -1983 911 SC Cab -1984 944 N/A |
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08-09-2021, 09:18 AM
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Based on your earlier remarks...
It seems that it is likely that neither is faulty (according to your ohm meter), although the design of the 220 ohm one would allow it to dissipate heat a bit better. As far as the voltage measurements. On the battery side of the solenoid, you should measure 50V. With the resistor in place, the other side should be almost the same @49.8ish. With the resistor disconnected, this should not be the case (unless you engage the solenoid).
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74 Targa 3.0, 89 Carrera, 04 Cayenne Turbo http://www.pelicanparts.com/gallery/fintstone/ "The problem with socialism is that you eventually run out of other people's money" Some are born free. Some have freedom thrust upon them. Others simply surrender Last edited by fintstone; 08-09-2021 at 11:08 AM.. |
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08-09-2021, 09:41 AM
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The green one might be super-cheap borderline design resistor. Or it might be perfectly good, but designed to operate forever at that temperature.
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"A machine you build yourself is a vote for a different way of life. There are things you have to earn with your hands." |
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08-09-2021, 09:45 AM
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My other ride is a C-130J
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Without the resistor connected in the voltage is zero on the far side of the solenoid.
Perhaps the green resistor is designed to operate at that HOT temperature, but I’m not comfortable with that. I may leave the white one in place and monitor it to see if it fails.
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1975 911 Targa S 3.0 2000 911 Carrera Cab 2005 Cayenne Titanium Metallic 2022 Mercedes-Benz E450 Coupé 2020 Mercedes-Benz E350 2006 ACG Hummer Previously Owned Art from Stuttgart 2000 Boxster -1983 911 SC Cab -1984 944 N/A |
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08-09-2021, 10:46 AM
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That should be correct when the solenoid is not activated...so that tells you your solenoid is not stuck closed (on)...is there a chance it is stuck open (off) or has burned contacts so that the voltage runs through the resistor vice the solenoid?
Are you measuring voltage from ground to these points? Although I don't know anything abut your cart...that does not look like the case.
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74 Targa 3.0, 89 Carrera, 04 Cayenne Turbo http://www.pelicanparts.com/gallery/fintstone/ "The problem with socialism is that you eventually run out of other people's money" Some are born free. Some have freedom thrust upon them. Others simply surrender |
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08-09-2021, 11:13 AM
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My other ride is a C-130J
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Brand new solenoid. It is not stuck closed (on)
Voltage as you can see in photos are taken over the top of the solenoid posts.
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1975 911 Targa S 3.0 2000 911 Carrera Cab 2005 Cayenne Titanium Metallic 2022 Mercedes-Benz E450 Coupé 2020 Mercedes-Benz E350 2006 ACG Hummer Previously Owned Art from Stuttgart 2000 Boxster -1983 911 SC Cab -1984 944 N/A |
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08-09-2021, 11:58 AM
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Platinum Member
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Quote:
What is the power rating of the green resister??
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08-09-2021, 12:27 PM
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My other ride is a C-130J
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Quote:
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1975 911 Targa S 3.0 2000 911 Carrera Cab 2005 Cayenne Titanium Metallic 2022 Mercedes-Benz E450 Coupé 2020 Mercedes-Benz E350 2006 ACG Hummer Previously Owned Art from Stuttgart 2000 Boxster -1983 911 SC Cab -1984 944 N/A |
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08-09-2021, 01:37 PM
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UnRegistered User
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Resistance is futile...
You will be assimilated.
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Bill K. "I started out with nothin and I still got most of it left...." 83 911 SC Guards Red (now gone) And I sold a bunch of parts I hadn't installed yet. |
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08-09-2021, 03:15 PM
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You do not have permissi
Join Date: Aug 2001
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Quote:
(but there are other combos which might work better.) Ohm's Law: Ohm's Law Calculator A couple long wires with alligator clips and tape the DVOMs on the dash. See the voltage change when it gets hot.
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Meanwhile other things are still happening. Last edited by john70t; 08-09-2021 at 04:16 PM.. |
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08-09-2021, 04:05 PM
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Quote:
Yep. 
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08-09-2021, 04:11 PM
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My other ride is a C-130J
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It would appear the green resistor is 10w 470 ohms.
That is what the volt meter indicates The 10w 220 ohm resistor is still cool.  
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1975 911 Targa S 3.0 2000 911 Carrera Cab 2005 Cayenne Titanium Metallic 2022 Mercedes-Benz E450 Coupé 2020 Mercedes-Benz E350 2006 ACG Hummer Previously Owned Art from Stuttgart 2000 Boxster -1983 911 SC Cab -1984 944 N/A |
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08-09-2021, 04:41 PM
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A "shorted" resister - would measure 0 ohms. It would be a short - so no resistance - it would literally be the same as a piece of wire. Typically - resistors fail by going open - which is an infinite resistance - and prevents current from flowing.
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Mark '83 SC Targa - since 5/5/2001 '06 911 S Aerokit - from 5/2/2016 to 11/14/2018 '11 911 S w/PDK - from 7/2/2021 to ??? |
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08-09-2021, 06:30 PM
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Dee
1974 Porsche 911 3.0
Yellow 911
1983 Porsche SC
