7020 alt light comes on above idle
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Category: Allis Chalmers
Forum Name: Farm Equipment
Forum Description: everything about Allis-Chalmers farm equipment
URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=144226
Printed Date: 27 Jul 2025 at 10:20am
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Topic: 7020 alt light comes on above idle
Posted By: Leon B MO
Subject: 7020 alt light comes on above idle
Date Posted: 07 Nov 2017 at 10:13am
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Started it the other day and noticed the alt light got brighter with higher rpm's. I chopped stalks for 3 hours with fan on, seemed to be ok. What do I look for, could it be overcharging?ThanksLeon B Mo ------------- Uncle always said "Fill the back of the shovel and the front will take care of itself". |
Replies:
Posted By: DougS
Date Posted: 07 Nov 2017 at 10:31am
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Put a voltmeter on the battery. It's overcharging if the voltage is 14.5+ volts. It isn't charging adequately if it's much below 14 volts. It seems to me your problem is a wiring issue. Check the voltage first. |
Posted By: Gerald J.
Date Posted: 07 Nov 2017 at 1:52pm
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Usually the alternator light is in the wire that starts the alternator excitation and once the alternator fires up the light goes out because it has 12 volts on both sides. Sounds like the power source for the light isn't there. Gerald J. |
Posted By: JoeO(CMO)
Date Posted: 07 Nov 2017 at 4:31pm
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have you checked the belt? ------------- |
Posted By: Steve in NJ
Date Posted: 07 Nov 2017 at 4:58pm
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Loose belt...... Check it out. ------------- 39'RC, 43'WC, 48'B, 49'G, 50'WF, 65 Big 10, 67'B-110, 75'716H, 2-620's, & a Motorhead wife |
Posted By: wekracer
Date Posted: 07 Nov 2017 at 8:42pm
| 3rd on loose belt. |
Posted By: MACK
Date Posted: 07 Nov 2017 at 9:30pm
| Light comes on above idle because of a bad diode or rectifier bridge. MACK |
Posted By: Leon B MO
Date Posted: 08 Nov 2017 at 7:43am
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Belts are fine, where would I find the diode or rectifier, inside altenator?Leon ------------- Uncle always said "Fill the back of the shovel and the front will take care of itself". |
Posted By: DougS
Date Posted: 08 Nov 2017 at 7:45am
| What kind of alternator? Is it a Neihart? It's the fact that the light gets brighter as the engine speeds up that puzzles me. Neihart alternators are a strange breed. I'm somewhat familiar with how they work and I know that conventional wisdom does not apply. |
Posted By: Leon B MO
Date Posted: 08 Nov 2017 at 8:02am
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Don't know off hand the brand but the tach rpm's are measured by sensor in alternator, so not any alternator will work. Leon
------------- Uncle always said "Fill the back of the shovel and the front will take care of itself". |
Posted By: Gerald J.
Date Posted: 08 Nov 2017 at 8:10am
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All alternators generate Alternating Current voltage and then rectify that with internal rectifiers to produce DC for battery charging and DC equipment on the machine or vehicle. The tachometer can use the frequency of that alternating current voltage to read out engine speed. Not all alternators bring out the AC voltage for the tachometer. And the alternator pulley diameter has to be the same as well as the internal alternator construction has to have the same number of rotor poles to generate the same frequency proportional to engine crankshaft speed. In early alternator days there were alternators that used external rectifiers. I suspect the feed wire to the alternator lamp is not connected to the battery but has broken and has gotten connected to ground. It should be connected to the battery with the ignition switch so its disconnected when the engine is shut down. Gerald J. |
Posted By: Leon B MO
Date Posted: 08 Nov 2017 at 8:24am
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Thanks Gerald, I'll start there.Leon ------------- Uncle always said "Fill the back of the shovel and the front will take care of itself". |
Posted By: DougS
Date Posted: 08 Nov 2017 at 10:06am
| Neiharts use an internal generator and regulator to control the field of the alternator. They are a bit peculiar, and expensive. |
Posted By: CAL(KS)
Date Posted: 08 Nov 2017 at 10:12am
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arnt they Neihoff? not Neihart? ------------- Me -C,U,UC,WC,WD45,190XT,TL-12,145T,HD6G,HD16,HD20 Dad- WD, D17D, D19D, RT100A, 7020, 7080,7580, 2-8550's, 2-S77, HD15 |
Posted By: DougS
Date Posted: 08 Nov 2017 at 10:16am
| Whichever. As long as it isn't a Newhart. They cancelled him. |
Posted By: CAL(KS)
Date Posted: 08 Nov 2017 at 10:19am
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lol my wife makes me watch reruns off youtube with her ------------- Me -C,U,UC,WC,WD45,190XT,TL-12,145T,HD6G,HD16,HD20 Dad- WD, D17D, D19D, RT100A, 7020, 7080,7580, 2-8550's, 2-S77, HD15 |
Posted By: MACK
Date Posted: 08 Nov 2017 at 9:59pm
| 7020 should have a Delco. In todays world, it may be cheaper to go to local auto supply and buy a alternator than pay some one to check and repair the one you have. MACK |
Posted By: JoeO(CMO)
Date Posted: 08 Nov 2017 at 10:29pm
Don't be poking fun at Newhart..he was funny without a garbage mouth ------------- |
DougS wrote:Posted By: LeonR2013
Date Posted: 08 Nov 2017 at 10:34pm
| If you do buy another alt. get a 21SI. All the rest of them are J.U.N.K. |
Posted By: JoeO(CMO)
Date Posted: 08 Nov 2017 at 10:52pm
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Do the headlights come on ok and keep the same brightness all the way to full throttle up and stay that way? or dim until you bring it back to idle? When the lamp comes on what is happening to the alternator belt? Does it keep its tension all the time or does it start to show slack? Does it track good, consistently or want to come off? How about the alternator pulley, is it true all the way in or grooved? How is the belt for wear and cracks, is it smooth and shinny, can you see the cord in the belt? The alternator takes a lot of power and the belt requires GOOD friction to drive at high speed and when they get warm the tend to stretch. Does the tach drive from the alternator ? does it indicate correct speed or is it dancing around like its looking for a number to land on? ------------- |
Posted By: Leon B MO
Date Posted: 09 Nov 2017 at 10:20am
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Belts are tight ang only 2 years old, the tach is not jumping, seems to follow the actual rpm's pretty close. I'll put a volt meter on it first if it's not over 14.5 or so I'll look into a bad wire at alternator light. I can test to see if it's charging by turning off the battery cut off since it has electric solenoid at injection pump. I forgot about that. Thanks for all the suggestions. Leon B Mo ------------- Uncle always said "Fill the back of the shovel and the front will take care of itself". |
Posted By: DougS
Date Posted: 09 Nov 2017 at 11:11am
| Yikes! Do NOT run the alternator with the battery disconnected! |
Posted By: Leon B MO
Date Posted: 09 Nov 2017 at 11:46am
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If the alternator is bad and not charging, tractor will die instantly, if tractor continues to run , I'll turn battery cut off back on within 5 seconds. Will that hurt anything? Leon B Mo ------------- Uncle always said "Fill the back of the shovel and the front will take care of itself". |
Posted By: HoughMade
Date Posted: 09 Nov 2017 at 12:19pm
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This isn't a permanent magnet alternator is it? Where is the rotor going to get the electricity to be an electromagnet? ------------- 1951 B |
Posted By: Brian Jasper co. Ia
Date Posted: 09 Nov 2017 at 12:53pm
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Since Mack says it should have a Delco, repair kits can be had off the internet usually about $30. I'd say with the light on and still keeping the battery up, the diode trio is probably bad. ------------- "Any man who thinks he can be happy and prosperous by letting the government take care of him better take a closer look at the American Indian." Henry Ford |
Posted By: DougS
Date Posted: 09 Nov 2017 at 7:12pm
Why such a mammoth alternator? I'm not certain, but the SI series probably doesn't have a tap off the diode pak that would work as a tach signal. |
Posted By: LeonR2013
Date Posted: 09 Nov 2017 at 7:57pm
| Dang, I've got my diagrams down at the shop and don't think I'll get out to go get them. But that may right, so you'd be best to check that out before getting any alt. |
Posted By: Acdiesel
Date Posted: 09 Nov 2017 at 9:04pm
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THAT HAS A MECHANICAL FUEL PUMP SHUT OFF SO EVEN IF YOU UNHOOK THE BATTERY IT WILL STILL RUN. GET A VOLTMETER AND CHECK OUTPUT. DAN ------------- D19 Diesel,D17 Diesel SER.3 2-D14, 2-D15 SER.II WF/NF D15 SER.2 DIESEL D12 SER.I, D10 Ser.II 2-720'S D21 Ser. II Gmc,caterpillar I'm a pharmacist (farm assist) with a PHD (post hole digger) |
Posted By: DougS
Date Posted: 10 Nov 2017 at 5:24am
Which tells you little or nothing with the alternator not under a load. Doing this may well blow what's left of the diodes. Unloaded alternators can produce voltage spikes well in excess of 100 volts. He knows the alternator has an output. He's measuring 14.5 volts under load. It's a matter of figuring out why the light is coming on. If a single diode is bad you'd generally only see it with an oscilloscope. If the wiring is good take the alternator to a rebuilder - one that offers a warranty with his work. Of course you can purchase a rebuilt alternator, but if it has a different number of poles the tach will not be accurate. |