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Discussion Starter · #1 ·
Hello, iv run into a small problem and am in search of outside help. Iv got an 89' 7Mgte, 5spd, targa car and recently, my battery light, "taillight out" light and E-brake light have been fluctuating on and off at different points in the rpm range. Found the alternator was weak so i just replaced it. Battery is only a few months old. After replacing the alternator, i started the car and all was well till i gave it a little gas to pull out of the garage. Then all 3 came back on and will only turn off a few times for a split second before all 3 light back up and are on steady. Does anyone know what would cause all 3 to be connected like that and all turn on and off at the same time? Ik its not voltage drop bc i have LED front signals and they work fine with and without the car running. Doesnt seem to be a big problem but its just bugging me. Any help big or small is greatly appreciated. (Pictures for reference)
Speedometer Motor vehicle Vehicle Tachometer Gauge

 

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92 TURBO
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^ Have you read your battery voltage with engine running? You should see 13.5- 14volts depending on the state of charge of your battery. The reason those 3 lights are on is because they are interconnected by diodes on the printed circuit on the rear of your combo meter ( gauge cluster) and "bleed" over thru the diodes when you're not charging.
Check your voltage and check your 15 amp engine fuse which is for your alternator charging. It's located on driver's side kick panel behind the fuse box access door. Should say ENGINE and be a 15 blue 15 amp fuse.

Check out these 2 pics, 1st one shows the 3 lights you have on and how they are connected by diodes. Charge light in pic is the red Battery light. Second pic shows the 15 amp fuse you need to check. If blown there will be no charging:

See how Charge which is red Battery, brake and rear lights are connected in this pic: When all 3 are lite, You are Not charging properly. It usually means your alt. is toast or that 15 amp fuse is blown.

Rectangle Slope Font Parallel Schematic


The 15 amp Engine fuse that if blown will prevent the alternator from charging.

Rectangle Font Schematic Slope Parallel
 

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Discussion Starter · #3 ·
I haven't checked voltage after replacing alternator, is brand new and just put it on today (8/5/22). Checked the fuse and its good. I swapped the front signal lights to LED. When my old alternator was getting weak, they wouldn't work properly unless i kept it above a certain rpm. With the new alternator installed, They're working properly without any voltage drop and car starts quick after a second or 2 of cranking. Since the LEDs are working properly, set tells me the alternator's charging properly and my engine fuse is not blown
 

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I haven't checked voltage after replacing alternator, is brand new and just put it on today (8/5/22). Checked the fuse and its good. I swapped the front signal lights to LED. When my old alternator was getting weak, they wouldn't work properly unless i kept it above a certain rpm. With the new alternator installed, They're working properly without any voltage drop and car starts quick after a second or 2 of cranking. Since the LEDs are working properly, set tells me the alternator's charging properly and my engine fuse is not blown
^ What are you reading for voltage at your battery? Do you have a volt meter to read it?
I've seen that same thing with those 3 lights lit when the charging systen is not working correctly. Check that the 3 wires going into the rear of your alternator regulator plug to see that none broke off at the round black regulator plug. Those wires dry out from heat and time and break off the plug. It happened to me on my 92. The Y-G ( Yellow with green stripe) wire broke off flush at my alt. regulator connector plug. I made up a new 3 wire plug.
 

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I had the same problem after replacing alternator. I needed up getting it bench tested and found it to be bad. Had to get another one.
 

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Discussion Starter · #7 ·
^ What are you reading for voltage at your battery? Do you have a volt meter to read it?
I've seen that same thing with those 3 lights lit when the charging systen is not working correctly. Check that the 3 wires going into the rear of your alternator regulator plug to see that none broke off at the round black regulator plug. Those wires dry out from heat and time and break off the plug. It happened to me on my 92. The Y-G ( Yellow with green stripe) wire broke off flush at my alt. regulator connector plug. I made up a new 3 wire plug.
Connector and 3 wires are fully intact
 

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I just picked up a new one from O'Reillys yesterday and put it on today. Probably have to take it in and get it checked tomorrow
I would. I got mine from AutoZone and had it check while still in the car. T
hey said it tested fine. Then I took it to NAPA and their in car test said it was bad. Then I took it out and benched it at AutoZone and confirmed it was bad.
 

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Discussion Starter · #11 ·
I would. I got mine from AutoZone and had it check while still in the car. T
hey said it tested fine. Then I took it to NAPA and their in car test said it was bad. Then I took it out and benched it at AutoZone and confirmed it was bad.
Just had the alternator checked at O'Reillys quick before they closed and it wouldn't even register on their tester that the engine was running. So I would assume iv got a wiring issue somewhere?
 

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I would think that but the only thing that changed was me putting on the new alternator.
The rectifier assy which contains 8 diodes is inside the alternator behind the rear cover.

The rectifier has 8 diodes on it inside the alt. I circled them in red. If one or more diodes shorts out and grounds it allows a negative back feed up the Y-G wire causing the lights to light.




Rectangle Font Slope Schematic Parallel
 

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Just had the alternator checked at O'Reillys quick before they closed and it wouldn't even register on their tester that the engine was running. So I would assume iv got a wiring issue somewhere?
^Heavy black wire is connected on alt post, 3 wire regulator plug connected to back of alt., 15 amp Engine fuse good, 100 amp alt fuse on (GTE) or fuse link wire loop on the GE in engine fuse box is good, belt is on and tensioned. If all good I would assume you got a bad alternator.

When you changed out the alt, did you disconnect the battery negative post? If you didn't any chance the black alt. load wire touched engine or car body ground and you blew the 100 amp alt. fuse or fuse link?
 

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Discussion Starter · #14 ·
^Heavy black wire is connected on alt post, 3 wire regulator plug connected to back of alt., 15 amp Engine fuse good, 100 amp alt fuse on (GTE) or fuse link wire loop on the GE in engine fuse box is good, belt is on and tensioned. If all good I would assume you got a bad alternator.

When you changed out the alt, did you disconnect the battery negative post? If you didn't any chance the black alt. load wire touched engine or car body ground and you blew the 100 amp alt. fuse or fuse link?
When i changed the alternator, i disconnected the positive terminal and stuck it down between the battery and the radiator
 

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Discussion Starter · #15 ·
^Heavy black wire is connected on alt post, 3 wire regulator plug connected to back of alt., 15 amp Engine fuse good, 100 amp alt fuse on (GTE) or fuse link wire loop on the GE in engine fuse box is good, belt is on and tensioned. If all good I would assume you got a bad alternator.

When you changed out the alt, did you disconnect the battery negative post? If you didn't any chance the black alt. load wire touched engine or car body ground and you blew the 100 amp alt. fuse or fuse link?
I just took the car out and drove it for an hour. If i had a bad alt. the car would have died by now right? Its still running great with headlights on
 

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I just took the car out and drove it for an hour. If i had a bad alt. the car would have died by now right? Its still running great with headlights on
^ That's not true. You could still have a shorted or open diode on the rectifier assy and you will still charge BUT you WILL NOT get the alternator's full rated capacity and if 1 or more diodes is/are shorted you will be getting a "choppy" DC voltage out to your battery ( not good for the battery) instead of a clean DC voltage.
The diodes convert or rectify the AC voltage that the alt. makes to a DC voltage. All 8 diodes need to be good. There are 4 positive diodes & 4 negative diodes on the rectifier plate. It's in the direction they are installed. A diode can either blow open (infinity ohm readings in both directions) or short or ground out (meaning a 0 ohm readings in both directions).

The Battery light on means you most likely have a grounded negative diode allowing a negative back feed voltage back up on the Y-G wire which lights the light. The + side is already present to the battery light from the 7.5 amp gauge fuse once the ignition is on.
 

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Discussion Starter · #17 ·
^ That's not true. You could still have a shorted or open diode on the rectifier assy and you will still charge BUT you WILL NOT get the alternator's full rated capacity and if 1 or more diodes is/are shorted you will be getting a "choppy" AC voltage out to your battery ( not good for the battery) instead of a clean DC voltage.
The diodes change or rectifier the AC voltage that the alt. makes to a DC voltage. All 8 diodes need to be good. There are 4 positive diodes & 4 negative diodes on the rectifier plate. It's in the direction they are installed. A diode can either blow open (infinity ohm readings in both directions) or short out or ground out (meaning a 0 ohm readings in both directions).
The Battery light on means you most likely have a grounded negative diode allowing a negative back feed voltage back up on the Y-G wire which lights the light. The + side is already present to the battery light from the 7.5 amp gauge fuse.
When i changed the alt, i disconnected the positive terminal from the battery instead of the negative, would that have done anything?
 

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Discussion Starter · #19 ·
Nope, as long as you disconnected one side, you were OK
Ok. So the diodes that you're referring to in the dash is probably the problem. I dont think theres anything else it could be. Il double check the 3 wires in the back of the alt in the morning when i can see to make sure none are disconnected. But everything else seems fine. Both fuses good, black wire on top is connected, and belt is tight. I should just take it out of the car and have it bench tested, if it passes, then il know it has to be the diodes. Nothing else it could be. And i would assume the diodes are a pain to get to...? Only thing that has me scratching my head is (if its the diodes) how could they just short out that quickly?
 

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Ok. So the diodes that you're referring to in the dash is probably the problem. I dont think theres anything else it could be. Il double check the 3 wires in the back of the alt in the morning when i can see to make sure none are disconnected. But everything else seems fine. Both fuses good, black wire on top is connected, and belt is tight. I should just take it out of the car and have it bench tested, if it passes, then il know it has to be the diodes. Nothing else it could be. And i would assume the diodes are a pain to get to...? Only thing that has me scratching my head is (if its the diodes) how could they just short out that quickly?
^ NO, I'm referring to your diodes inside your alt. What I circled in red in post 12.

"What happens when alternator diodes fail?"


"When diodes fail, the charging output of the alternator drops. If only one or two diodes have failed, the alternator may still produce enough current to meet the vehicle's electrical needs, but it may not be enough to keep up with higher loads or keep the battery fully charged."
 
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