My brake light suddenly decided to stay on with brake off, took two alternators to have tested and this is what they showed. Does this look right any ideas? I tested alts on vehicle with multimeter and they seemed low at 12.2V (1980 Fj40)
Alternator troubles....again 😞
- Thread starter Buffchief
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Not all voltmeters read the same, you should use two meters to confirm readings - if they are more or less the same, then good chance the meters are good. Engine rpm determines alternator output - if it is too low then output will be low. Battery tells alternator how much power it wants - a fully charged battery will tell the alternator it doesn't want much power, a depleted/discharged battery will tell the alternator to send power, but if the rpms are to low the alternator might not be able to send power. Then there are the other loads to consider like head lights, blower motors for heater/air conditioning that can suck up all the power the alternator can make leaving nothing for charging or even depleting the battery.
The test say the alternators are good. I believe them.
The brake light switch on the brake pedal has/had an issue. It can be bad (shorted internally) or out of adjustment - the brake lights remained on. Depending on the bulbs power draw and the storage capacity of the battery, it could go dead over night to a few days. You run a lead acid battery dead about 5 times it will be toast.
The test say the alternators are good. I believe them.
The brake light switch on the brake pedal has/had an issue. It can be bad (shorted internally) or out of adjustment - the brake lights remained on. Depending on the bulbs power draw and the storage capacity of the battery, it could go dead over night to a few days. You run a lead acid battery dead about 5 times it will be toast.
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- #3
I tested it at idle and at 1500rpms with all lights etc ... Off. Normally the brake light dims just a bit when I release the brake handle and then start, a couple seconds later the light goes out as the ALT provided power. It just suddenly started staying on after brake release and running. What does diode current leakage mean on the one sheet? Other doesn't show that
Diodes are like a one way valve for electricity. Nothing is perfect, current leakage is the back flow of electricity threw the diode,
Are you talking about the brake light on the dash or the ones by the rear bumper. The dash one is controlled by the hand brake and the sensors on the master cylinder.
Are you talking about the brake light on the dash or the ones by the rear bumper. The dash one is controlled by the hand brake and the sensors on the master cylinder.
An external voltage regulator is adjustable - procedure is in the manuals. When asking for help year/model/market helps.
14.4 is the top end of charging voltage and 13 is the low end IMHO.
Load test the battery. Make sure all the connection are clean and tight.
14.4 is the top end of charging voltage and 13 is the low end IMHO.
Load test the battery. Make sure all the connection are clean and tight.
- Thread starter
- #7
The one on the dash that is next to the seatbelt light, same housing
- Thread starter
- #8
I did put 1980 fj40 in the original question. Maybe it should have been at the beginning .
Sorry I didn't see it for some reason.
- Thread starter
- #10
No worries! I've been known to put information in the wrong spot, wrong page or not at all!
That late of unit should have an internal voltage regulator - they might be adjustable by a build/bench tech.
So corrosion causes resistance which is a voltage drop. This corrosion can happen at any connection. It can crawl under the insulation at crimp connectors. You have look carefully to detect that. Your wiring is 45 years old and likely has corrosion issues. Take every connector apart, inspect and clean it, apply a light coat of dielectric grease applied with a q-tip will help keep them clean. Good plan to disconnect the battery ground while cleaning connectors.
Resistance also causes heat - heat causes more resistance and can lead to a run away - that's why fuses melt - on a good day they melt first and protect the circuit/load from letting out the magic smoke. On a bad day the $500 chip burns out to protect the nickel fuse.
Coolerman's wiring diagram and parts
So corrosion causes resistance which is a voltage drop. This corrosion can happen at any connection. It can crawl under the insulation at crimp connectors. You have look carefully to detect that. Your wiring is 45 years old and likely has corrosion issues. Take every connector apart, inspect and clean it, apply a light coat of dielectric grease applied with a q-tip will help keep them clean. Good plan to disconnect the battery ground while cleaning connectors.
Resistance also causes heat - heat causes more resistance and can lead to a run away - that's why fuses melt - on a good day they melt first and protect the circuit/load from letting out the magic smoke. On a bad day the $500 chip burns out to protect the nickel fuse.
Coolerman's wiring diagram and parts
- Thread starter
- #12
Had the alternator tested on vehicle @2500 rpm lights on and off. Says it's fine. Still have that dang light on the dash after releasing brake. Guess I'll start checking wires somewhere!
This stinks.....was heading to Colorado this Thursday for some SOLO camping! Guess I'll have to take the Tacoma
Look at the wiring diagram and see what all connect to that light bulb. I'd look for a switch on the parking brake lever and pressure sensors on the bottom of the brake master cylinder. Could be some sort of relay in the actual parking brake assembly.
How’s the alternator bracket to engine block connection? Starter to frame ground wire? I actually run an extra battery ground wire straight to the alternator bracket. It’s extra insurance for the starter/frame ground wire and ensures the alternator sees a good ground.
I’m not positive that a weak ground would cause your issues, but I’d check to make sure your alternator bracket is seeing a good ground since that’s easy enough.
I’m not positive that a weak ground would cause your issues, but I’d check to make sure your alternator bracket is seeing a good ground since that’s easy enough.
I agree with you hobbes. I put on 2/0 battery cables red + to starter and - black right to engine block. I took the old 2 black cable and ran from that bolt on the engine block down to the frame ground under the brake line tee. From that tee I ran a #10 black ground wire to fire wall. She cranks over much better with the nice fat wires
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- #16
So it looks to be fixed with everyone's advice. Added another ground wire from Alternator to battery and for whatever reason it's testing at 13.4 to 14.2 depending on rpm's and what is on!! Next my brake light for hand brake on dash. To the switch apart and found the brass ring around the plunger was corroded and the two contact "ears" where spread wider than the should be. Cleaned and reshaped and ops check good!
Awesome fix!