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'79 Bertone has dim idiot lights when running

sisu_slay

Member
Joined
May 11, 2016
so i finally was able to drive my car home today, on the way home it developed said issue where all the idiot lights except for the oil pressure warning light would light up dim. It used to do this once in a blue moon after starting it up after sitting a few days, one blip of the throttle made them fade and stay off.

this current issue developed where they would fade on dimly when i came to a stop, occasionally and instantaneously going off, then got worse fairly quickly where it only instantaneously went from dim to off around highway speeds until the current situation where now they are on dim all the time.

i did do an aw71 swap so the engine does have a lower rpm at highway than it did before, i also had to undo the wires to pull the harness off the intake for unrelated repairs (this is a b27f), i did put them back as they were and verified snug connection. the car runs fine, none of the other lights around the car flicker, checked battery voltage when i got home standing and running, nothing seems to indicate that it's not charging or draining the battery (it is an optima yellow top though). the small red wire on the back of the alternator did have a break in the sheath but was not touching anything. i dont know if i should solder a new wire and lead, replace the regulator or the whole alternator. from what the green books tell me is that this is the 70 amp version according to how it has two wire posts.

i have looked around a little but nothing seems to be an exact match to what is happening here
 
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The most likely causes are either a bad ground to the instrument cluster or bad diode(s) in the alternator.
 
The most likely causes are either a bad ground to the instrument cluster or bad diode(s) in the alternator.

wouldn't a bad ground in the cluster cause the oil warning light to do the same thing along with the hifh beam and all the other gauges twitch? i would lean towards the diode problem then.
 
wouldn't a bad ground in the cluster cause the oil warning light to do the same thing along with the hifh beam and all the other gauges twitch? i would lean towards the diode problem then.

Try the simple thing first and give fuse 13 a spin. Any resistance at that point causes the voltage drop to the instrument panel.
 
well i tried giving fuse 13 a sandpaper massage on both contacts, it seemed to work but they did start to creep on after a prolonged stop at the end of my drive to work, it's a little more dim than before. maybe I'll try making a new exiter wire lead and see what happens.
 
well i tried giving fuse 13 a sandpaper massage on both contacts, it seemed to work but they did start to creep on after a prolonged stop at the end of my drive to work, it's a little more dim than before. maybe I'll try making a new exiter wire lead and see what happens.

Try cleaning the fuse panel contacts for 13. Get in the mind of your dental hygienist. Keep in your mind the fact sand from sandpaper is an excellent insulator. I predict the replacement of the exciter lead will be wasted effort.
 
the contacts where the fuse meets or the wire contacts for the fuse bracket? because i already sanded the box contacts for the fuse too, and this was super fine grit stuff, like meant for electrics and body wet sanding. I'll go to a file next then.
 
the contacts where the fuse meets or the wire contacts for the fuse bracket? because i already sanded the box contacts for the fuse too, and this was super fine grit stuff, like meant for electrics and body wet sanding. I'll go to a file next then.

Normally a spin will break through the microscopic insulating barrier. Try that again; it worked once, right?

You might check the spade lug tightness, but the usual problem is caused by dissimilar metal (galvanic corrosion) at the fuse ends. No sandpaper of any sort can reach into the cup and the rim of the hole which keep the fuse ends in contact. Even small wire brushes can fail to remove the "tartar" and a sharp dental pick becomes useful. Best to replace the fuse with a copper or brass one, also.

Just to prove it isn't the fuse, you could connect a short jumper wire with alligator clips between the two fuse holder contacts, bypassing the fuse and its corrosion.

Another way to prove it, if you have a meter, you can make a measurement at the contact itself looking for voltage drop across that fuse. If you do this while the idiot lights are glowing, you'll find the resistance.

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well you said think like a dentist, good excuse to use those tools. :-D

and it's already a brass fuse so got that covered
 
yeah, i tried fiddling with the fuse more, problem still persists.

now if the diode is shot, the battery isn't going to die right since it's not dying already? or cause extra issues beyond the mild annoyance? as in i will have to wait 2 more weeks for alternator funds
 
wishful thinking but they attach differently since bimmers have their alts burried and not used as a belt tensioner. I'll hit up fcp euro in a few days for one unless you know of some place cheaper that doesnt use some vatozone quality rebuild.
 
yeah, i tried fiddling with the fuse more, problem still persists.

now if the diode is shot, the battery isn't going to die right since it's not dying already? or cause extra issues beyond the mild annoyance? as in i will have to wait 2 more weeks for alternator funds

It saddens me you would spend money on an alternator without proving first it is the culprit. Oh, for the lack of a geek with a voltmeter and ten minutes of his time. :-(

Make sure the belts are tight and pulleys not glazed. Pull the regulator and look at the brushes.
 
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Brushes are cheap and easy enough to solder. No need for an alternator. Before you spend the big bucks.
 
No, the diode plate is an internal part of the alternator.

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Google "bridge rectifier". It's what turns AC into DC.
 
alternators arent really big money to me being around 200. the car runs fine, apparently some guys with bmw e28s have ran around with a similar problem for over a decade as a minor annoyance. this car has also gone a surprisingly long time without complete maintenance so if something around a problem is happening, best to just update what i can. considering the cooking it gets from the hot tarmac and the exhaust manifold on top of any oil from a valve cover seep, it's fairing ok. im going to pull the regulator when it stops raining here and see if there is any carbon to clean off.
 
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