1970s 1973 Eldorado convertible issue

[JerRita]

Hi My 1973 Eldorado convertible the gas gauge won't work and the GEN light stays on. We checked and the alternator is charging well. I had the same issue a few years ago and an old mechanic fixed it but he's gone now. Any idea what it could be. PS the problem returned after we replaced the seat and blew a fuse in the process. The seat is working fine now. Thanks for the help JeRita

[TJ Hopland]

I would have to pull out the diagram to see what specifically would have to be out but something is still blown and things are back feeding through other things. There are a lot of things that can intermingle related to the seats especially when it comes to the buzzer.

I think I know where my diagrams are so if no one else gets to it I should be able to look tonight.

[Big Fins]

If I remember correctly, doesn't one of the diodes in the diode trio control the dash light?

["Cadillac Kid" Greg Surfas 15364]

Did you check the fuses?
Greg Surfas

[JerRita]

Yes Greg we did and they all seem good Last time I thought he said it was a link or breaker that was bad But unfortunately the old timer is gone 

[TJ Hopland]

My diagrams were not where I thought they were so I can't look and find specific details yet.  One thing you can try is unplug the small 2 pin plug on the alternator.  What happens?  Is the light still on?  Does it still output over 13v with the engine running?  Light should be out and you should be just at battery voltage of less than 12.6, possibly below 12 because you just started the engine.  IF the light is still on something is grounding that wire between that connector and the gen bulb.  If the alternator still works its had an internal failure and will likely kill the battery overnight.


The gen light is an interesting bit of the circuit.  One side of the gen bulb is connected to I belive the gauge fuse which is a key on circuit.  The other side of the gen bulb is connected to the closest to the output stud on the alternator.  When the alternator isn't turning this terminal appears as a ground so key on one side of the bulb gets system + voltage from the fuse and the the other side is a ground so bulb lights.

Just spinning this style of alternator doesn't make it create output.  For it to do that it has to 'see' voltage on that pin.  The bulb just acts as a resistor so most of the voltage from the key/fuse gets to the alternator so it turns on can creates voltage.  When everything is working the alternator side of the bulb is at system voltage and so is the key/fuse side so bulb doesn't light.

For the bulb to light with the engine running that means there is an imbalance between the two sides.  It could be the system/battery voltage is higher than the alternators output but it could also mean the alternator is higher than the system.  It could not 'start' that way but lots of things kinda fluctuate during cranking while the starter drawing the whole system down.

Since seat work and electrical issues were mentioned plus a similar past 'repair' there are a lot of things that could be happening.  Its possible that the previous repair was more of a patch than a repair.  Something like a fusible link had failed but wasn't properly diagnosed and instead a jumper was installed somewhere to back feed the failed section from the working section.  We have seen this a few times here on this forum.  73 was a year there is a connector in the harness down to the starter that contains the 3 fusible links / main power feeds.  Over time and starter replacements this connector can easily have issues.

The seats is also an area that gets messy can easily confuse people.  73 has pressure sensors in the seats and sensors in the buckles to sound the buzzer if someone is seated but the belt is not buckled.  That in itself isn't too complicated where it gets messy is at the buzzer. 

Single buzzer is for the seat belts, key in ignition, and engine over temp.  If and when those thing buzz depends on what position the gear shift is in and if the door is open or closed.  No problem for a body control module, that just takes a few lines of programming but to make all that work with no electronics it can be a bit of a mind bender. 

If no one messes with anything you don't really have to understand it but people often messed with this system because they didn't use the seat belts.  You can't just pull the buzzer out.  I don't remember what quits but they apparently intentionally connected something you wanted to keep that quit if you removed the buzzer so people that didn't know what they were doing did all sorts of creative things to stop the buzzer with varying degrees of success and reliability. 

Some of the things in that buzzer circuit work to ground while others are on the + side.  Some things are always on and others are only with key on so you can start to imagine is is one place where a connection where it should not be can get all sorts of things crisscrossed and back feeding.  You need the diagram to start tracing and isolating things till you find the problem.           
         

[scottsdaleaz]

Sounded like an interesting challenge so i traced it out.



is it like this:  When the ignition is switched on, but the engine (and thus the alternator) is not running, the battery's 12-volt supply flows through the warning light and through the resistor and at that tap to the alternator. At this point, the alternator is not producing any power, so the circuit is completed back to the battery's ground through the resistor , allowing the lamp to light up.

When the engine starts and the alternator begins to generate power, it produces a voltage that closely matches the battery's voltage. This means the voltage on both sides of the lamp is nearly equal, leading to a drop in current flow through the lamp, causing it to go out. Essentially, the light goes off because there is no longer a significant voltage difference across it.


oh.. now i also see the Fuel Gauge and all the Tell Tale Lamps getting hot from ...  the 5amp Gauges /trans fuse..  for the bank of lamps Coolant, Oil Pressure, Gen, Trunk and Low Brake..  possibly missing that when you add fuel gauge symptom.
The hot side of the Tell tale Bulbs goes to the 5 Amp Gauges/Trans fuse.   

So wherever that joint is.. that dot on the center of the schematic..  tracing left of that dot.. that needs to produce 12v at the alternator and have a good circuit back to that dot.. or else the resistor is good enough to make the circuit for ground path so the the Gen lamp will glow (?).

I would try pull the connector at the alternator and feed the brown wire with 12v and see if the lamp goes out eliminating the wiring and blaming the alternator internals.  This is my guess - so verify before you inject 12v.

Maybe the Seat short spiked the Alternator?

But now i see the lamps  and the fuel gauge get the 12v from the 5 amp Gauges/Trans fuse.. so changing my guess on that source.

[bcroe]

The rotating field is initially energized by some
current from the GEN light, sometimes a big alt
will have a second bulb in parallel to give more
start current (like my diesel).  Once the rpms
produce output thru the main rectifiers to the
output stud, the small diode trio also produces
voltage to fully energize the field, and GEN
goes out.  Sometimes a main rect diode opens,
causing a voltage spike on that phase, which
can flow thru the diode trio and light the bulb. 
Max output also decreases. 
I would check all the diodes.  Bruce Roe