No Battery Light, Alternator should be good

[DPDISXR4Ti]

Checked voltage across the battery terminals while running and I get 14.45V, so clearly the alternator is doing it's job. Measuring output at the alternator stud I get 14.47V, so very little difference. I actually have two of these Nissan spec Mitsubishi alternators and both perform the same.

I removed the battery light bulb in the cluster and then started the engine. I had 11.8V at the battery, so clearly it was not charging with no bulb in place.

Frankly, everything seems to be working as it should. Were it not for the dim bulb I'd be wrapped up and moving on. I don't like the idea of living with the dim bulb, especially since when it burns out my alternator would stop working.

[DPDISXR4Ti]

Got this off an old Beetle forum....

measure the voltage to a good ground on each side of the lamp. Just curious what it would be? In an ideal world, it would be 14 volts on the ring and maybe a little bit more on the tip of the bulb. If it has, say 15volts on the 61wire - the tip of the bulb - and say 10 volts or something somewhat less on the ring the 12-volt side then you probably have a bad connection on the battery cables or something.

Just did this ^^^ test and it's telling me something, I'm just not sure what.
With key in on position I get 12V on one side of the bulb and .87V on the other. Bulb is on bright.
With engine running I get 13.9V on one side and 8.8V on the other. Bulb is dim.

[cgifool]

Looking at the diagram, it looks like the "8.8v" side is connected to the "start/run bus". Seems like a bad connection there somewhere.

Looks like are multiple places you could check the voltage to maybe narrow down the bad spot, somewhere between (and including) the ignition switch and the alternator light.

Do you have the ETM for your car? If not, send me a PM with the year of your car. It's a must-have for this kind of thing.

-David

[320iAman]

Thanks for the input Randy. I've checked the integrity of the D+ wire end-to-end, right up to the bulb contact point at the back of the instrument cluster and it's showing .7 Ohms. Mind you, that was going through a couple alligator-clipped connections on my test leads. I don't think that's the problem.

I suspect I'm getting a bit of a voltage drop to my cheap cigarette lighter gauge, so I imagine I've got 14.x at the battery, but I will verify. Either way, it does show that the alternator is working and charging the battery.



I do get a full bright illumination of the battery light with key on (before start-up), and then it goes to dimly lit once running.

Disconnect the D+ line to the alternator, turn the key on to light up instrument lights and measure the voltage on the D+ line at the terminal end.

Should be around 12 Volts, ohms say the line is not broken measuring from point a to point b , it does not tell how much voltage is present, which is critical to the alternator working.

What is the voltage on this line with the key on ?

Having a dim lit alternator bulb says you have an imbalance between the battery and the alternator , the bulb is between the two and must be good for the alternator to work or charge the battery.

Randy

[DPDISXR4Ti]

Looking at the diagram, it looks like the "8.8v" side is connected to the "start/run bus". Seems like a bad connection there somewhere.

Looks like are multiple places you could check the voltage to maybe narrow down the bad spot, somewhere between (and including) the ignition switch and the alternator light.

Thanks for chiming in David. After studying the schematic some more (yes, my EVTM is well worn for this car), I also came to the possible conclusion that something in the Engine Run Relay circuit had to be the culprit. So I went to work...

Integrity of ground from relay socket pin 85 to G1005 is good - checked via resistance and potential voltage drop.
Integrity of G1005 ground to battery and an assortment of ground locations (including alternator case) is good
Integrity of Lamp D+ circuit through relay pin 86 in either direction is good
Incoming 12V+ 30-2 circuit to pin 30 on relay is perfect. Literally exactly the same 12.55V as measured at the battery.
Fuse #8 to relay pin 87 is good

Next, as advised by someone smarter than me, I confirmed that the warning light circuit is working properly by disconnecting the D+ circuit from the alternator and temporarily connected it to the main B+ alternator post. That applied +12V between the two components, resulting in 12V across the relay coil, making it switch on. And 0V across the warning light bulb, making it switch completely off. (Note, this is a "test mode" only as without an input to the D+ Lamp circuit the alternator doesn't charge. And also, with the key off, the battery light is on full bright.) In theory, this test proves the issue is with the alternator.

With everything reconnected properly, I thought of one thing I hadn't checked - output of the D+ Lamp circuit right at the alternator with the engine running. It measured 9V, essentially the same as the 8.8V I measured the other day (with the engine warmed up a bit) at the back of the cluster. Neither 8.8 or 9 is enough to shut the light out. Given that both Mitsubishi alternators I've tried have operated the same way - and one of them was a recent rebuilt unit - I'm wondering if they just both work that way? I've never heard of such a thing, but I don't know what else to believe. The OEM alternator donor is a 2006 Nissan 350Z.

[DPDISXR4Ti]

Disconnect the D+ line to the alternator, turn the key on to light up instrument lights and measure the voltage on the D+ line at the terminal end.

Should be around 12 Volts, ohms say the line is not broken measuring from point a to point b , it does not tell how much voltage is present, which is critical to the alternator working.

What is the voltage on this line with the key on ?

Having a dim lit alternator bulb says you have an imbalance between the battery and the alternator , the bulb is between the two and must be good for the alternator to work or charge the battery.

Thanks Randy. See my reply above. I didn't do your exact test though, so that's on the docket for tomorrow.

FWIW, I have tried a couple relays and a couple different bulbs with no changes at all.

[DPDISXR4Ti]

Disconnect the D+ line to the alternator, turn the key on to light up instrument lights and measure the voltage on the D+ line at the terminal end.

What is the voltage on this line with the key on ?

Battery light is dim and I get 7.8V. Same value I obtained when I probed the D+ side of the bulb in the cluster.

With the D+ circuit reconnected to the alternator, key on, engine not running, battery light on bright and I get .87V at the D+ circuit in both locations.

[320iAman]

Should be 12 volts to cancel the 12 V from alternator when the car is running-this extinguishes the alternator light in the instrument cluster. Cgi said same thing,, You can hook up a 12 volt line from from any where, where 12V is present, fuse box any battery bus line with 12Ve on it and this will shut off the indicator light, if it does not extinguish the light-- one the diode trio is giving up the goat most likely and leaking AC voltage down the line.

AWG 18, 20, 22 wire size is enough as amperage is very small--string it up-many connectors to use-connect the other end to the alternator D plus male terminal and check it out..

One more point--Bosch alternators are kinda of picky--on mine I had 12Ve on the D plus line and the alternator light was dimly lit--I hooked up an 68 Ohm resistor in series with the D plus line and it shut off the light--Bosch electrical manual calls for a 68 Ohm resistor on some of its alternators--way back when.. Resistors cut amperage down yet leave Voltage alone--12V in--12V out,, 1,2 amp in - 1/4 amp out..... Food for thought.

D plus line-----68 Ohm resistor----- D plus line with female terminal 6.3mm ---male 6.3 mm terminal-Alternator---solder the D plus line to one end of the 68 ohm resistor, solder the other end of the 68 ohm resistor to the D plus line with the female terminal end--tape the entire soldered ends or use heat shrink tubing--the 68 ohm resistor is in series as described above.

Randy

[DPDISXR4Ti]

Should be 12 volts to cancel the 12 V from alternator when the car is running-this extinguishes the alternator light in the instrument cluster. Cgi said same thing,, You can hook up a 12 volt line from from any where, where 12V is present, fuse box any battery bus line with 12Ve on it and this will shut off the indicator light, if it does not extinguish the light-- one the diode trio is giving up the goat most likely and leaking AC voltage down the line.

Not sure if you're suggesting something different than what I already did. I mentioned it above, but maybe not clearly so let me try again. I disconnected the D+ wire from the back of the alternator and connected it to a good 12V+ source; conveniently, the main B+ wire going to the back of the alternator. With things wired this way, I started the engine and the battery light goes off. I believe that confirms that everything else in that circuit is working, right?

Incidentally, the above wiring is for test only. The battery does not charge when wired this way.

[DPDISXR4Ti]

One more point--Bosch alternators are kinda of picky--on mine I had 12Ve on the D plus line and the alternator light was dimly lit--I hooked up an 68 Ohm resistor in series with the D plus line and it shut off the light--Bosch electrical manual calls for a 68 Ohm resistor on some of its alternators--way back when.. Resistors cut amperage down yet leave Voltage alone--12V in--12V out,, 1,2 amp in - 1/4 amp out..... Food for thought.

I've read this several times now and I'm not understanding why adding resistance to the D+ circuit could help to turn the warning light off. If the voltage is unaffected, how/why would that change the warning lamp from lighting (dimly or otherwise)?

On a somewhat related note, I have considered adding a resistor in parallel to the bulb. On other cars, I've seen this done as a running change from the factory. It would allow the alternator to keep charging in the event of a bulb failure.

[320iAman]

As too the first part-this shows that your not getting 12 V on the exciter wire D+, loose connection , broken strands or low voltage feed wire to the D+ wire--trace out the wires and find where your tap on the 12 Volts is coming from. As too the 2nd part a resistor contains the imbalance by back and forth reduction in amperage, where one side has more amperage it will push the other side back causing a slight ground and thus a dim light, its amperage that lights the alternator light up not voltage, voltage is pressure, amperage is electron quantity flow per time increment. a resistor has two legs and cuts amperage to specifications on either side. Diodes are one way- voltage/amp flow, resistors are two way-voltage/ equal amp reduction flow.

Randy

[DPDISXR4Ti]

Thanks for the detailed info Randy. In the mean time I did another test. I disconnected the D+ wire from the alternator and replaced it with a temporary warning light circuit by putting a bulb in-line and then directly to 12V+ at the battery. The light goes fully on when connected and then fully out once the engine is started. I measure 14.5V on either side of the bulb. The cigarette lighter voltage gauge confirms the battery is charging as does the 14.5V measured across the battery terminals. I think we've confirmed that the alternator is not the problem.

Thinking this through for a simple diagnostic mind like me, I guess I should think of the 12V+ as coming from the car down the D+ wire towards the alternator. We know the alternator can receive and properly process that signal, it just has to get it.

[DPDISXR4Ti]

After studying the schematic some more (yes, my EVTM is well worn for this car), I also came to the possible conclusion that something in the Engine Run Relay circuit had to be the culprit.

I was on the right path here ^^^ but needed to keep going. So I started thinking about the Engine Run Relay and why it even exists. Studying the schematic, it basically exists only to make sure you don't open the fuel filler door or rear hatch while the engine is running; not really a big deal as I see it. So I pulled the relay and guess what happened? Dim light in the instrument cluster turns off and now I get 14V at the D+ circuit as I should!

I still needed to get power to those two solenoids so I made a fused jumper cable to go between 30 and 87a, and now I'm in business. It certainly doesn't look factory, but it works.

In summary, there is something incompatible between the Engine Run Relay and the Nissan spec alternators (Hitachi or Mitsubishi). Fortunately the Engine Run Relay doesn't support an important function on this vehicle so it was an easy solution to just remove it from the equation.