Voltage Drop

[DoubleYOOHZ]

Hello Classic enthusiasts. 

My 1977 datsun 280z car is almost up and running. The problem is a little hard to describe. I replaced the battery and the car ran fine. I notice the car started to lose cranking power after 8ish car start ups. Around this time, the car would struggle to crank. I charged the battery right up and the cranks beautifully. At first, I thought my alternator is not charging my battery (which seems to be the case). I went through the "Alternator not powering battery" thread and could not assess my situation. 

I did some testings on my car with a multimeter and found some questionable data. 

From the battery,
Trial #1 
12.6v - Engine Off
13.6v - Engine Idling
11.88v - Engine Idling w/ headlights on
11.90v - Engine Idling w/ headlights on and max fan
12.90v - Engine Idling w/ lights and fans turned off 

Trial #2 
12.42v - Engine Off
13.34v - Engine Idling
12.06v - Engine Idling w/ headlights on
12.01v - Engine Idling w/ headlights on and max fan
13.36v - Engine Idling w/ lights and fans turned off 

It is weird how I got different voltage readings for when car is at idle and when car is under load.. 

I do have to bring up that about a year ago when I was working on my car, when I tried to start my car the fusible link caught fire. Weirdly enough, it was the alternator and Relay fusible link. I replaced the link and the car starts up fine but then the car battery would not be charged by the alternator. The 1977 came with a voltage regulator that is located below the fusible link box. Would there be any chance that when the fusible link caught fire, it shorted my voltage regulator? What could be the problem being that my car is not charging while under load? If I read through a series of forums correctly, I believe that the the voltage should be around 14V while idling or underload. 

[siteunseen]

My first guess would be a loose belt, not super loose but loose enough to slip when you're off idle.  I've always tightened mine until they will twist very little.  Think of the top flat part being at 12 o'clock and then twisting it to 3 o'clock.  That's what I've always done. 

Try testing it at the alternator, you may have corroded connections.

[Dave WM]

If its like mine the alt is pretty marginal, I bet your different responses were do to slighty diff RPM's. What is the RPM and do you have air cond? Agree with the belts. Did you replace the alternator? It could be losing some diodes (internal). If the fuse link to the alt smoked then there is a problem, did you find the problem or just replace the fuse link? (its possible the fuse link was bad but not likely).

Make sure all the bat/alt/starter connection are good and clean, esp under the bat terminals.

You can jump the field coil to the battery connector on the alt to bypass the VR, do not rev the engine with this, and see if the voltage at the battery comes up. If so then it would indicate the VR may not be working. Do not rev up more than about 1500 since there is NO regulation with the F connected to the B on the altenator, if its working and you rev the engine you could overvolt the system and fry your electronics.

As I mentioned before, mine is marginal at idle, with the AC on (fan on high), lights on etc... I can barely make 12.5 that is with the fast idle servo engaged (AC) pulling up to about 1k. over about 1.5k and its all fine (over 13.5v and well regulated). So the only issue would be if a sat around idling all the time with the AC on.

Yours should have the voltmeter built in (my 75 uses an amp meter), is there a charge lite and does it work properly?

 

 

Edited July 13, 2016 by Dave WM

[SteveJ]

First, I'm glad you're using a meter to diagnose the problem. You'd be amazed at the number of people who try to diagnose electrical problems without one. (Kind of like balancing the carburetors without a flowmeter.)

After following Cliff's suggestion to adjust the belt, download a copy of the FSM. The link is in my signature. Look at the EE section, and follow Nissan's procedure for testing. Let us know what the results are.

 

[Zed Head]

Looks to me like everything is working correctly.  As noted, the alternators don't put much out at idle RPM.  If your eight starts were just to get the engine idling so you could work on it, it might be that you just need to start driving it.

The burning link though, from the past, might be because of an intermittent short.  It could be the short that's your problem, not the charging system.  If you rev the engine to ~2000 RPM and charging voltage increase to ~14 volts, there won't really be anything to fix in the charging system.

[DoubleYOOHZ]

@siteunseen - From a visual inspection, the belt looks tight onto the alternator. I will give it a tug and see if it is loose of some sort. 
@Dave WM - My Idle RPM is around 1100. I know this is high because the RPM stated on the hood of the car says it should be around 800- 900 RPM. I dont know if I have air conditioner. I have fans that just blow air. I dont see an air compressor near my block.  The alternator has not been replced. When I replaced the fusible links, the car ran fine but battery was not getting charged. Some thing may be fried since my radio is not working anymore. It used to work (aftermarket radio) I thought my voltmeter should jump if I used my headlights to compensate for the energy usage? But it dipped. Are you referring the the tachometer (the three circles?) There is a red light charge before starting the car. 
@SteveJ - Will do. Your site looks informative. I will go thru this and see if I can assess the drop. 

@Zed Head - I do drive the car around the block for maybe 20 mins. But I do see a drop in battery voltage in its ability to crank the motor. I just had tested the car when at 2000 RPM and it read 12.07v. 

 

Edited July 13, 2016 by DoubleYOOHZ

[Dave WM]

it sounds like the alternator or the VR, check ALL the fuses in the fuse box (pull them out and check for continuity).

review the FSM see the part about bypassing the VR (I mentioned the F to the Batt on the alt). this will tell if its the VR or the Alt.

Report back after the F to Batt check.

If I had to guess the alt is fried (the smoked fuse link), make sure the fuse links test good as well you cant tell them by looking at them. they  can be open and look fine. Just need a few diagnostic test to know for sure, you don't want to start replacing parts unless you know they are the problem.

Edited July 13, 2016 by Dave WM

[beermanpete]

Another issue related to the belt slipping is oil on the belt from an oil leak. Make sure everything is clean and dry.

I would say it is normal for the system voltage to drop when you turn on the headlights and/or heater fan while at idle. The alternator will only output one third to one half its rating at idle and can't "keep up" with the load. When you race the engine does the system voltage rise?

[Dave WM]

you can also check the continuity of the field (F) and the Stator (Bat) with a meter if you disconnect the wires from the alt 1st.

Note the Stator (bat) is connected to the stator coil thru a diode pack so you meter will only show continuity in one direction. here I am showing how reversing the polarity of the meter will show continuity one direction and open the other. In my case the diode pack shorted out, burning the fuse link. You could also have an open diode pack that would not blow the fuse link but would keep the alt from charging.

 

[Dave WM]

2K RPM at 12.07v sounds like a problem. Fuses 1st then Fuse links, then bypass F to Batt on alt in that order. to see if any improvement. Of course the most obvious is check and clean the battery cables. Does the battery terminals feel warm? I have check the voltage on battery terminals and had 14v on the cable and 12v on the post inside the cable, and a LOT of heat at the junction.

[DoubleYOOHZ]

@Dave WM - When you are referring to the F to battery check, I am assuming you are doing an alternator test from the factory service manual. I am a little hesitant to do this for some reason. Am I just removing the F connector from the alternator and then measuring the voltage at A and E? Or do I remove the nuts from A and connector at F and put A wire to F? I dont want any blow fuses. 

I checked out all of the fuses in the box and it was okay. Dont know how to test fusible links? Measuring Voltage at the connector? 
what I am really confused on is "connecting A terminal to F terminal" part. Can you guys elaborate? 

Edited July 13, 2016 by DoubleYOOHZ

[SteveJ]

Physically remove the link and test it for continuity. They can blow internally and still look intact.