alternator with low voltage after warmup

[jls_frog]

I replaced my alternator in December due to the voltage regulator going out in it, I bought the replacement from a local alternator rebuild place. The other day my radar detector shut off while i was sitting at a light, so i switched it over to display voltage. While driving my car was around 12.9 - 13.5 volts, but idle it would go all the way down to 11.9 volts. But as the car warmed up the voltage would slowly go down, even as i was driving. So yesterday i pulled the alternator and took it to get a replacement, the shop tested it and said it was perfectly fine, but just in case it had a problem after it was warming up they gave me another freshly rebuilt alternator that they tested also.

I installed the alternator last night and took the car around the block when i left my place it had 14.1 volts, but by the time i got home it was reading 11.5 volts. I have tested the voltage with a multimeter at the alternator terminal and it is reading the same low voltages maybe .2 volts higher then the battery. Today i checked the belt, and all the plugs on the alternator and everything seems to be fine. When i started the car this morning it had 14.3 volts again but after a few minutes of ideling it drops to 12 volts. The red wire in the alternator plug has 12v constantly to it. The charging relay is a new factory relay replaced in November. The are no dash lights on, scanned the ecu with a obd2 and no codes come up. If i turn on the lights, heater/ a/c and put a draw on the system it now drops to 10.5 volts.

Also once the voltage on the alternator drops the idle on the car goes up to 1500-1700 rpm.

any ideas?

 

[luv2rallye]

Sounds like a short somewhere - possibly in your battery. Have the battery tested.

 

[jls_frog]

The battery has been tested and is fine, its a optima yellow top. I just found out when i unplug the 4-pin connector from the alternator nothing changes the charge stays the same aprox 11.5 volts now.

 

[Lou98GSX]

maybe your idle is to low

 

[MJBean420]

I had a similar problem when I tried to have one rebuilt for more amperage. Company kept telling me it was me or my fault. Had same issue: low voltage at idle. they told me I damaged it when I installed it. I told them it was broke when I picked it up from them. Autozone, Advance Auto, and Oreillys all tested it and said it was the bearings. To make a long story short, the company was from Texas and I ended up just buying a Denso for the Galant. Installed it about a year ago and no issues what so ever.

 

[luv2rallye]

1) Is the alt relay you replaced the "Mitsubishi generator relay" next to the BCS?

2) Does the battery (charge) light come on when you initially turn the key from off to on (but not start)?

3) Does the alt white wire have near 0v or near +12v?

 

[jls_frog]

1. The relay i replaced is bolted to the top of the trans( auto), and part number was MD128752, which came up as alternator relay.

2. battery light did not come on when i just went and checked

3. will test voltage of wire in the morning

 

[luv2rallye]

If your battery light or generator relay is not working, the alt will never charge. Check if your battery light is burnt out.

 

[jls_frog]

the battery light is not burnt out, also the a/t temp light is not working, it is also not burnt out, also the white wire seems to be sitting at 0volts even when car is running

 

[ktgurl87]

I've actually seen a couple people at the store having a tough time when it cones to charging a yellow top battery because they do not charge the same as ab acid filled battery. I believe those are the "spiral gel cell" batteries and they just charge differently. Not sure how long you've had the battery, but I know the GST had a tough time with a red top, so I'm just throwing it out there.

 

[jls_frog]

was the battery just not charging? Was his alternator putting out 14v?

Also on the alternator connector the b-y wire has 11.4 volts while running, red has 12 volts, white wire .2mv

 

[ktgurl87]

We actually ended up having to buy a new battery, and then a new alternator because it had just quit. It was ridiculous.

 

[jls_frog]

Just found the white wire were it connects to the ecu at pin 33 in connector p54(from electric manual) and it also reads 0 volts there. The e-brake light works.

 

[CrawlinZX14]

I have had a very similar issue with my 91 talon. Replaced the ALT after the voltage started dropping while sitting in traffic one evening. No battery light came up at all, even when my voltage hit 10v. I then pulled over into a parking lot and shut the car down. Replaced the alt the next day. New alt, same issue. I am about at my whits end.

 

[jls_frog]

Is there a sensor for the trans temp light, I'm just trying to figure what would cause the dash lights not to work. Ok found trans temp sensor, still trying to find out why it wouldn't light up when the key is first turned on.

 

[luv2rallye]

Just found the white wire were it connects to the ecu at pin 33 in connector p54(from electric manual) and it also reads 0 volts there.

On the 2g alt, the white wire ("G signal") is a control signal from the ECU (pin 33) to the alt which forces the alt to output 12.5V maximum (feature of a 2g alt). The ECU grounds this signal when it determines it needs maximum/more hp (eg. WOT) so the alt won't use hardly any hp thereby providing more to the drivetrain.

If this is at 0v then either you have it shorted to ground or the ECU is holding it low which it should not be doing under normal conditions (eg. idle).

 

[jls_frog]

ok thanks, I'm going to try another ecu and see how that goes. Also does that mean if I disconnect the white wire from the alternator harness should the alternator put out normal voltage?

 

[luv2rallye]

Also does that mean if I disconnect the white wire from the alternator harness should the alternator put out normal voltage?

If the alt works properly and its an oem one, AND if the terminal on the alt where the white wire connects floats high which I think it should (to +12v instead of 0v - you can measure this with your voltmeter), AND there is no other problem with the charging circuit, THEN yes.

But try another ECU first since that may be the problem (and if it is, there are probably other things its not doing right also).

 

[CrawlinZX14]

Do u know anything about 1g's in that situation? I know my ECU is good, caps are recently replaced, and it has been gone over very well. It is a chipped Eprom in my case. My alt really seems to drop voltage in traffic BAD. Even with a different ALT. I have used 3 total now. all have the same effect. They charge great on start up, but as I drive the output seems to lower. Could this be an issue of heat soak from my downpipe? This is really driving me nuts.

 

[luv2rallye]

The ECU is not involved at all on a 1g alt. Your problem could very easily be a heat issue as you suspect as alts are VERY susceptible to heat. Do you have the exhaust heat shields on? Also a leaky PS fluid dripping on an alt will severly damage it.

 

[CrawlinZX14]

No heat shields, and the PS pump is not leaking, I have checked multiple times. I am thinking about making up a heat shield for the DP and seeing if that helps, or atleast wrapping it with some thermo wrap.

 

[jls_frog]

I'm still waiting on the other ecu to come in, but in the mean time put all new grounds on the car, and put a new plug on the 4pin plug for the alternator. When i measure the voltage on the 4-pin connector the green, black, and red wires all measure 12.34 volts and the white one still measures 0, that's with the key in the on position and the plug disconnected from the alternator. I also wrapped my down pipe and O-2 housing to make sure i don't cook my alternator. Just tested all the ecu ground wires with voltmeter and all have good connection(pin 13,26,92. also the alternator green wire(alternator fr terminal) is putting out 12.x volts at the ecu and the white wire (alternator g terminal) is still reading 0 at ecu.

 

[luv2rallye]

When i measure the voltage on the 4-pin connector the green, black, and red wires all measure 12.34 volts and the white one still measures 0, that's with the key in the on position and the plug disconnected from the alternator.

By black I assume you mean black with yellow stripe (or yellow marking - some cars have a blue wire instead) - this is the wire between the red and green. The red and black/yellow I can understand having 12v. The green only connects to +5v inside the ECU however so I can't understand how it could be +12v. The white wire gets pulled up by the alt but since you now have it disconnected it would float, so I could see it measuring anything, even 0v on the harness side. The white wire inside the ECU is the output of a transistor with no pullup resistor, so either the transistor is on pulling it to 0v (which it should not be) or more likely it is off allowing it to float to 0v (since it is disconnected from the alt where a pullup resistor would be).

 

[jls_frog]

Yes the black wire is the black with yellow stripe, sorry. Would you say that because the green wire has 12volts something in the ecu might not be right, any way for me to check this. Also tomorrow I will disconnect the harness plug from the ecu and see if the ecu pin at were the green wire is reads 12v.

 

[luv2rallye]

Quite possible and good idea. Also make sure ECU pins 13 and 26 have a good connection to ground (frame chassis).