I am having a problem keeping my voltage up while driving in my FD and could use some help.


A while back, my battery light came on and I saw I was under 10v on the PowerFC. Car barely made it home. The battery was old and tired but still tested ok, but I picked up another AGM anyway. I also added a beefy new ground strap between the firewall and upper intake manifold. Measuring voltage directly at the battery with a multi-meter matches the PowerFC, or maybe 0.1v higher. Voltage between the alternator and battery negative was about 0.1-0.2v higher than measured directly at the battery, which seems to be in spec.


I also followed the service manual's alternator wiring testing procedure and it passed. I did notice the B terminal on the alternator looked melted but figured it was age and engine heat.


When driving after that, I would start with 14v on the PowerFC commander, and after 15 minutes of driving I'd be down to 13-13.3v and steadily dropping.


Next I bought an ARD 150amp alternator and installed it and saw 14v on the PowerFC, 14.2 measured at the alternator. I idled for a while and the battery eventually dropped to 13.7v but the alternator was still putting out 14.1v. Took it for a short drive and then re-checked:13.9v from alternator, 13.7v at the battery, 13.5v shown on PowerFC.


At this point it at least seems to be dropping slower, but I am not sure if this is 'normal'. The only thing I can think of is that there is high resistance in the wire between the alternator and battery, which caused the melted terminal in the original alternator and may be causing the gap I saw in alternator vs. battery voltage after idling (14.1v vs 13.7/13.8v). The car has very little that should be drawing more current over stock, just a supra fuel pump and upgraded injectors.


Any thoughts?

 

What is the condition of the 2 pin connector going to the alternator

 

Seems perfectly fine to me, and i got the expected readings when I back-probed it to follow the service manual testing procedure

 

Static test results aren't always indicative of a possible fault. You have to touch and manipulate things sometimes. If it's the original wiring to that connector it could be the whole problem. The alternator circuit is extremely simple and there just aren't many possibilities for issues.

I would advise inspecting the wiring itself specifically to that connector and doing a continuity test to very integrity. Based on what you're describing and what you have already done, this is my bet as to where your problem lies.

 

I would like to add that the OEM chassis ground is terrible. It grounds to the fuse block bracket, which is then bolted to the chassis, not to mention the bracket is painted from the factory, I think the engineers were smoking crack that day. Anyways what helped me was making my own chassis ground point and buying a thick gauge cable, I used the one JP3 supplies in their battery tray kits. From the same new chassis ground point, I have a second cable going directly to the engine block.








Crappy ground from factory.

 

Alternators are designed to have higher voltage when cold and it drops when warm. Your voltages sound good. If you're 13.5 or higher when warm you are fine.

Dale

 

as Dale mentioned both the alternator will put out less as it heats up and the charge wiring will increase in resistance and cause more of a voltage drop to the battery as the car warms up. losses are part of the design and perfectly normal. you will never see the same voltage at the alternator and at the battery.


if you keep trying you will keep chasing your tail.

 

Michael Gagne makes these for us and they work great: https://jp3motorsports.com/products/...t&_ss=e&_v=1.0

 

I see you watermark your photos on your website @JP3 Motorsports , I couldn't even download the image. Had to print screen it, please don't sue me. Lol.

 

@CREEPENJEEPEN all good. We had to implement some backend protections as we were dealing with some spoof websites that were using our images. We've actually had a handful of people who emailed asking why they didnt get their order yet - come to find out they had purchased from one of these spoof sites.

 

Thanks for all the advice. After driving the car around a bit I am not seeing the voltage drop below 13.6v on the powerfc. Idling when hot, I see 13.9v at the battery and 14v coming out of the alternator, and it seems stable. I do see about 0.3v drop as the car warms up but it stabilizes.

Grounds and wiring all look good to me, though I am suspicious of the tiny ground cable going to the shock tower. I'll replace that next time I order wiring harness materials