SR20DET voltage dropping when under electrical load

[Quiche2192]

I have an S13 SR20DET in an S14 chassis.

Electrical power has been pretty bad around idle. My idle is around 850RPM. With the car at idle, lights on, high beams, interior lights on, radio on, my battery voltage slowly just keeps going down and down at idle.

The issue does not show up when the car is not under electrical load. With lights off etc. voltage holds up just fine at around 13.8 V at idle.

I have replaced the alternator, didn't help. I have checked wiring, all good connections and good condition. Replaced the battery, didn't help.

I tested something. I had the car on, no electrical loads. Disconnected battery. The alternator can run the car just fine at idle. But when I turn on the headlights, car just dies.

I have tested for ground faults by checking current draw with the car off. 0.01 A draw.

Tested grounds, all good.

I have found sliiiiightly higher than ideal voltage drops when measuring positive alternator to positive battery and negative battery to grounds.

Would appreciate any help.

[jr_ss]

What are the discharge amps of your alternator? Assuming it’s a stock alternator, you should have 90amps at idle.

[slider2828]

I've had a similar issue of bad ground at the battery.... maybe take a pic of all the grounds you have? There should be about 5 in the engine bay.

[Quiche2192]

Originally Posted by jr_ss

What are the discharge amps of your alternator? Assuming it?s a stock alternator, you should have 90amps at idle.

Dunno I gotta measure it. I assume I can't measure it with a regular multi meter but I'll need a multimeter capable of measuring way higher currents than the usual 10A.
Stock alternators, yes.

Originally Posted by slider2828

I've had a similar issue of bad ground at the battery.... maybe take a pic of all the grounds you have? There should be about 5 in the engine bay.

Ok can check that and see. But I did measure resistance and voltage drop between grounds and batt negative and it does look like all grounds are good. BTW I have a Wiring Specialties Pro Harness.

Grounds in the engine bay are:
- One behind each headlight for the chassis harness
- One on the back of the engine head for the engine harness
- One on the intake header for the engine harness
- One for the MAF through the engine harness
- And then I have a dedicated grounding harness that grounds the chassis, alternator body, intake manifold, ground behind the headlights.

One thing I have noticed, I'll hear a whining sound coming from the speakers when the car is running and it kind of gets louder when I turn on the lights.

Right now, I'm thinking about removing that grounding harness to see if that makes a difference. IDK if I'm properly grounding the alternator. There's a bolt that screws into the body and I have a wire going from that to the chassis. I have good continuity between the alternator and the chassis. I don't have a good contact from the alternator body through the bracket to the engine, but that grounding wire should still be fine, right?

[jr_ss]

Originally Posted by Quiche2192

Dunno I gotta measure it. I assume I can't measure it with a regular multi meter but I'll need a multimeter capable of measuring way higher currents than the usual 10A.
Stock alternators, yes.

A clamp on meter can read the amperage.

Originally Posted by Quiche2192

Ok can check that and see. But I did measure resistance and voltage drop between grounds and batt negative and it does look like all grounds are good. BTW I have a Wiring Specialties Pro Harness.

Grounds in the engine bay are:
- One behind each headlight for the chassis harness
- One on the back of the engine head for the engine harness
- One on the intake header for the engine harness
- One for the MAF through the engine harness
- And then I have a dedicated grounding harness that grounds the chassis, alternator body, intake manifold, ground behind the headlights.

One thing I have noticed, I'll hear a whining sound coming from the speakers when the car is running and it kind of gets louder when I turn on the lights.

Right now, I'm thinking about removing that grounding harness to see if that makes a difference. IDK if I'm properly grounding the alternator. There's a bolt that screws into the body and I have a wire going from that to the chassis. I have good continuity between the alternator and the chassis. I don't have a good contact from the alternator body through the bracket to the engine, but that grounding wire should still be fine, right?

If you are relying on just the threads at each ground location, I would advise you to remove the connection, sand the paint until bare metal is visible and add a dab of dielectric grease. Reinstall grounds.

[Quiche2192]

Ok gonna have to get a clamp meter. I cleaned all ground spots down to shiny metal, but no dice. I'm taking it to an automotive electrician tomorrow. Fingers crossed

[slider2828]

Originally Posted by Quiche2192

Ok gonna have to get a clamp meter. I cleaned all ground spots down to shiny metal, but no dice. I'm taking it to an automotive electrician tomorrow. Fingers crossed

Quick question where is your battery grounded to?

I assume you have an injector harness ground right?

[Quiche2192]

Originally Posted by slider2828

Quick question where is your battery grounded to?

I assume you have an injector harness ground right?

Battery is in the trunk. Grounded to the chassis in two spots with I think 2AWG wire. It's pretty solid.

The engine harness is a wiring specialties one. If I recall correctly, it does have an injector ground.

The electrician couldn't really solve the problem. Just gave me some advice that I already had in mind. Check wiring (battery positive and grounds).

Problem is still here. And it has an effect on the performance of the car.

I have some things in mind, but it's like grasping at straws. Things like redoing grounds, rewiring the main battery supply. I'll give that a try and hope for the best.

[PoorMans180SX]

You don’t need all those ground wires.

Simply run a 2awg wire from battery negative to the engine block. I’m betting it solves all your problems.

The chassis is not a good ground path, and by adding all those grounding wires, you’ve created ground loops, which increase the resistance in the ground circuit.

One 2awg from battery negative to block, and one 4awg from intake manifold to chassis. The rest you can get rid of or tie back to the block.

No engine harness should have ties directly to chassis ground outside of an aftermarket four or five pin ignition coil harness. All other engine harness items should ground to the ECU, and the ECU has it’s own chassis ground. This should be tied to the engine block or the negative battery terminal.

[slider2828]

Originally Posted by Quiche2192

Battery is in the trunk. Grounded to the chassis in two spots with I think 2AWG wire. It's pretty solid.

The engine harness is a wiring specialties one. If I recall correctly, it does have an injector ground.

The electrician couldn't really solve the problem. Just gave me some advice that I already had in mind. Check wiring (battery positive and grounds).

Problem is still here. And it has an effect on the performance of the car.

I have some things in mind, but it's like grasping at straws. Things like redoing grounds, rewiring the main battery supply. I'll give that a try and hope for the best.

Move around your trunk ground for your battery, make sure you grind off paint when attached.

You say you relocated the battery, is your positive lead to the front like a 0 or 2 gauge? Is it a good quality cable?

[Quiche2192]

I still have not resolved the issue. Tried a couple other things, but I still need to clean up the ground. Planning to just take a 2AWG wire from the battery negative to the engine block like PoorMans180SX said and see if that helps.

One thing I have noticed, the 75A fusible link gets noticeably hot. IDK if anyone has any thoughts on that. The plastic has broken so the internals are open to the air. And the metal inside looks corroded. But it still works, just gets hot. For testing I did replace it with a temporary jumper, but didn't help. Maybe the gauge wasn't good enough. I'll order a replacement regardless.