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RPMs drop when braking and/or when under electric load

I have a 93 Nissan 240sx that exhibits weird electrical behavior. I’ve had the battery and alternator checked at two local Advance Auto Parts locations with results indicating no problems.

The car starts and drives fine. Immediately after start, my automatic seatbelts are engaged and the RPMs drop to about 400 and the car shakes violently (while pressing brakes and even when not pressing brakes). RPMs drop anytime accessories are in use. When braking at a stop light, the RPMs drop and the car shakes again. Using my headlights causes the RPMs to drop but not a point where I feel shaking. The problem is worst at night when I am pulling up to my apartment complex with lights on, braking, and using my power windows to access my gate card reader. I don’t know if this problem is related, but when I am listening to the radio and turn the volume all the way up, the radio keeps cuts on and off. I have a suspicion it’s the grounds, but before I start tracing all my wires. I’d like to hear everyone’s input. I determined it was the brake booster vacuum because it alsp happens when not using the brakes and pinching the line has no effect on idle.

You may very well have a problem with the idle air control valve. It is suppose to keep a steady rpm, unlike what you’ve got going on.

Thank you, db4690, for taking the time to read and answer my question! I’m sorry that I forgot to mention that I already replaced the idle air control valve and gasket last Thanksgiving. That was my first suspicion. At that time, I also replaced my vaccuum lines as well.

I noticed the last sentence in my original post has a typo, correct typing should be:

“I determined it wasn’t the brake booster vacuum because it also happens when not using the brakes and pinching the line has no effect on idle.”

Any other thoughts?

So you definitely don’t hear any kind of hissing at idle with the hood up?

You might have a PCV problem. PCV operation definitely affects idle.

How old is that battery? If it’s older than 5 years, it’s time for a new one.
Have it and the alternator checked. If your charging voltage isn’t steady, you might have all sorts of weird problems. It’s possible that the alternator “can’t keep up”

Thanks for all the good input.

No hissing when the hood is up. I can’t remember if I replaced the PCV valve two years ago, but I’ll check that. The battery is a little over 2 years old. I had it and the alternator checked twice. I’ll check it myself with a multimeter this or next weekend. The alternator was replaced 2 years ago as well. But remanufactured parts can’t be expected to last… (This has been a problem ever since I’ve had the car, just was busy fixing everything else. When I installed the new alternator, there was no change.)

Here’s what I’ll be trying when I get to working on it this or next weekend.

Check alternator output. Check wires between alternator and battery, perhaps there’s a problem in supplying enough power to the accessories because of old wires (if alternator output is good). Add a new ground from the battery to chassis and add a new ground from the engine to the chassis. Install new battery terminal clamps. I may also just get a new battery as well and use the old one on my Corolla which has a dead battery. So I’ll try a fresh battery on this as well to narrow it down.

The feedback is helpful in finding things I may have overlooked. Again, thank you so much.

Try and narrow down the events to a replicable cause. Does it happen if the seat belts are not engaged? Just for kicks I would replace the brake light bulbs.

It happens whenever there is any electric load on the system (when accessories are being used) - brake lights, headlights, automatic seatbelts, power windows… I can replicate this by using either of the mentioned accessories.

There may be a faulty throttle positioner of the engine to body ground is rusted or missing. Toyota used to attach them over thick paint; it would work for a few years. Maybe Nissan had a similar setup.

Problem was resolved. Thank you all for your help.

I repaired a short caused by two exposed wires near the battery which slightly alleviated the symptoms. Then I replaced the battery cables and redid the grounds. The final action fixed the problem!

While I didn;t contribute on this one, I thank you for the follow up post. It’s good to hear good results. Congratulations on your diagnostic and repair work.

Sincere best.

@MrStutterZ Grounds just don’t seem to be what they used to be. It is great knowledge, thanks.

This is the exact same thing my 2001 Prelude has been doing, you may have just came up with the solution, now I know where to start looking. Thank you sir.