"Bouncing" Tach + Other Issues
 

Hey everyone –
I’ve found a few threads that contain some similarities to the issues I’m having, but none that are quite the same.

Here are some symptoms I’m trying to untangle, and I can use your help in figuring out where to start, what is related/unrelated, etc.:

1. “Bouncing” tach – has bounced back and forth between 1k RPM and 5k RPM without engine speed changing. Has done this 2x so far, that I’ve noticed, both times while the car is stopped/idle.
2. Bouncing tach STOPS when I turn off the headlights – resumes when I turn the headlights back on.
3. Headlights dim/get brighter with engine speed
4. Running a bit rougher – especially at low engine speed
5. Battery idiot light on dash illuminates at low engine speed while the above symptoms happen.
6. Parking brake light on dash came on a few weeks ago and stayed on until I cleaned the bulb contacts. (not sure if this is related?)

I haven’t had time to check battery voltage at idle and 2,500 RPM, but car off the voltage is: 12.8V. Also, the battery is coming up on 11 years old.

I appreciate any tips on where to start diagnosing!

Sounds like classic symptom of alternator going south

Bill

I agree with Bill that your symptoms sound like alternator going out. I'd also check positive and grounds at alternator, and ground on the nose of the transaxle.

Make darn sure to disconnect your battery before removal ( and installation ) of alternator. Easy to forget


Bill

This is a mistake I've done exactly one time 20 years ago, and its now burned into my mind, haha.

Follow up question - I also need to figure out if I have an internal/external voltage regulator.

Any POV on replacing both given my symptoms? Headlights dimming seems to relate to voltage regulation, at least to my very tired dad brain...

IMHO I'd put in a new tech alternator like the one from Classic Retrofit or the one from Powerlite.

I'd check the cheap/easy stuff first (the wires). Your voltage regulator can't keep up with the voltage demand/changes if there isn't enough capacity for the current from the alternator, so it *might* be the voltage regulator, or it could just as well be that the wires are broken or compromised in some way that makes it so the voltage regulator can't do its job properly.

Good points, and easy enough to check over. Thanks!

I had the same issues. pulled my alternator out and all was seemingly ok. Finally realized the bulb wasnt coming on at startup. The bulb was falling apart and connections corroded.

My money's on the regulator.

Change it out before the battery starts spitting acid.

I'm sorry - which bulb are you talking about? Taillight/headlight?

I was out of town for a few days and didn't have a chance to do much before last night.

That said, I have the below numbers pulled, with a bit more info below. Let me know your thoughts!

Car off, at battery terminal: 13.01V (note: since I'd been gone for a few days I had the battery on the tender prior to the below)
At Idle: 13.58V
Engine At 2500 RPM: 13.83V

I was having my wife help me by watching the multimeter while I started/held idle - and she mentioned at 2500 RPM, the meter climbed and bounced until it hit 13.83V and stopped climbing as I increased throttle. We checked this a few times by increasing the throttle to 3,000-ish and the numbers never climbed higher.

I'm not sure I've ever seen that one happen.

It's supposed to do that. The voltage regulator drives the output to a particular value, 13.83 in your case. Most early alternators flatten out between 14 and 14.5, but a little lower is easier on the battery. Alternators in newer cars actually change their output to be higher voltage right after you start (to recharge the starting load from the battery) then go down into the mid to upper 13-volt range as the battery is recharged.

The alternator charge light in the gauge. Its what excites the alternator. It wont charge without it. If it turns on with the key on and goes out when its started then it is working right but the connection could still be corroded and intermittent

I cant believe I didnt think about that...thanks for the reminder/explanation.

Ah, yes - mine turns on with key/off with start, but will plan to pull it and check all that too. Thank you!