Cluster LCD pulsing

[Stingray1967]

Hey gang,
So I was sitting at a stop light the other day and noticed that the center square LCD (the one that has the speed on it) was pulsing. It doesn't seem to be in line with the turn signal clicking. I tried taking a video of it, but the camera can't really pick it up.

Battery was fully charged up two days ago (trickle charger for 8 hours), and the battery is only 6 months old anyway.

Initial thoughts? Anything I need to worry about?

[viper107]

check charging system?

[IM28702]

My 97 Suburban does that as well, I'm positive it's the alternator in mine.

[Young1]

My 91 did the pulse. Then the alternator rotor came apart at high rpm. New alternator no more pulse.

[Stingray1967]

I did a little reading online, as it has been a bit since I have done any alternator testing on my own. Is the process still this?

1. Start car
2. Disconnect battery
3. Car should run with alternator and not die?

[32valvZ]

Originally Posted by Stingray1967

I did a little reading online, as it has been a bit since I have done any alternator testing on my own. Is the process still this?

1. Start car
2. Disconnect battery
3. Car should run with alternator and not die?

NO!
Here is a simple procedure to follow... I found this in my first search...

https://www.wikihow.com/Check-an-Alternator

[Stingray1967]

I checked the voltage across the battery prior to starting the car. I had driven it twice the day before, no issues. The voltage was 12.5. I started the car and backed it out of the garage and tested the voltage again with the car running. It was steady at 15. I'm assuming this is ok.

[32valvZ]

Originally Posted by Stingray1967

I checked the voltage across the battery prior to starting the car. I had driven it twice the day before, no issues. The voltage was 12.5. I started the car and backed it out of the garage and tested the voltage again with the car running. It was steady at 15. I'm assuming this is ok.

Sounds like its ok. Another easy way to determine if the alternator is full fielding, put on all your accessories with the car running and see if the volts increase and hold. My cluster does the same as yours, not real bad, but my alternator is brand new and my battery is a dry cell about a year old... I know my grounds are clean and tight, cause I cleaned and tightened them... My car always starts and I never have an issue... I chalk it up to age of the car at 30 years old...

[Stingray1967]

Ok thanks. I'm heading on a three hour trip with the car in a couple weeks and just wanted to make sure everything was OK.

[Paul Workman]

No secrets surrounding lead acid batteries (including AGMs). But, to your fluctuating illumination question:

Fluctuation of the dash (LCD) screen is due to fluctuating current which can be detected indirectly by monitoring voltage.

Some facts about (car) batteries:

Each of the 6 cells at rest and fully charged produce 2.1 volts and are series connected to produce 12.6 volts at rest when fully charged.

A 12 volt car battery that measures 12.1 volts at rest is fully discharged, far as working to capacity goes. NOTE: A 12 (car) battery with a shorted cell will read ~ 10.5 volts, even after having been charged sufficiently to normally charge the battery fully. However, a battery can read much lower too if allowed to become severely discharged, e.g., leaving the headlights on over night. A battery discharged (I've seen some as low as 1 or 2 volts after the headlights ran the battery down over night. The battery's capacity to be restored to full (current load)

With the engine running and the headlights OFF, normal charging voltage is 14.6 volts, when the engine bay is cold, i.e., engine is first started and will recede to approx 13.8 after the engine bay heats up and the battery capacity if fully recovered after starting the engine is complete.

The capacity of a battery can only be done once it is fully charged and then the voltage measured after a specific time while a specific load has been applied to the battery. (Even a fully discharged "dead" battery will read 12 volts!)

So! with all that in mind, onto the fluctuating LCD (dash) light...

• If the LCD fluctuations are do not seem to be in coincidence with the turn signals, i.e., they're on a different circuit, that would suggest to me that the circuit for the dash is experiencing voltage fluctuations that are related to a poor connection, either a source or a ground path connection affecting only the (LCD) dash circuit.
  
• Just to eliminate the source for that circuit, I'd want to shine up the fusible link connections (behind the battery) first, and coat them with a film of dielectric grease against future corrosion/oxidation (these are old cars...).
  
• Then, I'd want to service the ground buss connections, paying particular attention to the dash ground connections. Again, dielectric grease.
  
• If the problem isn't fixed by now (and there are no voltage fluctuations at the battery terminals), pulling the dash cluster out and servicing the bulk connector on the back of the cluster assembly. (Mine were "frosted" with oxidation.) But, a quick wire brushing and scraping with a knife blade and (again) dielectric grease was the solution to my random fluctuation issue.
  
• If all that fails, then the cluster board might need to be pulled apart and the ground points shined up. But, too, before going that far, also pay attention to the LCD board connector.

Chances are 99%(?) that you'll have fixed the fluctuation issue before getting this far! Beyond that and it is starting to look like maybe a component failure (the LCD board itself?) may be in question. That being the case, then perhaps BANTEE.com may be your solution. (And, if you haven't already replaced the tach voltage divider board, having them also do that while they're in there my be well worth the few bucks to kill two birds with one stone!)

Good Luck!

P.