1979 620 Charge Light / Instruments Issue

[Mitchell]

Hi,

 

I've had the following happen twice. All instruments, tach, fuel gauge, turn signals (try to flash but don't, charge light dimmed), parking brake light will stop working and the Charge light will come on, but the plugin lighter outlet meter shows 13.7 to 14.2, however the hazard flashers work, truck runs and starts normal, turn signals seemed like not enough power because the charge light dims. The first time it happened, I plugged and unplugged all the relays and alternator plug as well as the round connector and all came back on and worked fine for about 2 months.

The second time, this didn't work. What did work was while the truck was running, removing the positive terminal five consecutive times while running each time with different results until it seemed to 'clear up' on its own. The first time I removed the terminal the truck stopped running. The second time it remained running but the charge light was still on. The third time the parking brake light came on, tach and gas gauge all came on and the charge light didn't, but as soon as I used the turn signal the charge light came back on. Repeated these steps one more time and all was well and remains working fine, even after multiple restarts, very strange....! Driven daily, and just trying to avoid a possible future tow bill....

[datzenmike]

Intermittent problems are almost impossible to sort out. Could be anything.

 

The 6 fuse box will unplug and it can be removed. Keep track of the fuses, even get new ones. Fill a suitable sized bowl with lemon juice and salt mixture. Wash the fuse box with soapy water and rinse. Submerge over night in the juice and rinse and dry. Replace the fuses and install.

 

Further, clean battery posts and clamps.

[thisismatt]

Before running, the charge lamp gets +12v on one leg from B+ and ground to the other leg through the alternator. Once the alternator is energized and operating, it puts +12v to the lamp instead, causing an equal potential on both sides of thr lamp, turning it off. If there is a bad connection of battery voltage to the cluster/instruments, the charge lamp may see a change in potential as other components in the cluster area try to use the power coming through the bulb, causing the bulb to illuminate, even though the charge circuit is working fine.

Edited August 27, 2023 by thisismatt

[Mitchell]

Thanks to both of you, much appreciated. Forgot to mention the Fuse Block looks good, not corroded, no prior overheated melted connections and all 6 fuses were powered on both ends with parking lights on which were also tested during and after the issue had cleared up. But a cleaning is worth a try. Where would be best the way to power the instruments temporarily to test, (after removing the fuse) the next time this occurs?

After posting the above, I took a 10 mile test drive and the next time had occurred sooner than expected, the trigger for the charge light coming back on and all gauges stopping functioning was when I put my turn signal on. I stopped removed the positive terminal once and 'reset' and all began working again. Once I returned home, I tried the turn signal and no effect, charge light did not come on. Mike you're right these intermittent problems are the worst...

Edited August 27, 2023 by Mitchell
More Information

[datzenmike]

Never trust a glass fuse on looks. If there is any question throw away and replace.

 

All gauges quit? Gas and temp as well as the brake warning, charge and oil pressure are all powered from the same 10 amp fuse on the left side rear corner of the fuse box. Take a test lamp with you and next time it quits test for power on both sides of the fuse. If OK then there is a plug on the back of the fuse box that might not be making good contact. Unplug and plug back in several times to clean the contacts. The only thing left is the round 12 pin connector on the back of the gauge cluster. It's hard to get at but grip it and unplug/plug back in to clean the connections.

 

 

[Mitchell]

Thanks Mike, Will do that...

[iceman510]

Clean all your circuit grounds, especially any associated with the instrument cluster area.

[EDM620]

11 hours ago, iceman510 said:

Clean all your circuit grounds, especially any associated with the instrument cluster area.

Grounds are notorious for causing intermittant problems. Check them out, make sure you've got clean metal contact and even apply some silicone overtop after to help keep moisure out.

[datzenmike]

This is true...

 

However all the dash lights and gauges have different grounds.... temp sender on the head, gas sender in the tank, oil pressure sender on the side of the block, charge light in the alternator, brake warning on the hand brake. What they do have in common is they are powered by the same fuse.

[thisismatt]

Yes, and why the charge lamp would illuminate when things start acting up, as it's grounding through those other devices that usually would get 12v from the battery.

[datzenmike]

OOoo that's a good one. Unplug the alternator

[Mitchell]

Unplugging the alternator will be interesting, I'll definitely do that and report back to hopefully settle this, once it acts up again, I was tempted to add grounds to the instruments, but I have a full width AC evaporator attached to the dash that makes it difficult to access. Yesterday I ran everything aok, AC, Turn Signals, Fuel Gauge, Tach and Temp all working. One thing I have observed is the plug in volt meter is reading higher, closer to or at 14 more often than usual. Probably improved contact from all the plugging and unplugging. I really appreciate all of the feedback and suggestions!

[thisismatt]

Anything 13.8-14.4 is good 👍

[FrankRizzo]

On 8/27/2023 at 7:46 AM, thisismatt said:

Before running, the charge lamp gets +12v on one leg from B+ and ground to the other leg through the alternator. Once the alternator is energized and operating, it puts +12v to the lamp instead, causing an equal potential on both sides of thr lamp, turning it off. If there is a bad connection of battery voltage to the cluster/instruments, the charge lamp may see a change in potential as other components in the cluster area try to use the power coming through the bulb, causing the bulb to illuminate, even though the charge circuit is working fine.

 

It tickles the ballz exciter!

 

been wanting to say that...

 

[Mitchell]

Following up, the next occurrence while the charge light came on when running, I carefully unplugged the alternator t plug and the charge light went dim but remained on. After this, to get everything back working I repeated taking the positive battery terminal off and on while running to get everything working again. Even though the battery terminals looked clean, went ahead and gave them a fresh cleaning while I had them off and back to basics been 5 days without the issue so far. We've had an extremely humid summer might have something to do with the grounds acting up. Thanks so much for everyone's input! 

[banzai510(hainz)]

buy something like this and you can monitor before the light comes on.

98% of the time you can see the alternator not working when lights or wiper on and putting a real load on there and then prematurely start finding a solution to the proplem before you run on battery then die on side of the road

 

https://www.amazon.com/Battery-Charging-Condition-Alternator-Display/dp/B07422ZPT4/ref=sr_1_1?crid=2DWQGVSMGEGGZ&keywords=12volt+cig+lighter+alternator+battery&qid=1694178514&sprefix=12volt+cig+lighter+alternator+battery%2Caps%2C158&sr=8-1