Dash lights dim to dark with lights & radio on with low RPM's (1 Viewer)

[cvan1971man]

What could be the culprit for this.

If the lights are not on, this doesnt happen. As soon as I turn the lights and radio on with low RPM's the dash lights dim to off.

Bad alternator or ???

 

[igotta40]

Do the lights go back to normal if the RPM increase?

 

[cvan1971man]

Do the lights go back to normal if the RPM increase?

Yes

 

[brian]

Totally normal

 

[cvan1971man]

Totally normal

No, the dash lights should not fade to 100% dark when the headlights and radio are on!

 

[middlecalf]

Voltmeter

 

[cvan1971man]

Voltmeter

Sorry, what should I check with the voltmeter? The alternator?

 

[middlecalf]

Yeah, and the terminal connections at the battery and fuse box to see if you’re getting a big drop from one of those sinks - battery, lights, radio, …. It’s normal for the voltage output of the alternator to drop as rpms drop as @brian alludes to, the question is are there other sinks that are drawing more than normal.

 

[rlong]

Also take voltage readings, how low does it go?

 

[pb4ugo]

Clean your connections, along with the light plug and grounds. The dash light reohostat could be corroded. You can try to spin the light knob back and forth several times to knock some of the corrosion off, or remove the switch and clean it up.

 

[igotta40]

Yes

The reason I asked- I have a similar (not new) issue, but it’s the turn signals instead of the dash lights. At low rpm, with the headlights, heater blower, wipers etc running, the turn signals don’t blink. If I give it a little gas, it’s fine. I’m guessing the voltage drop is related to 51 year old wiring, and possibly the connections, as previously mentioned.

 

[mattressking]

What is 'low rpm' and what is voltage at idle.

Then take a measurement of voltage at 1500rpms.

Quite simply, at idle you want 13v minimum, but these trucks have low amperage alternators so yes, they do dim a little at the ~650-700rpm range. Increasing engine speed starts charging and adding voltage back to system.

We can't diagnose without empirical data from your actual truck.

 

[peesalot]

The flashers get corroded , I opened mine up, they are easy to clean and just snap back together. CLEAN the connections at the terminals on fuse block and both sides of fuse, It has been my exp that this symptom is from normal build up, clean all connections. If stock its easier, if it has been buggered up any butt splice or other could have corroded and is high resistance. check the main power at starter and back of ameter, at ign switch, & at fuse main in. Check fusible link as well, if its stiff and crusty it may be all gray& green inside. If you are in a humid climate the corrosion is accelerated.

 

[Living in the Past]

What is 'low rpm' and what is voltage at idle.

Then take a measurement of voltage at 1500rpms.

Quite simply, at idle you want 13v minimum, but these trucks have low amperage alternators so yes, they do dim a little at the ~650-700rpm range. Increasing engine speed starts charging and adding voltage back to system.

We can't diagnose without empirical data from your actual truck.

Agree on needing specs. User name has 1971 but doesn't a 71 FJ40. Toyota an AM radio that bolted to the bottom of the dash. Most audio today is newer and draws much more power. Also later light switches have a dash light dimmer. I ran a 78 light switch in my 68 for yeears. It had a dash light dimmer. The dimmer could be corroded helping cause the issue. I know my 68 I bought in 1974 would dim the headlights and slow the blower motor at idle. Half a century Flagstaff where I lived at the time would get really cold in the winter. The 40 amp alternator couldn't keep up at idle.

 

[cvan1971man]

Agree on needing specs. User name has 1971 but doesn't a 71 FJ40. Toyota an AM radio that bolted to the bottom of the dash. Most audio today is newer and draws much more power. Also later light switches have a dash light dimmer. I ran a 78 light switch in my 68 for yeears. It had a dash light dimmer. The dimmer could be corroded helping cause the issue. I know my 68 I bought in 1974 would dim the headlights and slow the blower motor at idle. Half a century Flagstaff where I lived at the time would get really cold in the winter. The 40 amp alternator couldn't keep up at idle.

Born in 1971

FJ is a late 1973, so it is a 1974 FJ