Radio cuts out when I hit brakes

[Gluteus Maximus]

74 620

Not much of a problem, but for the last week I've noticed my radio cuts out when I step on the brake pedal.  It turns back on after one second or so, even if I hold the pedal down.  Being an electrical idiot, any ideas what I can check?  The radio is an old Blaupunk that came with the truck, so I don't know how it's wired in there.  The cigarette lighter has never had power, so I'm wondering if the radio was connected to that power source.  Thanks!

 

Brett

[datzenmike]

Does the starter turn at the normal speed when starting?

 

See if it does this when you turn the headlamps on.

[Stoffregen Motorsports]

Probably a ground issue.

[banzai510(hainz)]

Does the radio turn on with the key switch off??

then maybe yes you cig wire could be used as it’s ON all the time but why when there is a Radio wire

I love the old Blaupunk am/fm radios.

 

Find theCommon Fuse and clean it

521s the fuses get wet and loose connection

most times I just move the fuse around and fixes most electrical problem even the common fuel/temp gauge /brake or running lights in rear not working.

[Gluteus Maximus]

Starter cranks over just fine.  I believe I had the running lights on but not the headlights while this was happening.  All other lights are working (except the dome light, which has never worked).  I've replaced all the fuses recently, although I'm not sure of the quality of the Harbor Freight fuses.  Radio doesn't turn on unless I have the ignition on or turned to the left.  So the only electrical things not working are the dome light, cigarette lighter, and the useless dimmer knob which doesn't seem to do a thing.  

[datzenmike]

Radios are sensitive to low voltages. Putting the brakes on, lights the tail lights and on that circuit there is a sudden but short dip in the line voltage. I was wondering if turning the headlamps on did the same thing.

[Gluteus Maximus]

Thanks...I'll try and see if turning on the headlights reproduces it.  I suppose my alternator may be weak and cause this....The idiot light doesn't come on though.  Perhaps when current alternator dies I'll put in something more potent...till then, I hope this is the worst of my problems.

[klam07]

2 hours ago, Gluteus Maximus said:

Thanks...I'll try and see if turning on the headlights reproduces it.  I suppose my alternator may be weak and cause this....The idiot light doesn't come on though.  Perhaps when current alternator dies I'll put in something more potent...till then, I hope this is the worst of my problems.

I recently retrofitted a nissan sentra 100 amp alternator in my truck. I think its the easiest upgrade thats mentioned the least, no major modification just some knucklebusting and a bad time. If you decide to go that route I can share the details

[banzai510(hainz)]

idiot light not coming ON is a issue.  I had a alternator that would blow the fuse.

 

get a volt meter and run this vehicle and get some voltage reading if lights not on. then lights at say 1500/2k rpm and on check the volsage again and record them down.

 

a no cig light power, dome light poer or buzzer power is all on the Common fuse(means it has power when all things are off and not thru the key switch)

[Gluteus Maximus]

Well I believe the radio issue was just the battery being super low.  The truck started...barely.  After driving 10 miles and stopping...dead...had to get a jump to start and made my way home.  All I have is a 3 amp "Battery Tender" smart charger I use on my motorcycles.  Battery took about 8-10 hours to fully charge, but truck starts and no more radio cut out when I step on the brake pedal.  Took some voltmeter measurements:

Truck off                 13.0 volts 

At idle                      12.68

At idle with lights   12.23

2 K rpm with lights 12.32

 

The battery is only a few months old, so it appears the alternator is kaput?  Connections and belt all look good.  I know others mod in bigger more stout alternators, but I'm not going to run any big lights or stereos...just the radio.  Best option for a plug and play alternator?  I know A-zone had new and rebuilt ones at 35 amp that are inexpensive...any brands that are superior?

 

B.

[thisismatt]

Alt needs to put out about 13.5 to properly maintain the battery.

[datzenmike]

 Is the external voltage regulator tightly grounded to the body?

 

Does the red charge light come on dimly or flicker? at idle?

 

Are the battery post clamps eroded and worn out? Are they clean and tight? Is the ground connection on the head behind the fuel pump clean and tight? the positive cable on the starter? Most important is there a good ground to the cab sheet metal?????

 

 New battery? For now assume it's OK.

 

Could be the alternator, but maybe the external regulator. Take the alternator in and have it load tested. If battery and alternator pass then all that''s left is the regulator.

 

When this truck was just 10 years old you could, for $5, buy new brushes which was about the only wearable part on them. The ball bearings lasted forever not like the cheap crap today. Try to find an original one in a '73-'77 620 in a wrecking yard.

[Gluteus Maximus]

Now that you mention it, I did notice the lid to the voltage regulator was sloppy, with one screw missing and the other loose. I tightened the remaining screw and found a poor replacement screw for the other side. The red charge light always comes on at start and goes out once it's running.   Loose voltage regulator lid cause a bad ground?  Guess I'll have to go test again!

 

[datzenmike]

Yes it will. The regulator has battery plus and has to 'sense' that compared to ground. Now make sure there is a ground wire from the negative  terminal to the body.... or a wire from any part of the engine will also do. I have a 10 gauge wire from the engine bracket bolt to the block to one of the engine mount bolts on the body. Sorry on a truck the frame won't do, try a screw on the inner fender.

 

The frame is insulated from the body with rubber mounts, engine/transmission mounts are rubber, leaf springs have rubber so do shocks. 

[Gluteus Maximus]

Just an update.  I checked all the connections and grounds and was still getting around 12.3 volts.  An alternator shop near me had a rebuilt for $38!  Once in, it was generating 16+ volts to the battery, so I needed a new voltage regulator too!  Once that was in, I now have a steady 14.3 so I guess I'm good to go for now!  Thanks for all the help!