Radio and lights cut when letting off gas pedal

[chrisd0729]

Last week I started having an issue with the radio cutting out for a few seconds when I let off the gas pedal. It comes right back on after a few seconds and works fine until next time I let off the gas. I was out a few nights ago and noticed the headlights also dim and sometimes cut out when letting off the gas.

I have a red top Optima that I'm not sure about the age as it was in the truck when I bought it 2 years back. I replaced the voltage regulator 1 year ago as it was overcharging and I haven't had any issues in the last year.

Alternator is newer with the reman engine... I assume about 3 years old.

What are thoughts on the cause or how to troubleshoot?

 

[half cab]

This may sound crazy but I once knew a guy with a '64 galaxy 500 that was having the same problem your having and he finely found what the cause was or at least it stopped when he fixed it. One if his motor mounts was broke and was causing some kinda wonky ground problem when he got on or off the gas.Don't remember which.

He fix the broken motor mount n solved his problem.

I would check all my grounds and have that alternator checked out.

 

[spap]

is the gas pedal innanyway htting wires or contacting anything underneath.
I really like the story of the engine mount, totally possible

 

[Toddpole]

If it is the engine mount that would indicate the lack of proper grounding cable between the engine/body/frame. Verify you have all these ground wires and that they are clean and tight. An engine replacement could definitely be a reason for them to be missing or loose.

 

[broncobilly72]

test voltage going to the battery when idling, should be between 13 - 14.8 volts if the alternator is working right. Shut off the engine and check battery voltage, should be above 12 volts like 12.6. if the alternator is putting out the proper volts and the battery reads low the your battery needs to be chcked out. Also if your running the old stock alternator with original wiring then you are losing power at low rpms. I had a similar problem andsolved it with an alternator upgrade...more amps.

 

[chrisd0729]

Great advice, thx!
Idling 650 rpm with lights on, I'm getting 12.8 volts at the battery.
Idling with lights off, I'm getting 14+ at the battery.
After shutting it down, I got 12.8 and it slowly dropped to 12.4 after a few minutes and stayed there.

 

[Rustytruck]

Matbe a loose belt.

 

[Boss Hugg]

I pulled the entire dash harness and engine bay harness from a 76 with similar problem. With lights on, if you hit the brakes, the engine would die. After finding the harness undamaged, we put it back in. Hooking up the battery, we found the ground cable was not connected to the body. The lights were grounding back through the coil, which was causing a loss of voltage and killing the engine. Dumbest 6 hours of my life.

 

[September 1972]

Check the throttle linkage for burn marks, the starter might be using it for it's ground. Very good call half cab!

 

[chrisd0729]

If it is the engine mount that would indicate the lack of proper grounding cable between the engine/body/frame. Verify you have all these ground wires and that they are clean and tight. An engine replacement could definitely be a reason for them to be missing or loose.

Any chance someone can list the various grounds I need to veify on this?

 

[half cab]

I generally try to ground body to frame, motor to frame and body to motor.

 

[September 1972]

Bronco ground circuit.

Negative battery terminal to engine. All ground circuits must find the negative cable at the engine block to complete their respective circuits.

For example, the instrument cluster, lights, alternator, every circuit in the bronco must find the negative battery terminal attached to the engine block. The motor mounts are rubber and if there isn't a motor mount bolt or bracket that touches the frame and motor then the alternator and starter can find the negative cable but what about all the other circuits which are grounded to the frame or body?

Those circuits will look elsewhere to find the cable. The transmission mount or exhaust system hangers or any metal attached to the engine or driveline will be the target. The metal throttle bracket is usually used if it has the lowest resistance to ground.

I used a one OT negative battery terminal and placed a 1 wire cable and a 4 wire cable inside the terminal. A liberal amount of Di Electric grease was applied to both stripped cable ends. Next my 10.000 pound press crimped the negative terminal and a heavy shrink wrap was installed. The 1 gauge was installed on the engine lug bolt with a 1 gauge cable attached to the frame. The 4 gauge was attached to a fender bolt on the threaded part and a washer and nut was used to secure it. Another 4 gauge cable was attached to a fender bolt nearest the firewall and bolted to the firewall.

Always use a lock nut and bolt to attach grounds and glob a dose of Di Electric grease on both sides of the connection.

At this time my Bronco has a ground system that is sufficient to provide a solid ground path for any circuit that uses the frame or body for it's ground. Not quite.

If a circuit uses the body for it's ground and there is corrosion between two attaching body panels it might have to start looking around and the associated resistance might cause a voltage drop. Therefore another couple of grounds from the body to the frame might be advisable.

 

[chrisd0729]

Checked every ground today and cleaned them up just for good measure. Slightly better as lights don't cut out but, they still dim when letting off the gas and brighten up when hitting the gas.

Wondering if it's the alternator and planning to go test it and pep boys. How do I tell what amp this alternator is and what's the most I can get without hurting something?

 

[September 1972]

That's a good start.

It's critically important not to do anything that might fix the problem such as wiggling the wiring or cleaning connectors until you have tested all the circuits because the worst thing that can happen is that the problem is intermittent and goes away without determining the cause and it might resurface on a road trip which would not be good.

An amp meter using a solid shunt between the negative battery terminal and cable would be a prudent first step. A video camera aimed at the amperage meter with a voice commentary as to what you are doing is the easiest method to record data.

When the lights brighten up and dim then it's easy to see what's going on.

Record the voltage across the battery and the amperage draw with everything off at idle and slowly increase to 1500 RPM on each circuit test. If the lights flicker at 1200 RPM then the amperage should drop and the voltage rise during the flicker, if the amperage increases and the voltage drops during a flicker then there may be a soft short.

Turn on lights then high beams, the high beam switch is unprotected from water and corrosion and may be an issue so look for that. Notice that the connector is in a position so that water will fall directly into it and stay there.

Turn on wipers with microfiber towel taped to the blades then the blower motor and every other devise one at a time and video record the amperage and voltage readings.

Post your video to utube and your friends here will offer opinions and possible solutions. I don't mind helping because it keeps me sharp and I'll learn something for sure once your issue/issues are solved.

Resist the temptation to do anything insofar as a fix until you get the insights from your braintrust here!

 

[Okbroncoman]

Don't let off the gas, problem solved.


Had to say it. Ha ha ha

 

[September 1972]

Good one!

Once the electrical is 100% dismantle all the connectors one at a time and clean with electronic cleaner and grease the connections and outside the connector for easy removal and recheck the electrical circuit, one at a time!

I guarantee that you will find a gremlin hiding in one or more of the connectors and after you are done the electrical reliability of your bronco will increase 10 fold.

45 to 50 years will take it's toll on connectors and there is green corrosion hiding about, don't wait until you have a problem.