Let's talk about body grounds

[Matt Harwood]

I'm having a bit of a confusing issue with my dash lights, specifically two LED indicator lights that I've installed recently. Just ONE of my LED turn signal indicators and my newly installed fuel pump warning LED both flicker slightly when the engine is running. It's not visible in the daylight and looks much worse in the video, but it's clearly tied to engine speed. Check it:

 

 

However, the fact that my LED taillights, my instrument lights, and the RIGHT turn signal indicator do not flicker suggests something else is amiss (EDIT: now that I've watched the video, you can see that the right turn signal indicator is also flickering slightly but it's invisible to the naked eye in daylight). Even more curious is the fact that the fuel pump LED is on a toggle switch and is 100% disconnected from power when the switch is off, so the only possible way it can be getting power to flicker is through the ground wire instead.

 

This leads me to think that there's some stray current bleeding into my dashboard grounds somehow. It may also be introducing some noise into my radio, which is a little buzzy. I'm not savvy enough with my electrical engineering to quite figure out how that happens, but there has to be a way to attenuate it. Is it as easy as adding a few ground straps between the body and the frame and/or a ground strap from the instrument panel to chassis ground? If there's stray current bleeding through the ground path, would better/more grounds eliminate it? I already tried a resistor on the offending turn signal indicator and it had no effect, but then again, I tied it to the same instrument panel ground as the fuel pump indicator.

 

There's stray current somewhere and I think it's in the ground path. Would improving the grounds between the body, the frame, and the battery improve things? Any thoughts?
 

Thanks!

Edited June 18, 2020 by Matt Harwood (see edit history)

[Ben Bruce aka First Born]

Will not hurt to improve/increase grounds. In my opinion , it is definitely a ground problem.

 

  Ben

[TerryB]

The battery terminal that’s grounded is ground reference point. How it’s attached to the body/frame might be creating a high resistance ground that you are seeing in your flickering LEDs.  Incandescent bulbs mask the blinking because they are generating heat to make light. The variation in voltage is less noticeable due to the hot filament retaining heat.  LEDs are instant on, instant off, no filament to cool or warm up.  They will flicker if the on voltage dips a little.
 

Using a digital voltmeter I would measure the ground points from a point of ground right at the battery itself.  Any ground not measuring 0.000 volts would be suspect.  For digital, which is an LED is part of that family, all grounds are returned to the main ground point to eliminate electrical noise or poor grounding things like you are seeing.  Too many grounds in digital create “ground loops” that disrupt the operation of digital signals.  This is very apparent in high speed digital data circuits, not so much in LEDs as indicators.

[Bloo]

I am less sure of that.... but would still check the grounds first.

 

If the ground cable goes to the frame, another similar one needs to go to the frame from the engine/transmission. If the ground cable goes to the engine/transmission, there needs to be some sort of strap or wire to the frame.

 

In either case, the body should have a ground wire. Considering all the trouble you have been having, maybe the instrument cluster should have a dedicated ground wire too.

 

With all that out of the way, the problem might not be grounds. It takes very little voltage to light an LED. It varies with color, but is probably less than 2.5 volts (at the LED itself, not necessarily the bulb assembly) to begin lighting.

 

Old car electrical systems are very noisy. Every time the points (or electronic substitute) open, the coil unloads into the low voltage wiring as well as the secondary wiring, just at less voltage. Every time any other coil gets it's circuit broken, like the voltage regulator for instance, it also unloads into the wiring. So does the generator field. It might be sitting there buzzing as it regulates voltage. It probably is. The commutator in the generator is cutting loose coils and reconnecting more as it spins. So does the heater motor, the wiper motor (if electric) the horn, and every little relay in the car. The amount of electrical noise is unbelievable. The only filter "capacitor" in the system is the battery, and lead acid batteries make terrible filters. It is also located on wires several feet away, limiting it's effectiveness. If you are lucky there may be some little capacitors to help kill radio noise, maybe one at the generator/alternator, and one at the coil. By the 60s they were usually present. In the 30s/40s there is no guarantee they will even be present, and if present might be shot.

 

I have personally measured voltage spikes over 200 volts out in the wiring of 12 volt cars. I'm guessing about half that on some 6 volt cars.

 

There is also radiated noise from the ignition coming through the air. If you have solid core plug wires and non-resistor plugs like the old days, it could be pretty severe.

 

These spikes are effectively AC because they are pulses. You could have capacitive coupling in the wiring since all the wires run right next to each other in the harness. It could have been there all along, but due to the high current draw and slow response of incandescent bulbs you would have never seen it. Look up "glowing LED problem" on youtube to see examples of this happening in AC house wiring.

 

Anyhow I would verify the grounds first. Its much simpler. If it doesn't work, you might need some sort of resistor or capacitor in parallel at each bulb.

 

Edited June 18, 2020 by Bloo (see edit history)

[DonMicheletti]

I'm pretty sure that on your Buick, the battery is grounded to the engine, and that is on rubber mounts. So a good engine to chassis / body ground is important.

Once I had a friend whose car suddenly started overheating. he spent a lot of time finding the cause.

 

Turned out the body / chassis ground had failed and the car was using the temperature gage capilary tube for the ground connection (heating it up) and giving a false overheat condition.

[JACK M]

23 minutes ago, DonMicheletti said:

I'm pretty sure that on your Buick, the battery is grounded to the engine, and that is on rubber mounts. So a good engine to chassis / body ground is important.

Once I had a friend whose car suddenly started overheating. he spent a lot of time finding the cause.

 

Turned out the body / chassis ground had failed and the car was using the temperature gage capilary tube for the ground connection (heating it up) and giving a false overheat condition.

 

Similar story when a customer was telling me that his foot feed (gas pedal) was sparking under his foot.

[Matt Harwood]

I looked under the hood and the battery is indeed grounded to the engine. Fortunately, it's a big, healthy cable, so I don't have to worry about that and the sucker always starts strong, hot or cold, so there's a good ground path. However, I'm going to have my friends at Certified Auto Electric make me some big ground cables to go from the engine to the frame, the battery to the frame, and from the frame to the body. I'll use bolt lugs so I can bolt them in place easily, as well as connect one from the frame to the battery ground terminal. I'll add some smaller ground wires behind the dash and connect them to whatever bolt I use to ground the body to the frame. That should give everything a good, clean ground path and perhaps cut down on some noise.

 

I also ordered some radio capacitors for the generator and coil to see if I can kill some of the buzz in the radio. I don't use the radio very often, but as long as I'm chasing electrical noise, it can't hurt, right?

[Ben Bruce aka First Born]

.Typically the straight eight Buicks had a body to engine ground strap integrated into the coolant sensor from the head. It is a small webbed strap similar to the battery to engine one. Much smaller, maybe 1/2 ".  It is attached to the part of the sensor that screws into the head and the other end attaches to cowl. Only about 6" long.

 

 One way to check for "enough" ground.  A length of about  16 Ga wire long enough to reach from the Neg battery post to somewhere on the body. Turn the headlight on . While holding the wire touch one end to the battery, the other to the body.  Does the wire get hot?  Not enough ground. 

 

 What you are going to do may be overkill, Matt, but can't hurt.

 

  Ben

 

[Matt Harwood]

Ben, haven't you learned by now that my middle name is "Overkill?"

[edinmass]

Toss an alka seltzer in the top of thr radiator, and see if the light stops flashing.......if your coolant is acidic it will generate voltage and push backwards through the grounds..........you can also check for voltage from the coolant to the Battery using a DVOM and look for how many miliamps it’s reading. In mid 90’s Honda’s you can get a hunting idle due to the battery effect of the coolant acting like a battery back feeding the grounds. I use to toss in a alka seltzer when the Honda’s came in the shop with a fluctuating idle......told people their car had the sniffles...............and ten seconds later it would flatten out. Worked so good one lady I tried it on has been living  with me for 22 years. Now, I wonder about the trick and how good of an idea it was...........🤔