Battling voltage drops when switching on lights... Options

[jcwells]

Looking for some advice or solutions to fix what I think is a result of a voltage drop issue. As you can see in my sig, I've got a '73 2.0 running SDI EFI. Had to replace the fuel pump a couple years ago, switched to a two-port NAPA unit that Clay Perrine recommended as a suitable replacement for stock.

Car runs perfectly when headlight switch is fully off, with the exception of a little bit of idle balkiness that requires me to add the slightest bit of pressure to the throttle to keep it running at stoplights. That behavior goes away as the car gets thoroughly warmed up. However, when I switch on the parking lights, I'll start to get some more pronounced stumbles as idle and low rpm. Headlights switched on result in a lot more problems at idle, low rpm, and even some stumbling at anything under 3000 rpm. I have a pressure gauge mounted in the fuel rail, but no sensor, so can only read it when standing outside the car looking into the engine bay.

With the car on jackstands, I hooked leads to both sides of the fuel pump, to measure voltage drop across the pump directly. I see that I get between 0.5 and 1.0 volts drop (from 12.3-12.5v with lights off to 11.5-11.8v with lights on. When I try to get it to idle with lights on, voltage drops below 11v, car stumbles badly, and when trying to recover I'll sometimes get a very exciting backfire for my troubles.

I've run independent grounds from every lighting ground point to a common ground bus bar connected directly to the relay board ground point, hoping that would solve the problem. No joy so far. Volt meter in the car is hooked up in the traditional, stock manner, and shows the typical response of bouncing when turn signals are on, and showing 10v when headlights are on. It never reads above 12v.

This problem has gotten worse over the years, to the point where I cannot drive the vehicle at night, since it won't reliably stay running unless I keep the revs above 2000. So, I guess I'm looking for some advice that would point me towards a way to get away from the voltage drops happening when the lights are switched on. Would running separate power leads and relays straight to the headlight/parking light assemblies do the trick?

[ConeDodger]

You have joined Resistance...

[Spoke]

Voltage drop of 1 or 2 volts shouldn't be catastrophic. It's not good but not totally bad. The wiring used in the 914 isn't the greatest and any oxidation on connections will make it worse.

I would suggest doing more analysis of your electrical system with lights on and off. Start at the battery and measure its voltage with car off, car on with and without lights. Then measure voltage at the fuse panel. Also measure the voltage from the + connector on the coil to ground.

With engine running, the battery should be at about 14V. At rest, the battery should read 12.6V.

Also clean all grounds, especially grounds from the battery and the transmission ground strap.

You wrote:
I've run independent grounds from every lighting ground point to a common ground bus bar connected directly to the relay board ground point, hoping that would solve the problem.

Not sure what you mean here. Do you mean you've grounded the headlights back in the engine compartment? If you're going to make a ground wire for each component, ground them as close to the component as possible with as heavy wire as needed.

[jcwells]

<snip>Not sure what you mean here. Do you mean you've grounded the headlights back in the engine compartment? If you're going to make a ground wire for each component, ground them as close to the component as possible with as heavy wire as needed.

Thanks for the response! In an effort to get around 45 year old ground wiring, I've run individual 12 ga. ground wires from each of the front headlight buckets (stock chassis ground studs by headlight cover motors in the front trunk) back to a ground buss bar that I installed inside the cabin. That ground buss bar is connected directly with twinned 12 ga. wiring back into the engine bay, terminated on the chassis ground stub by the relay board. I've run another 12 ga. ground wire from the fuel pump directly to this same ground buss. I have yet to run ground leads from the rear trunk ground points for the tail lights, since I was hoping to see some improvement from what I've already done.

[Mike Bellis]

How old are the battery cables? they have a life limit and corrosion can happen under the insulation. I once saw a car where the mechanic changed the battery, starter and alternator only to find the car would still not start and run. The actual problem was bad battery cables.

All the different ground wires run around can cause ground loops and noise on the electrical system. Excessive noise will effect the radio and sensitive electronics.

I would upgrade the negative battery cable. One to the chassis and one to the engine case or transmission. The chassis should still conduct better than 20 different wires.

You may also have a bad battery or charging system.

[914Sixer]

Also if you upgraded starter, make sure you go to a larger gauge custom cable.

[TheWeatherMan]

Replace all your lights with LEDs. Solved ALL my problems

The LED headlights will change your life and voltage issues

https://www.superbrightleds.com/vehicle/197...1&year=1973

[danschy]

(Been lurking on this forum a while due to my love of 914's, although I currently do not own one.) I don't think it is wise to start replacing random things - this issue should be pretty straight forward to diagnose. As mentioned, you should be getting around 14V across battery with car running. Not sure how many amps the alternator has, but if you turn the lights on you should still see around 14V across the battery - the load from the lights should be easily handled by the alternator. If not, you need to check the voltage right at the alternator. If it is 14V there (but not at battery) then you have loss between the two which needs to be addressed by better cables, removing loss due to corrosion, etc. If you are not getting around 14V at the alternator then it is likely bad. Alternators can lose a diode and still output some current, but less than rated.

[Bartlett 914]

(Been lurking on this forum a while due to my love of 914's, although I currently do not own one.) I don't think it is wise to start replacing random things - this issue should be pretty straight forward to diagnose. As mentioned, you should be getting around 14V across battery with car running. Not sure how many amps the alternator has, but if you turn the lights on you should still see around 14V across the battery - the load from the lights should be easily handled by the alternator. If not, you need to check the voltage right at the alternator. If it is 14V there (but not at battery) then you have loss between the two which needs to be addressed by better cables, removing loss due to corrosion, etc. If you are not getting around 14V at the alternator then it is likely bad. Alternators can lose a diode and still output some current, but less than rated.

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