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After hooking up my electric fans. All lights are brownish. I have a fairly new headlight switch, I have ground from the back of the engine block to body, I have ground from battery to block and I have ground from battery to bottom of radiator support. Is there a separate ground coming from the headlight wire???

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The 20-25A load of the fans is just one reason I don't like them. I assume you have a stock alternator? I think mine is 35A. Since you have a serpentine now, it can certainly drive a bigger alternator. There are always unintended consequences to a change. If you are driving on a rainy night, a stock car is pulling around 55A, and you just added the fans to that. The attached Excel file gives the breakdownMustang amps.xls

Any mods to the headlights? I believe the headlights have individual grounds to the core support- Mid will know without us having to lift a hood. Adding a relay like RPM says should help.

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It's merely because your fans draw so much amperage.  Like Mach1 Driver talked about, you'll likely need a higher amperage alternator.  Along with that a larger size charge wire from the alternator to the battery (starter solenoid for stock wired Mustangs).  Adding a new charge wire will be more involved if your car has the original ammeter gauge in the dash.  Using a fan relay is a better and safer method of connecting the fans.  But it won't reduce the amount of amperage they draw.  That is the downside of adding electrical components to an older car or truck.

I'm with Mach1 Driver in that this is one reason I don't use electric fans on my old cars or trucks.

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1 hour ago, 1969_Mach1 said:

 Along with that a larger size charge wire from the alternator to the battery (starter solenoid for stock wired Mustangs).  Adding a new charge wire will be more involved if your car has the original ammeter gauge in the dash.

Correct, you need to plan this out to do it right. It would be a good time to add a mega* fuse and a fuse link. I would suggest a 3G Ford style alternator (not PA Performance) of around 100A, unless you will be adding a big stereo and power windows. The thing with the ammeter is that it has to be disconnected, but you could have it converted to a functioning voltmeter and no one would ever know. The secret here is that it already is a type of voltmeter,  but not what you need.

*haha, the first time I actually spelled that maga (Make America Great Again), not mega. 

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Yes, the headlight plugs have their own ground lines, which end up attaching to the frame with a ring connector close to the voltage regulator plug.  The ring connector are also grounds for parking lights and side marker lights (69 side marker lights) and the washer plug. 

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To understand the problem completely, we need voltage measurements.  With dim lights, the voltage is dropping more than it should somewhere.  I would not automatically assume the problem is the alternator, it can be elsewhere. 

Do you have halogen lights that require a lot of current? 

You need to measure the voltage at your battery during several different times.  Warm up the car.   1. with the lights and fans off, what is the battery voltage.  2.  with the fan off and the lights on, what is the voltage.  3.  with the lights off and fan on, what is the voltage?  4, with both on, what is the voltage?  

The answers for a good operating system is the volages are all over 13.5 volts.  I would guess the answers are 1.  14.2 volts,  2. 14.0 votls, 3. 13.5 volts, and 4. 13 volts. 

If the voltage at step 4 is more than 2 volts below what it is in step 1, then you have too much load on the battery for the alternator to keep up.  I would guess this is not the case. 

If your lights were bright before and now dim, you have a problem with the charging system.  It might not be the alternator, but probably is. What alternator are you using?    

Let us know what you find. 

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I have a new one wire 140 amp alternator.

I have dual fans and each fan has its own relay. CFMs on fan is 1500 each, on startup each draws 30 amps, when spinning each draws 20 amps. 
 With the fans on it off, the lights are still dull.

Ill try grounding the light ground (Per Midlifes description) to the battery ground

I’m attending a car show this weekend and the car is already parked at the convention center. I’ll do the voltage check when I get back home.

Thanks!

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Your alternator is certainly large enough.  How large is the wire from it to the battery?  For that much current, you need at least a #8 wire. I assume this is ok.  A small wire will create a voltage drop and contribute to dim lights. 

So with the fans off and the engine running, you should easily have more than 14 volts at the battery.   Measure it, but I do not think that is the problem.  You either have a bad ground on the headlights ( as midlife suggested) or a weak connection to the lights.   I assume you do not have the headlight relays that so many of us have.  If you are using the factory wire harness and halogen headlights, the lights will be dim.  If the lights used to be brighter and you made no other changes, then you have a weak connection somewhere.  Perhaps even in the light switch.  Try turning on the lights on high beam, and disconnect the harness to one side to shut off both lights on one side.  If the other side gets brighter, than you definately have a weak connection or high resistance in the wires. 

Check these things after your show and let us know. 

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Idle 14.10-14.50

headlight on 14.5

headlight and hi beam 14.36

fan no lights 14.27

fan and headlight 13.40

fan and high beams 13.30

I have stock wire and stock headlamps

low beam are Wagner

high beams are Sylvania Halogen

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There's something odd about fan and headlights together in drawing down voltage.  Either one by itself is not bad.  Both are high amperage devices, so I suspect the alternator is running at its limit.  That's what Mach1Driver and 1969_Mach1 were hinting at above.  I concur.

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There is ample volts at the battery- install relays and things should brighten things up. You are loosing it in the harness, light switch and connectors along the way. Stock low beams are 35w/35 w and the high beam is 50w. Your halogen high beam is probably 50w?

Don't skimp on the relays, get a good brand like Bosch, and 40A like 0332019157. You want short runs from the battery to relays to lights with heavy gauge wire, something like the attached

 

 

HEADLIGHTS.pdf

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