Engine Idles Down When Electronics Are Switched On

[Justin]

I have an early 90's GTD and whenever I switch on any electronics like the headlights or the fans the engine idles down and wants to die. The voltage holds true on the gauge but clearly there is an electrical problem.
Any ideas on where to start looking first to help narrow down the problem? It has a new battery and what looks like a fairly new alternator and the ground from the battery to the chassis looks good. Thanks for any input.

 

[Bill Kearley]

What size alternator are you using? Pully size? Current draw should be checked when acc. are turned on as well.

 

[JP Verweij]

Probably one of those common 35 Amp Lucas alternators you'r running. When electrics are on, there's probably more draw than you alternator can handle.
Easy to upgrade as for less then £100 you can found a 90 Amp Lucas alternator.

 

[Randy V]

I’ve had MSD ignition boxes start to fail when insufficient current is available. Likewise high pressure electric fuel pumps on EFI engines. Tell us more about your car - pics also may help..

 

[Mike Pass]

Agree with JP and Randy. Check the voltage at the ignition and the battery. The headlights or fans will each be taking about 20amps. So if the alternator can't keep then up the voltage will drop. We had an MSD box with similar symptoms and also non starting. We found that if the voltage at the box fell below 9volts it wasn't happy. We had 12volts at the battery so in our case better cabling did the trick.
Cheers
Mike

 

[Ian Clark]

Hi Justin,

I'd like to add to all of the above a couple more things to consider. Engine tune and battery grounds. A well tuned small block Ford has a strong idle a 650rpm with a stock cam, might need 800-850 with a cam upgrade. Handling a normal headlight or fan load shouldn't stall the motor, although I've been there.

So have look at your spark plugs, if there black and sooty or almost white there carburetor or ignition issues to solve first. Pop in a new set of plugs if yours are not in great shape. Timing or ignition system parts are suspect here.

The alternator most likely has an internal regulator the will not start charging the battery until a minimum rpm has been met, depending on the pulley ratios probably in the 1000-2000 rpm range. So the battery is getting no help at idle if the voltage regulator hasn't joined the party.

The best battery ground is directly to the engine block along with the usual chassis ground. Both have to in good condition, check for corrosion, frayed wires and loose connections. The 12v power output lead from the alternator also has to be in good shape and adequate gauge for its' length.

In the old days (pre EFI) carbureted engines had idle up solenoids to help maintain the normal idle speed (and charging) at high pumping or electrical loads such as A/C, lights and fans.

That said, turning on the a/c, headlights and fans usually will pull the idle not more than 100 rpm if all else is in good shape. A higher base RPM target may be required...

To add to Randys' tell us more about your car, what's changed since the last time it idled well?

Cheers
Ian

 

[Justin]

Fellas,
Thank you for all the replies this was all extremely helpful. I actually recently acquired the car a little over a month ago and bought it from a fellow member on here. Car had been sitting for awhile so I am in the process of going through the car while making some changes and updates. There is definitely still a lot of work to do but over all it is in good shape.

As I am going through the car I have recently replaced both fuel pumps, plugs, wires and all of the fuel lines. The car did struggle to idle initially but the idle was set to low so I raised the idle to about 750rpms and now it idles much better.

The carburetors definitely need to be rebuilt and one of them is spray a large amont of fuel in even at idle. All of the old plugs were super black when I changed them so once I work through a few of these other issues I plan on taking the car to Olthoff Racing to have them go through the carbs.

 

[Mike Pass]

As you have changed the fuel pumps you may need to check the fuel pressure. Weber carbs need no more than 3psi or the pressure will push the float needles open. This will result in fuel overflowing leading to the black plugs you describe.
Once the fuel pressure is correct the next thing to check is the float level on all 4 carbs. This is easy to do using a 10mm drill as a feeler gauge. These two steps may be all that is needed.
Use a piece of tubing to listen to the idle airflow to balance the carbs or use an air flow meter. Check the idle screws are all set the same by screwing in and counting the turns to close and then open again so they are all the same. That will get you pretty near until you can get the car on a rolling road to do the fine tuning.
However Weber IDAs are easy to strip and rebuild if needed. Rebuild and setting info can be found on line.
Cheers
Mike

 

[Mark Boyers]

You would probably be well advised to change the routing on the two wires rubbing on the pole at the back of the alternator also could be a future problem,car looks nice