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Juan Crespo
Juan Crespo, Tech Trainer
Category: Ford
Satisfied Customers: 1524
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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I have a ford mustang gt. 89. I'm only getting 5 volts tops

Customer Question

I have a ford mustang gt. 89. I'm only getting 5 volts tops at the coil when measured across the 2 terminals that go to the coil but if I use the battery ground I get 14 with the engine running. it hunts and will stall after the volts get under 2. replaced the distributor, coil, tps and iac sensor.
Submitted: 10 months ago.
Category: Ford
Customer: replied 10 months ago.
if I jump a ground to the neg. side of the coil it will stall like right now as well.. really noticed stalling and voltage drop when the ac or head lights are on. this problem seems to show up after running it for 15 min or so
Expert:  Juan Crespo replied 10 months ago.

Hi there.

Let me see if I have it right: the engine will start and run; then stalls when the A/C or head lamps are turned on. Is that correct?

If yes, is your battery showing more than 13 volts at idle with no accessories on? Does voltage drop after 15 minutes?

Customer: replied 10 months ago.
batter is 14 with all the stuff on and if I go pos. on the coil to neg on the battery I get 13/14
Customer: replied 10 months ago.
but pos. to neg on the coil I get 5 tops
Expert:  Juan Crespo replied 10 months ago.

OK. Let's not worry about voltage at the coil - if there is 14 volts at the positive side with the engine running, then we're good to go.

Let's put our attention on the battery. Does voltage go down once the A/C and the headlamps are turned on? Does it affect the brightness of the headlamps or the speed of the A/C blower?

Customer: replied 10 months ago.
doesn't seem to... nothing dims or slows down... I just need to know were the coil gets its positive and ground from
Customer: replied 10 months ago.
I jumped from the battery pos and neg to the coil and it wouldn't start
Expert:  Juan Crespo replied 10 months ago.

The coil gets steady positive voltage from the ignition switch. Ground to it is applied intermittently by the onboard computer to match engine firing order - if you were to touch the coil negative terminal with a grounded LED test light, you would see it flash rapidly, similar to how the injectors are pulsed open.

Would it help you if I uploaded the factory wiring diagram?

Expert:  Juan Crespo replied 10 months ago.

Please do not ground the coil negative terminal. Doing so will burn the driver inside the onboard computer.

Expert:  Juan Crespo replied 10 months ago.

I'm sorry, but seeing that you disregarded my advice to leave the coil alone, I have no option but to opt out of this question. Please do not reply until another expert responds or the site will cycle it back to me.