Got Questions? I Got Answers

[midlife]

Hi there. I've been refurbishing Mustang harnesses since 2008, and have learned quite a bit: you never can expect Ford to stick with a design for very long!

I've had most of my business dealing with 64.5-70's, and always wondered why 71-73 had so little demand for wiring work, especially since there is so little being reproduced. Poor red-headed stepchildren...never get any respect. Well, not any more. Lately, some Mustang restoration businesses have contacted me about 71-73's, and I've gained quite a bit of experience in the past year. Based upon those awful Osborne Assembly Manuals, it looked like there was little difference between the 71-73's...NOT! This is true for the underdash and main headlight harnesses.

Anyway, I also specialize in helping folks with electrical problems via forums and e-mails; I think I've pretty much seen everything there is to see in the way of electrical problems using OEM equipment. Now, if you put in aftermarket stuff, I can help, but this modern stuff of EFI, electronic ignitions, etc. can be tricky.

So...you got questions? Here's the thread or forum to ask your questions. Believe it or not, whatever problem you are having, others likely have had or will have in the future. Let's make this forum an archive of problems and their solutions.

Randy aka Midlife

 

[cobrajet99]

That sure is Good News Randy, As we all know, electrical Problems can be a Nightmare. Youre knowledge will definately be an asset to the 71-73 community

 

[sm3570]

Ooooohhh yeah! I'm sure I'll have a ton of questions for you Randy! First off...why is my temp light always on? The sending unit is good and I verified wiring. I'm leaning toward a bad ground or the circuit board being goofed up. I disconnected the wire from the sending unit and the light stayed on (if I remember right, the sending unit grounds the circuit when temp goes up, right?).

Thanks Randy!

 

[Detector]

Great to hear Randy.

I got a problem with my 1973 Mach 1 in that in drains the battery over night if I leave it(battery cableconnectedtd. I've put a meter between the cable and battery and it has a draw even with the ignition off. Seems to me when I was working on it it was the big white/yellow? wire that goes to the switch that was causing the draw. I'll check it out today when the sun comes up for more details.

It is really a pain having to unhook the battery re-hookhook it every time I drive it.

 

[midlife]

Ooooohhh yeah! I'm sure I'll have a ton of questions for you Randy! First off...why is my temp light always on? The sending unit is good and I verified wiring. I'm leaning toward a bad ground or the circuit board being goofed up. I disconnected the wire from the sending unit and the light stayed on (if I remember right, the sending unit grounds the circuit when temp goes up, right?).

Thanks Randy!

The temperature idiot light? I thought all Mustangs had temperature gauges. Anyway, there's two signal lines for any idiot light: the sending unit line and a power line, usually powered by a wire tied to the ACC line at the ignition switch. When the temp line goes low enough resistance (high temperature), the light turns on. No real grounds involved, unless you've pinched the temp wire somewhere to ground. Try unplugging the gauge harness in the engine compartment and then turning the key to ACC: you should see no light. After that test, let's go from there.

Great to hear Randy.

I got a problem with my 1973 Mach 1 in that in drains the battery over night if I leave it(battery cableconnectedtd. I've put a meter between the cable and battery and it has a draw even with the ignition off. Seems to me when I was working on it it was the big white/yellow? wire that goes to the switch that was causing the draw. I'll check it out today when the sun comes up for more details.

It is really a pain having to unhook the battery re-hook/hook it every time I drive it.

What switch are you referring to?

The best way to trouble-shoot these kinds of problems is to proceed as follows:

Disconnect negative battery cable, and use a DVM to measure current between disconnected cable and the negative battery post. If more than 10 or 20 amps, you can blow the DVM fuse, so be careful.

Now begin by removing fuses one by one, measuring the current as above. If you finally get a current signal less than 100 milliamps, the last fuse you pulled will contain the offending circuit. If no fuses are installed and you still have high current, begin by disconnecting the Voltage Regulator (test), then the alternator (test), then the various other lines on the battery side of the starter solenoid. One of these will contain the offending circuit.

Once the circuit is known, we can go from there...

 

[Detector]

I seem to have a problem with my meter. As you can see I get a 12.66v reading when in volt setting but 0.00 in AMP testing. You can also see the spark when I touch the negative cable to the post. The picture of my Solenoid shows the big white/yellow wire(just after the red battery cable) that when I disconnect stops the drain. I believe the wire schematic says this is supposed to go to my ignition switch.

I remove all fuses and still show the 12.66v between the negative cable and battery post.

Voltage regulator wiring doesn't seem right.

 

[hyena429]

Thats great new...And its good to see some one willing to help who knows what they are doing when it comes to wiring...Nothing worse than getting bad wiring advice!! lmao

 

[sm3570]

I seem to have a problem with my meter. As you can see I get a 12.66v reading when in volt setting but 0.00 in AMP testing. You can also see the spark when I touch the negative cable to the post.

When testing amperage you need to put the meter inline with the circuit; disconnect the wire, then put one lead to that wire and the other lead to where the connection point is. If you still are reading 0 amps, check the fuse in your ammeter.

 

[midlife]

I seem to have a problem with my meter. As you can see I get a 12.66v reading when in volt setting but 0.00 in AMP testing. You can also see the spark when I touch the negative cable to the post.

When testing amperage you need to put the meter inline with the circuit; disconnect the wire, then put one lead to that wire and the other lead to where the connection point is. If you still are reading 0 amps, check the fuse in your ammeter.

+1. You typically have to move the red probe on the meter to another hole to measure DC current.

The original poster's first pic seems strange to me: there's only the battery cable, the alternator cable and a smallish wire going to the starter solenoid. I also see a contact junction box to the left and below, which typically should have a wire running from the starter solenoid to it and either one or two lines attached to the other side. I'm not totally familiar with this, so I could be wrong. What doesn't look right to me is that there's only one small wire to carry battery voltage to the rest of the circuitry. Doesn't the fusible link go to the battery side of the starter solenoid?

 

[sm3570]

The temperature idiot light? I thought all Mustangs had temperature gauges. Anyway, there's two signal lines for any idiot light: the sending unit line and a power line, usually powered by a wire tied to the ACC line at the ignition switch. When the temp line goes low enough resistance (high temperature), the light turns on. No real grounds involved, unless you've pinched the temp wire somewhere to ground. Try unplugging the gauge harness in the engine compartment and then turning the key to ACC: you should see no light. After that test, let's go from there.

Good info, I have the gauge cluster with the temp/alt/oil/whatever idiot lights. Looking at the wiring diagram it looks like the sending unit is a switch, I didn't know it's a thermistor. I'll have to recheck to make sure it's good, then check the circuitry at the gauge cluster. Thanks for the pointer, Randy!

 

[droptop73]

OK Randy, I'm finishing my 5 speed swap and am to the point of getting the car running. Since the car was an auto it has wiring for a neutral safety switch. I have looked at the '73 wiring diagram and cannot find the NSS, I'm sure it'll need to be bypassed somehow.

Can you help me out here?

Thanks,

 

[midlife]

On the headlight harness, there's a bunch of plugs in a cluster: wiper, coil/oil/water/carb solenoid, a couple of single female bullets, and a 4 prong square plug with two males and two females. One of the wires is white/purple, another black/red: these are your backup light in/out lines. The two other lines should be red/blue (starter signal in/out of the NSS). These wires are usually coated with that firewall sealant stuff, so they are hard to pick out. At any rate, take the mating plug and find the mating wires, break them at a convenient point and splice them together, coupling the male and female together (be careful---they may want to have children!). That will bypass the NSS.

The remaining two wires will need to go to your tranny backup switch.

 

[71_badmach]

Hey Midlife, how are you with Chevy trucks? :-/ 86 Chevy Blazer to be exact, with power windows and door locks...

Here's my little story, the wife brought this one home, has a swapped in later model 350, a '99 Vortec with the CPSI fuel injection and GM computer. Well, they hacked up alot of the "normal" wiring pretty good. In order to get the throttle cable and transmission cables right, I decided to put on an intake and carb.

I've traced most of the wiring under hood, and inside to the fuse panel. I'm having issues with things quit working, the windows decide to quit, heater blower motor quits working. Anyway, do you have a good source for electrical manuals? I need basic stuff like what wires hook to the back of the gauge panel. There is a green connector with "AC" on it that looks like it handles gauge lights and functions.

The fuse panel appears to have maybe 2 wires that run inside from the firewall connector to the rear of the fuse holder part of the connector. Maybe none of this makes sense the way I describe it. Another option is an aftermarket Painless kit I suppose for $400 or so. Any opinions would be appreciated. And you are free to say, "sell the Chevy!"

 

[Tnfastbk]

And you are free to say, "sell the Chevy!"

SELL THE CHEVY

 

[midlife]

Sell da Chevy. I know nothing about the competition except the basics of automotive electrics. Sorry I can't help you there.

 

[71_badmach]

Thanks fellas! :dodgy:

 

[sm3570]

coupling the male and female together (be careful---they may want to have children!).

You know, that does tend to happen. Birds and bees and all that. Then the male and female need to get a bunch of cars for all those children. In this group the cars are usually some vintage of Mustang. It's a mess.

Oh, and SELL DA CHEVY!!!

 

[71_badmach]

Only one problem, wife's name on the title...

 

[droptop73]

Thank you Randy!

Maybe I'll steal the wiring from the NSS and jump in that pigtail to bypass it and run the other 2 to the back-up light switch on the trans.