disconnect the plugs on each terminus of the circuit

place a load such as a test light, and measure for voltage drop in the circuit, you will narrow it down, and eventually find the resistance.

do NOT use ohm resistance tests

usually pin connectors are the source of the volt drop, poor or corroded connections

you have a vert, so think water has come in the past and probably corroded a connector

 

Ok, thanks. I have inspected and cleaned (with electrical cleaner spray) all of the connections and I didn’t find any obvious corrosion. I will check the voltage with a light. I have already done the Ohm resistance tests and they didn’t really help so I see why you advised against them. I appreciate the help.

 

Today I unplugged all the sensors and checked the pins on the plugs for the harness with an led test light. All the pins on all the front sensor plugs lit up brightly. All the pins except one purple one on the plug for the rear safing sensor lit up brightly and the purple one was very dim. I spliced a bypass wire from the sensor to the module. It was pin 23 on the module plug. Still very dim. I plugged in the sensor and the spliced wire was bright. All of these tests were done with the negative battery terminal disconnected. I plugged all the sensors back in and went for a drive. I pulled over and turned off the car several times and restarted it to get the module to pick up any error codes, hoping it didn’t. It did. Code 8 once or twice, but then code 6 for the reminder of my short drive home. When I got home and turned it off and then turned the key back on without starting it I got code 4 again. I have attached pictures of the purple wire that I was getting a dim light signal from and a wiring diagram if that helps. I have no idea what to do next. It’s very frustrating.http://www.veryuseful.com/mustang/tech/airBag/images/airBagSchem1991.jpg

 

you didn't test as i stated

re read what i wrote and re test as suggested

you should have voltage readings for us to look at

 

I’m sitting in the car right now. I just had my wife hold the test light in the plugs at the module while I checked the voltage at the sensor ends. We checked #11,12,15,16,17,19,20,21,22, and 23. All read 10.62-10.63, but #16 wasn’t giving a reading at first until my wife pushed hard into the plug. I had her push the wire (blue) into the plug at the module while pushing the test light in to get a better connection. That worked fine and gave a steady 10.62. So my problem is the connection on #16 at the module right? I’m cleaning the plug as best as possible and then I’m going to make sure that the wire in 16 is especially plugged in hard. I’m hopeful that solves it. Am I missing anything? Should I do anything else? Indy2000, thank you for your help. And thanks to anyone else who wants to add any advice.

 

a volt drop should be near 0 volts, if wire is good
I dont think you tested as i stated, maybe i was vague

both leads have to be on the same wire

 

the test light was on the battery negative. I connected the battery positive to the voltmeter positive and was using the negative probe to check the voltage figuring it was completing the circuit. #16 was reading 0 until my wife really pushed the light and it read 10.62. Do you mean that both leads of the voltmeter should be on the same end of the same wire? Or test light on one end of the wire with the ground from the voltmeter and the positive end of the voltmeter on the other end of the same wire? Sorry for my ignorance on this. I’m relatively mechanically inclined but not so much when it comes to auto electric.

 

that was incorrect test

i will draw it out later

the way you tested, your voltage of the light should be nearly identical to battery voltage

was it?

 

the test light was on the battery negative. I connected the battery positive to the voltmeter positive and was using the negative probe to check the voltage figuring it was completing the circuit. #16 was reading 0 until my wife really pushed the light and it read 10.62. Do you mean that both leads of the voltmeter should be on the same end of the same wire? Or test light on one end of the wire with the ground from the voltmeter and the positive end of the voltmeter on the other end of the same wire? Sorry for my ignorance on this. I’m relatively mechanically inclined but not so much when it

 

See Voltage Drop vs Resistance Testing (Flow Charts Suck!) - YouTube for a comparison of Voltage Drop vs Resistance test discussion.

 

Ok, thank you

 

I thought about it and both leads of the voltmeter on the same end of the same wire didn’t make sense. I had my wife hold the test light lead and the voltmeter negative lead on the ends of the wires at the module plug while I touched the positive lead of the voltmeter to the corresponding wire at the sensor end. I got 0 voltage on all of them. Then I switched the positive with negative on voltmeter just to check. Test light connected to the negative and positive gave the same results. However, I plugged all the sensors back in and the test light lit up brightly on pins 16-22, but not on 23 (the rear safing sensor which has been replaced twice now).

 

i dont have a chance to draw out what i mean

but here is a better explanation

watch as many you can, so you can understand the concept

the airbag module MEASURES VOLTAGE DROP, if there is a drop, it will request a DTC

once you understand the concept, you can be the module and find out what it is missing

 

That was very helpful. Thanks. I’m going to hit it up again this weekend

 

.

 

watch as many as you can, until the concept is second nature

this is a foundational concept, once second nature, all electrical diagnosis is easy

 

Should I have the battery connected and the module plugged in, key on, but the connections unplugged at the sensors? Then check for voltage drop at the connectors at the sensor ends? I also watched a video on voltage drop where the guy had the connectors plugged in and the key on and used a test light on the back of the connectors to find an open ground. I understand the concept of voltage drop, I just want to make sure I am testing correctly.

 

you can, but to be of any use current must be flowing, the module must be powered up

what i suggest is remove the connectors and use a test light to test voltage drop, you will need to obviously provide the ground

you have the leads on the same wire.

 

I was typing my last post as you posted this.
Just did a new test:
Battery disconnected, module unplugged. Test light connected to positive battery terminal and checking plug at sensor sensor end WITH voltmeter positive. Wife checking module end of plug. 11,12,15,16 & 23= 10.54

you have one lead on battery negative?

and you probing the wires with the other lead?

a voltage drop test on the same wire should give you NEAR ZERO VOLTS

that is the test i asked for

you have not reported one voltage drop near 0 volts

either you are doing the test wrong, or ALL the wires are OPEN.........

Yes negative lead of voltmeter to battery negative. Probing with positive. I will try your test light method again right now to try to find 0 volts

 

I’m back in the garage and have done every test I can think of. I MIGHT have found something. Here are the results:

I’m getting 12.05 at the battery terminals
Power to backup power supply - 11.91

Battery connected, key on, module plugged in, rear sensor plugged in. Checking voltage at the back of sensor plug piercing the wires:
15-11.09
16-.12
23-11.09
11&12 - 5.6

Could 11&12 be the problem since they are evenly drawing the voltage?

 

you have one lead on battery negative?

and you probing the wires with the other lead?

a voltage drop test on the same wire should give you NEAR ZERO VOLTS

that is the test i asked for

you have not reported one voltage drop near 0 volts

either you are doing the test wrong, or ALL the wires are OPEN.........

 

Mustang 90-91 Air Bag Diagnostic Codes

have you test according to above to the T for code 4?

 

I believe so, I will do it over again

 

Just did a new test:
Battery disconnected, module unplugged. Test light connected to positive battery terminal and checking plug at sensor end WITH voltmeter positive. Wife checking module end of plug with negative lead also connected to battery negative. 11,12,15,16 & 23= all 10.54

 

I think I’m getting this:


Low voltage at pins 11 and 12 can be caused by:

1. Resistance to ground on circuit 614 or 623. Circuits 614 and 623 should be open circuits to ground when the diagnostic monitor is disconnected from the harness. Resistance to ground on these circuits can cause a drop in the diagnostic voltage on both circuits.
2. Low resistance in one or more of the primary crash sensors. If the resistance of one or more of the primary crash sensors is lower than normal, the voltage on pin 11 will be pulled down too low,
3. A poor ground on pin 3 of the diagnostic monitor may cause voltage to appear on the diagnostic monitor ground reference. Any voltage on the ground reference will cause the diagnostic monitor to measure the voltage at pin 11 as low, even though the voltage at pin 11 with respect to the sheet metal is normal.
4. Intermittent battery voltage at pin 13 can cause the reference voltage inside the diagnostic monitor to fluctuate and cause Fault Code 4 in some circumstances.

Or this:


Fault Code 4D- Safing Sensor -- Circuit Shorted to Battery Voltage

Description: The diagnostic monitor measures voltage at pin 11 (circuit 614, GY/O). The voltage at pin 11 is controlled by a resistor inside the diagnostic monitor (along with the resistors inside the crash sensors -- see Fault Code 4A above for details). This voltage varies with charging system voltage (refer to the pin voltage chart at the beginning of this appendix for the correct voltages at the pins listed above). If circuit 614 is shorted to battery voltage, the voltage at pin 11 of the diagnostic monitor will be battery voltage and the diagnostic monitor will flash Fault Code 4 on the AIR BAG readiness lamp (Figure 89).

Possible Causes
Battery voltage on pin 11 can be caused by:

1. A short in the wiring harness between circuit 614 (GY/O) or 623 (P/W) and another wire. This can cause high voltage in these circuits.
   NOTE: The wiring leading to the safing sensor(s) has higher voltage wires bundled in the harness with circuits 614 and 623.
2. A short in the clockspring between circuit 614 and some horn or speed control wiring.
3. A short across the normally open contacts of the safing sensor(s).
   NOTE: The voltage on circuits 611 and 612 inside the safing sensor(s) is usually battery voltage. There should be an open circuit across the contacts of the safing sensor(s) if the sensor(s) is operating normally. If either of the sensor contacts are closed, the voltage on circuit 614 would be high (battery voltage).

By the way, thanks again for all the help. I owe you a beer, or a case of beer

 

One more test:
Battery on, module disconnected:
#1-11.23
2-.08
3-.08
4-.09
5-.09
6-11.23
7-.08
10-0
11-0
12-0
13-11.57
14-11.48
15-.01
16-0
17-.08
18-.08
19-0
20-.08
21-.08
22-0
23-0

 

looks good

i will get back to you with a step by step

4a-4d from that site

connect everything up for now

 

Awesome, thanks. By the way, I just did test light on positive battery to sensor plug, Voltmeter negative to negative battery. Voltmeter positive to module plug. All pins=10.52

 

one lead at pin 11
the other lead at pin 13

power it up, what is the voltage?

 

Voltage=0.00

 

pin 11 to battery ground and battery positive

pin 13 to battery ground and battery positive

again powered up

 

I will check tomorrow and report back.

 

sounds good

 

Pin 11: Positive-0.00
Negative-0.00

Pin 13: Positive-0.03
Negative-11.60

 

pin 12 to batt neg and batt pos

 

0.00 on both