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SRS light

R

rstarkie

Member
Joined
Jan 23, 2004
Location
Hamilton, Ohio
1994 940t 134K
The SRS light came on. I tried to pull the codes by plugging the lead into terminal 5 on box B. The LED on Box A illuminates for the one second push, but I get no response from the LED otherwise. Moving the lead to terminal 2 (cruise control) I get a 1-2-2 code so I know the lead functions there at least. I cleaned up the socket and the lead but still no signal from terminal 5.

Any suggestions on pulling the SRS codes and extinguishing the light would be appreciated.
 
Maybe you have lost power/ground to the SRS box. I would imagine if the box goes offline, it would turn on the light.

Confirm good power and ground with a test lamp. I would also inspect the box/wiring for any signs of water intrusion. It is mounted at a low point in the car, under the driver's seat.
 
ZVOLV said:
Maybe you have lost power/ground to the SRS box. I would imagine if the box goes offline, it would turn on the light.

Confirm good power and ground with a test lamp. I would also inspect the box/wiring for any signs of water intrusion. It is mounted at a low point in the car, under the driver's seat.
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Thanks for your reply. I will try to look at that possibility. It has an inoperable power seat so access to components might be a problem if I can't get to the bolts securing it.
 
You won't get a code back from a failed crash sensor. They actually do fail on their own occasionally. You may just have a bad connection where the harness connects to the sensor.
 
2manyturbos said:
You won't get a code back from a failed crash sensor. They actually do fail on their own occasionally. You may just have a bad connection where the harness connects to the sensor.
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Where are the sensors in a 1993 940?

You could measure resistance each sensor and the wiring at the SRS module. I would imagine a bad sensor would show a high resistance or open circuit?

The first thing to do when diagnosing a problem is a thorough visual inspection.
 
The only sensor is the module itself. It is an accelerometer. It measures the resistance of each of the restraint devices every time you turn the ignition switch on. That is why it takes a couple of seconds for the light to go out. If it isn't talking to the test box, it either has very low voltage going to it, a bad connection or it is dead itself. The crash module is under the driver's seat.
 
2manyturbos said:
The only sensor is the module itself. It is an accelerometer. It measures the resistance of each of the restraint devices every time you turn the ignition switch on. That is why it takes a couple of seconds for the light to go out. If it isn't talking to the test box, it either has very low voltage going to it, a bad connection or it is dead itself. The crash module is under the driver's seat.
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Got it.

In that case, the easiest thing to do would be to access the box, confirm power and ground with a test lamp between the power and ground terminals, and then go check each leg of the circuit with a meter.

I have had at least a couple instructors tell me never to put a meter on the airbag deployment circuits. I had a much more reliable/experienced instructor confidently tell me that a multimeter will not put out enough current to cause any chance of deployment.

Be careful. If you go a test lamp between the power circuit and accidentally probe a deployment circuit, that would probably set off the component.
 
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ZVOLV said:
Got it.

In that case, the easiest thing to do would be to access the box, confirm power and ground with a test lamp between the power and ground terminals, and then go check each leg of the circuit with a meter.

I have had at least a couple instructors tell me never to put a meter on the airbag deployment circuits. I had a much more reliable/experienced instructor confidently tell me that a multimeter will not put out enough current to cause any chance of deployment.
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There is no reason to be putting the meter across the connector of the restraint components. The crash sensor tests those and is very accurate in doing so. The crash sensor will point straight to any component that is out of spec. It's possible to have a problem with one of the cables going to a restraint and testing the cable is safe and easy. I agree with your instructor that says do not probe an air bag. It is not worth the risk.
 
2manyturbos said:
There is no reason to be putting the meter across the connector of the restraint components. The crash sensor tests those and is very accurate in doing so. The crash sensor will point straight to any component that is out of spec. It's possible to have a problem with one of the cables going to a restraint and testing the cable is safe and easy. I agree with your instructor that says do not probe an air bag. It is not worth the risk.
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This one was entertaining:




I just went and measured the resistance of an airbag on the bench with my Fluke 87. It measured 2.4 ohms.

I have used a kit with a bunch of different connectors with a 2.4 ohm resistor built in. You go install the 2.4 ohm resistor, instead of the suspect component, and then if the light goes out you know the problem is internal to the component and not the wiring, connectors, or the module.
 
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Thanks for all the knowledge everyone has shared. As was my fear the inoperable power seat prevents access to the box. I have restricted access to the bolts holder the rear of seat so I am not able to remove those. So at this point I am stuck with an inoperable SRS circuit. Actually counting on the airbags to deploy in a collision might be a stretch.
 
Was any recent work done in the steering column area or anywhere else?

Basic electrical diagnosis is needed at this point. Starting with a good visual inspection. Sometimes you get lucky and find something unplugged.

This thread got me motivated to work on extinguishing the SRS lamp on my 1991
 
UPDATE

I posted on 10/2 that the windshield wipers on this 94 940T weren't working (https://www.turbobricks.com/index.php?threads/94-940t-dead-windshield-wipers.380678/). I found that bypassing fuse 16 (blower motor) due to excess heat distortion restored the function of the blower motor but resulted in the failure of the wipers and radio. With this forum's help I learned that there was a hidden bus bar that supplied 12v to the feed side of the fuses for the radio and wipers (14 and 15). Wiring a jumper to feed 12v back to the male spade on the supply side of the now eliminated fuse 16 restored the function of the wipers and radio.

After driving the car I realized the SRS light was no longer on. Apparently altering the fuse block and eliminating fuse 16 had also somehow effected the SRS circuitry and caused the light to come on. Adding the 12v jumper back to the supply side of the original fuse 16 slot put the SRS back in service.
 
I don't understand what you did with your "bypassing" things and how that restored SRS.

What is a bypass?

Ok so you found a fuse with no power and added power somehow to that circuit. I hope you put a new fuse in your power circuit. So you bypassed the fuse block that was burnt for that circuit?

One diagram I have for a 1990 740 shows fuse 15 powers the SRS module that year. So your 14, 15, and 16 fuses were all dead?
 
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The "bypassing" I did was fuse 16. That is the blower motor and it is not unusual for that fuse to heat up and distort the area around the fuse as well as build corrosion and increased resistance. I pulled the fuse block and cut the wires on the underside. I then lengthened those wires and attached a fuse holder and installed a 30 amp fuse. What I was unaware of was that since fuse 16 socket was no longer getting a 12v feed it meant that fuses 14 and 15 were not getting power on the supply side because there was a (hidden) bus bar connecting the supply side of fuses 14, 15, and 16. that is why the radio and wipers were no longer working.

So the solution was to find a 12v source and attach it to the supply spade in the old 16 fuse socket. By adding 12v there it once again supplied 12v (via the bus bar) to the supply side of fuses 14 and 15. I got that 12v by tapping into the new extended lines for the replacement fuse 16.

Yes, fuses 14 and 15 were "dead" until I added the jumper wire to the old supply side spade in the burned 16 fuse socket. So it would appear that the SRS circuit get its power from 14 or 15 as you suggest.
 
rstarkie said:
1994 940t 134K
The SRS light came on. I tried to pull the codes by plugging the lead into terminal 5 on box B. The LED on Box A illuminates for the one second push, but I get no response from the LED otherwise. Moving the lead to terminal 2 (cruise control) I get a 1-2-2 code so I know the lead functions there at least. I cleaned up the socket and the lead but still no signal from terminal 5.

Any suggestions on pulling the SRS codes and extinguishing the light would be appreciated.
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Hey Randy, that black wire on fuse 15 that you cut carries on as power to the SRS module. The SRS isn't on that fuse, just uses that same black wire from the ignition switch with a branch at fuse 15. Sooner you get that black wire you cut restored to the hot side of fuses 14, 15 & 16, the less suprises you're likely to have. Edit: Just saw your later posts saying you've already figured this out
 
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