Greetings!!

I have few questions about volvo SRS system, I hope someone in this forum has some answers.

The problem is irrational srs light behavior. Situation is this, SRS light is on and I cannot read any fault codes from the SRS unit. I have tried to read the codes from socket B5 on diagnostic unit side B. Diagnostic led lights when I push the button, nothing further happens, not even 1-1-1 no fault code??

I checked all the connectors between B5 terminal and crash detector unit, circuit seems to be ok. SRS manual says that voltage should be measured between terminal B5 and ground. So I measured it and it was aprx 10 volts. Manual says that if voltage is around 10 volts -> replace diagnostic unit. If less than 10 volts -> check circuit for shorts or breaks. I don't know if there is any use to replace diagnostic box, because I am able to read codes perfectly from side A-connectors (ignition, fuel injection). I guess I have to do resistance check for those few other wire which goes to alternator D+, etc. Probably have to check crash det. unit grounding too.

If those connections are ok, is there a possibility that there is something wrong in instrument cluster? is there any specific component to replace?

Since the options are running short, I am beginning to suspect faulty crash detection unit, hence the next question is. Are those crash detetector boxes interchangeable? there is little differences in model numbers on top of the box, but othervise they seem to be similar.

Has anyone tried to mix those boxex together?

Kind regards.

-Merl1n

Dear Merlin,

May this find you Well. By "SRS manual", do you mean Volvo Technical Manual TP32231/1? If not, before you do any more tests or other work on SRS items, get this manual. An inadvertently-triggered airbag can be lethal.

If you do have the manual, then you're familiar with the precautions that must be taken.

Before you replace the sensor, try the following. Make a note of the radio security code. Then disconnect the negative battery cable clamp from the battery. Allow the car to sit for 30 minutes. Reconnect the cable and drive the car briefly. Do this two more times. If the SRS light remains "on", there likely is a fault in the sensor.

I would replace the sensor with one with the identical number (Volvo #9148793).This part was used in 1994 and 1995 940s. If bought at the dealer, this item will cost about US$1,000. You should be able to get once from a junkyard, for a fraction of that amount. So long as the airbags have not deployed - and the unit was not allowed to get wet - the unit should be usable. Install using all procedures in the above-mentioned Volvo technical manual.

Hope this helps.

Yours faithfully,

Spook

Hi again!!

Many thanks to Spook for your prompt reply.

The unit that is now in car has number Volvo #9148794 Bosch #0285001045.

The numbering is kind of confusing, The Bosch number might be exactly sama in different units from same era, but volvo number can vary a lot. For example there is one used box for sale and the bosch number match, but volvo number is totally different yet the box is from same model car with same manufacturing year??

Wiring is same so I wonder what difference might be?

I guess I just remove the box and dismantle it for inspection. If there is bad soldering or moisture.

SRS light behaviour makes me think the previous. Sometimes light turns off when I start the engine, it stays off sometimes pretty long time and then again sometimes it lights up after couple of minutes time.

best regards

-Merl1n

Dear Merlin,

May this find you well. It might be the case, that the intermittent SRS warning light is caused by a corroded or loose connection at the instrument cluster. Remove the cluster - see the FAQs for the procedure - and examine the connector and the flexible circuit card.

I'd would not do any work inside an SRS sensor box. While there could be a failed solder joint - and you might repair it - there is also a risk that the repair would look sound, but wasn't effective. As a sensor failure could have very nasty consequences - the airbags don't deploy when needed, or deploy when not needed - I think the risk is best avoided. Thus, while I'd substitute a used unit from a donor car - with undeployed airbags - I'd want that part to have the same Volvo part number.

It might be that units with the same Bosch part number - but different Volvo (or other manufacturer) part numbers - are differently programmed. Thus, a unit for a sedan might be set up to trigger at different crash impact energy levels, than a unit installed in a wagon.

Do not use the continuity tester, on the sensor unit, when it is connected to the rest of the system. The small amount of electricity that a Volt-Ohm-Multimeter puts into the wiring, could trigger an airbag deployment. In mid-1990s Volvos, all SRS components have to be replaced, because a triggering event causes both airbags (in 1994 and 1995 cars) to deploy and both seat-belt pre-tensioners to "fire".

Hope this helps.

Yours faithfully,

Spook