Hi guys,

I'm trying to diagnose my friend's Volvo XC90 air bag light and was told by the dealer that the connector at the seat belt tensioner was causing the issue. The computer is giving codes for "RF Seat Belt Tensioner Signal Too Low" and "RF Seat Belt Tensioner Circuit Fault".

Anyways I took out the two wire connector and inspected it and it turns out there are two capacitors built into the connector harness. One is about a 1/4" cube, running in series with one of the wires, and says "10" on it. It measures at .456 microFarads with my cap meter. The other is very small and runs in parallel between the two wires and says "103" on it, and measures .01 microFarads. When I test these caps with an ohmmeter, the larger one starts to climb to infinity when I test at the 200 megohm setting (with a 9v tester) and the other starts at 800 megohm and goes in infinity immediately. Can anyone help me verify if these components are working properly? Thanks

 

Capacitors should measure infinite resistance (open circuit) except for very large value capacitance (greater than 1000μF where the time constant is very high).

What you have measured is normal for low value capacitance.


Edit: We can discuss measuring capacitance but we are restricted in how far we can go with discussing automotive electronics. Please see the Terms of Service found at the bottom of this page.

 

Thanks for the reply that answers my question. Dealer diagnosed the problem in the connector because they got infinite resistance from the pin side of the connector to the wire side, but that makes sense if capacitors normally show infinite resistance. When the capacitor has voltage does the resistance go down? How does the circuit make a complete loop? If the cap really has infinite resistance shouldn't the circuit show open all the time?

 

A circuit can have infinite resistance at DC, and still form a loop. You need to consider the impedance of the capacitor over a frequency range of interest. The higher the frequency the lower the impedance.