Seems more likely that there is a vacuum leak.

 

If switch is not physically broken which it can from age, probably just a leak. Check the surface of the sensor on the brake booster, the one to the left of brake fluid reservoir. It cracks with age around the surface where electrical plug is.

 

The switch appears intact. I'll check out the position sensor tomorrow. Thanks for the tip!

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Well I couldn’t find anything on visual inspection. Tonight I’ll get the carb cleaner out and see if I can find a leak that way.

If it is a leak, could that explain some engine surging I occasionally experience when coming to a stop, especially after highway speeds?


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So I haven't had a chance to search for the vacuum leak but I was looking at the brake booster and it has some component that's not identified on any parts diagram I've seen.

This is the diagram VIDA gives me for my 2004 VR. However my car's booster has something in the area circled red that enters the booster with an electrical connector.

I double checked with Tasca and some online Volvo dealer parts sites and haven't seen any diagram with a component here.

Anybody have this mystery part too?

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the 5 wire plug there is on cars with DSTC, R models all have this. It is 2 wires for the active booster solenoid and three wires for the pedal apply sensor.

check the switch, 15 in diagram, as mentioned. That is the #1 problem I find. Next is the booster check valve on the hose at booster. It is 3a in diagram. Follow the hose and take that off the vac switch, turn key on and then go cover the port on switch where hose was removed. If pump stops cycling then booster is suspect. Usually you can hear the booster leaking (hissing) if you listen under dash after pump goes off and it gets quiet.

 

I was referring to item #7 - the switch with a rod. The rod is spring loaded IIRC and it pushes toward the connector. Right around connector, you may be able to see cracks in the housing from the pressure applied by the rod. I've had this happen to my car when troubleshooting leaks in brake booster as vacuum pump was coming on and off constantly like you're describing.

 

I disconnected the hose running from the switch to the booster and plugged the switch port with my thumb. The pump then started turning on in rapid fire bursts continuously. With the hose reattached the pump operates for a second or two then turns off for a second or two and then turns back on, continuously until I start the car. With the engine on the pump seems to stay off unless I rapidly pump the brakes like you would in pre ABS days.




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I went through this recently with my 2006. I had a similar symptoms: key in pos II, engine off, vac pump would cycle on and off every 5-10 sec. When the pump was off, I could hear a vacuum leak. I traced it to the pump, but it wasn't the pump. The pump makes vacuum, but does not hold it. There are check valves in the switch (on the pump line #16 in your figure, and the intake manifold line #18). The pump comes on, until there is sufficient vacuum at the switch, and goes off. The check valves hold the vacuum in line #14 to the booster. If you have a leak in line #14 or the booster or one of the check valves, the pump will cycle back on. Before replacing the switch, I checked that the booster was not leaking. I first connected my Mityvac to line #14, but couldn't get it to pull a vacuum. I was able use the pump pull a vacuum, then clamped the line, and attached my Mityvac and could see it holding a vacuum. I replaced the switch (there were several part #s available, which I cross checked with images on google to get the correct one), and no more cycling.

I disconnected the hose running from the switch to the booster and plugged the switch port with my thumb. The pump then started turning on in rapid fire bursts continuously. With the hose reattached the pump operates for a second or two then turns off for a second or two and then turns back on, continuously until I start the car. With the engine on the pump seems to stay off unless I rapidly pump the brakes like you would in pre ABS days.

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This makes me suspect the check valves in the switch. Connecting to the booster increases the volume of your vacuum reservoir, which takes longer to leak down (and the pump cycles slower). With the port plugged with your thumb, the volume is very low, and the pump cycles rapidly.

FYI - type "volvo 31273684" into google images, and you will see a booster with your electrical connector.

 

I did some mods in my car and one of them was the removal of this pump from the system and all the electrical part (connectors) of the region. The car works perfect, does not light any light on the dashboard and continues to brake very well.

 

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