If one of these relays fails away from home, it could be costly (high towing expenses, wild mechanic's diagnoses, etc.). 
Even though these are VERY common failures and SHOULD be widely known by anyone who works on these cars professionally, I have seen MANY mechanics insist a Volvo needs a new fuel pump, fuel injection computer, air mass meter and other expensive parts, when in the end it's discovered it was ONLY A SIMPLE INEXPENSIVE RELAY which you can plug in yourself. 

Spare Fuel Pump Relay:  I have experienced a fuel pump relay failure in FOUR of my Volvos over the years (240s and 740s).  After one particularly stupid $200 tow bill,  I made it a point to replace the original Fuel Pump Relays in my older Volvos with NEW ones, and I carry a used old one as a spare in my glovebox just in case.  If you have older or original relays (10 years and up) in your Volvos, or if you are experiencing intermittent or temperature related fuel or electrical issues, maybe it's time for a replacement before it leaves you stranded.

I had a 1990 740 Turbo a number of years ago.  When it was close to 8 or 10 years old, it began developing intermittent stalling, but would eventually start back up after a few minutes.  After it happened  to my wife twice, she refused to drive the car.  I could not re-create it when searching or testing.  I figured it had to be a relay or crank sensor, and THIS CAR HAS TWO RELAYS that are prone to failure.  So I replaced the Fuel Pump Relay (3523608), Noise Suppression Relay (1323592) and Crank Position Sensor (3547847), all at once. Sure enough, one of those was the problem and the car was good after that.   Because this is so common in Volvos, I keep all three of these items in stock here just in case any of you need them.  They fit '88 to '93 240's (except the 240 uses the 1323592 as the cooling fan relay), '88 to '92 740's and most '91 to '95 940's.

CAUTION Regarding COOLING FAN RELAY FIRES in late 240s
I was warned by a long-time Volvo tech regarding a number of instances where older Cooling Fan Relays (PN 1323592) were causing fires under the hoods of 240s (1988 to '92).  No explanation was given, but perhaps it was related to the high loads associated with the auxillary cooling fan.  He said it became common enough to recommend replacement of any Cooling Fan Relay over a certain age.

Here's a Brickboard post regarding a couple of Radio Suppression Relay replacements....
"I have had two cars which had a problem with this relay. It is black, usually on the passenger side by the anti-freeze expansion tank. The part number is 1323592-1.  My 740 had poor acceleration and hard cold weather starting here in Minnesota. The problem did not change with a replacement L-engine out of a 1993 940 (non-turbo). Changing the relay made a huge difference in both acceleration and cold weather starting. The reason I put in an L-engine is because the old K-engine froze with a broken intake valve. I have always wondered whether the relay, which supplies a ground to the fuel injection, could have had high resistance contacts leading to the injectors not injecting a rich enough mixture so the engine ran lean?  The second case of this relay going bad was fixed two nights ago. My 1995 940 (non-turbo) with 69K miles had poor acceleration and would surge slightly while running steady.  I thought the problem could be related to the years in storage even though I started it periodically and had a full tank of gas with stabilizer.  I changed the fuel filter, cleaned the IAC valve and throttle bottle, checked fuel pressure and volume, and I could not find anything wrong. Then I changed the relay and again, it was like a different car."

TESTING RELAYS
I have received a lot of emails over the years asking how to test these relays. Since I'm only an "amateur Volvo tech," the only test I know for a relay is swapping it with another (try a used one if you have one). 
As for the Crank Position Sensor (240: '89 to '93, 740: '89 to '92, 940: all 4-cylinder models), I hear of them going out often enough in later 240/740 models that I would consider changing it if you suspect it's original.  It will be MUCH HARDER TO CHANGE if it fails on some back-country road.  I don't know how they are tested or what to expect if one is tested.  The Crank Position Sensor may also be known as an Engine Speed Sensor or Crank Impulse Sender. 
Here is a good Brickboard post on these sensors http://www.brickboard.com/RWD/index.htm?id=807903

Here is some excellent info on Volvo fuel pump relays:
http://brickboard.com/FAQ/700-900/ElectricalCircuitsRelays.htm#Hot_Fuel_Pump_Relay

Relay Locations - DASH, 1984 240 (PDF)

Relay Locations - 1984 240 Turbo (PDF)

Relay Locations - 1987 240 (PDF)

Relay Locations - 1993 240 (PDF)

Relay Locations - 1987 740/760 (PDF)

I keep these in stock
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