I don't know the year because the engine is not original to the vehicle, but I'd guess 85-89, manual transmission. I had just arrived home and parked it, and noticed antifreeze leaking. It was late and I couldn't see where it was coming from, so I left it overnight. It started up again the next morning and I backed it up where the light is better and saw that it was the lower radiator hose. I removed the hose - from the lower left of the radiator (facing vehicle) to the engine. Mid-way on that hose is a T connector which houses the coolant temperature sensor. I was able to clean up the connector and replace both pieces of hose and reconnect everything successfully. I topped off the coolant, but now it won't start. It has a new battery, but when trying to crank over it almost sounds like it just doesn't have the juice to quite start. I tried jump starting it, but no success. I did not replace the coolant temperature sensor, just put the original back in. Is it possible that it isn't making a good connection (maybe corrosion on those leads) or is faulty - would that prevent it from starting? Any other thoughts? The only things done between when it last started and subsequently wouldn't start was the hose replacement and reconnection of the coolant temperature sensor.

 

On a B230 the coolant temp sensor in the head. The sensor you show looks like the sensor for the electric fan but older Volvo engines don't have one. ... One end of that hose connects to the radiator, the other to the water pump, right?
That then should not affect starting but I am not sure what engine you got in there to be sure.

 

If jumping the battery has not helped then I would suspect a bad electrical connection. Check the battery terminals, the battery cable and it's connection at the starter, the top terminal on the starter solenoid. Then check the battery ground connection to the chassis or engine. The engine should have a heavy cable from the block to the chassis or sometimes just a braided cable from the head to the firewall. Voltage measurements at those points are helpful.

 

Update
While poking around under the hood and checking connections as recommended, I discovered a fried relay!! I think it is the relay for the lights and did a little online searching. It is a Hella 4RA, 12V 003 510-36. Went to Autozone and their replacement one doesn't have the fuse in it, so I'm not sure if will work - it was a Novita RL44, SPST 40A/12VDC. The wiring diagram was the same for the RL44 as the original, but without the fuse. I'm guessing there's a wiring problem somewhere, because this relay shouldn't prevent the vehicle from starting...