My coolant temp gauge is not working, but grounding the yellow/red wire from the sensor under the intake manifold makes the gauge react. The Volvo dealer says it's a knock sensor, and it's only $90. First, what's the deal with having the temp gauge hooked up to a knock sensor, and is there a place to get one cheaper than forking over $100? Thanks in advance for any help.

Thanks for the info. I failed to describe the sensor in my original post. The sensor looks like a temp or oil sending unit, but the threaded part just looks like about a 5/16" bolt about 1/2" long, and it doesn't penetrate the block into the water jacket (as far as I can tell.) It doesn't show any signs of being in the coolant, and the cavity it goes in definitely bottoms out. I've been in several parts stores and when I asked about a temp sending unit they bring out something that looks like what you would expect, not this, and the threads are totally different. (Larger than what I have.)

Bear in mind that there is more than one "temp sensor" on that car. the one that is larger in diameter and has a boshe style clip on plug with 2 terminals is the temp sensor for the computer and fuel injection system...not your problem...if you ask for a temp sensor that may be what the ultra-informed parts department guys bring you...just make sure thay aren't confused...which if they are anything like the guys at my place will involve lots of slow clear speaking. the one that goes to the guage is much smaller than the engine temp sensor (at least the hex nut part that your wrench grabs)and will have a single (orangish to yellow wire on it) and will be attached with a female wire connecter end to the sensor. in that general area on the engine there are 3 sensors, the engine temp sensor, the guage temp sensor, and what I think is either a knock sensor or an oil pressure sensor(the only one coming out of the block aka the lowest on the engine...also has a one wire connection). The one closest to the front of the engine is the guage sensor and it is directly under intake runner #2. it goes into the HEAD...not the block. If your coolant level is really low that may explain why it looks to you that this sensor doesn't go into coolant. the passage in the water jacket portion of the head that this sensor sits in is not very deep (maybe 1 inch or so) but I assure you that it must be in the coolant to function. the sensor also sits very high on the engine so if the coolant level is low that is where it will show up first....just a possibility. good luck

well that unit...which goes into the head and is located under the #2 intake runner and has a single wire coming off of it is definately the temp guage senesor. Now if the guage reacts...aka reads hot when you ground the wire coming off of that unit then the sensor is bad...plain and simple. Last I looked the sensor was only about $15. I was able to replace mine without removing the intake manifold (I have been graced with small hands). I did have to remove the distributor cap however. I would recomend using a thread sealer or teflon tape to seal the threads, as mine began to leak after sever heatings and cooling...it was easily remedied by tightening it further but beware putting too much force into that as the head is aluminum and you will strip the threads if care is not exercised.

Oh yeah....and find a dealer who knows what he is talking about (easier said than done)