Sangerboy Posted October 21, 2015 Share Posted October 21, 2015 I am the original owner of my '95 Legacy Outback and am coming up on 330K miles. My question is which sensor provides the engine rpm to the ECM so that it then sends the information to tachometer in the gauge cluster. My guess is the crankshaft position sensor but I was just wondering if anyone knew for sure. Link to comment Share on other sites More sharing options...
987687 Posted October 21, 2015 Share Posted October 21, 2015 Well, the ECU uses a combination of the crank position sensor and the cam position sensor to properly control spark and fuel injection timing. Without either of those working the engine isn't going to run. I wouldn't say either technically sends the signal to the tachometer, the computer "knows" the rpm because that's its timing base for firing the sparkplugs and the injectors. If your tach isn't working but your engine runs, the issue is with the tach, not the computer or engine sensors. Link to comment Share on other sites More sharing options...
Sangerboy Posted October 21, 2015 Author Share Posted October 21, 2015 Thanks for the reply, the engine will not start. I've had the cam position sensor go out once before (about 250K miles ago lol) and it wouldn't start just like now until I replaced it. This time though I noticed symptoms in the tachometer for a bit before it failed to start and run. The tach would randomly fluctuate at times (drop a few hundred rpm and then go back to an accurate reading right away) the engine would not hesitate or miss though and I do not remember that happening to the tach the last time. I'm pretty sure the problem is in the crank position sensor or the cam position sensor, possibly both but I was just curious if one or the other was primarily responsible for sending engine rpm data to the ECU since the tach has occasionally been acting abnormally. Link to comment Share on other sites More sharing options...
Sangerboy Posted October 27, 2015 Author Share Posted October 27, 2015 Found my code reader, computer is not throwing any codes but car will still not start. Fuel pressure is good, have spark to the plugs, fuel injectors do not seem to be firing. Am beginning to suspect the ECM. The random, sporadic odd behavior of the tach should be a clue (see previous post). Any thoughts by anyone? Am considering buying a used ECM just to test rather than spending $600 on a new one in case that is not the problem. No codes thrown but scanner did list three MIL "ready's" Misfire, Fuel and Comp. After disconnecting the negative battery terminal and placing a new coil. The car started and ran perfectly for about 10-15 seconds then quit again and no start since then. Will welcome any insights Link to comment Share on other sites More sharing options...
ocei77 Posted October 27, 2015 Share Posted October 27, 2015 Exactly which codes were in a pending state? Numbers please. Pull an injector plug. You should be getting 12v on the white wire with key in the on position. Make sure your vacuum hoses are on and whole, especially the small hose from the intake manifold to the pressure solenoid on the IM. O. Link to comment Share on other sites More sharing options...
Sangerboy Posted October 27, 2015 Author Share Posted October 27, 2015 Thank you O., I'll check those things this weekend as during the week my job takes me away from where I keep the car. There were no numbers given on the scanner as no codes had actually been recorded by the system. The MIL "ready" codes are evidently "pending" in the system but the Actron scanner I was using won't give a code # until the system has actually decided to throw the code. Just the words- "Misfire, Fuel & Comp". Thanks again for the leads, Greg Link to comment Share on other sites More sharing options...
987687 Posted October 27, 2015 Share Posted October 27, 2015 If you're getting spark but not fuel, I'd suspect the cam position sensor. It's the same part as the crank sensor, so test the resistance of the crank sensor, then test the resistance of the came sensor. If they differ by more than 5% or 10% I'd assume one is bad, likely the cam. In addition to making sure power is going to the high side of the injector, you should test they're firing. The proper tool here is a noid light, if you don't have one either use a digital volt meter that has a true RMS frequency counter, or just an analogue meter on the 12v range. The needle should jump up and down as the engine is cranked. A regular digital meter won't work here because they don't react fast enough to give any helpful reading. Link to comment Share on other sites More sharing options...
Sangerboy Posted November 23, 2015 Author Share Posted November 23, 2015 OK everyone, here is an update thanks for all the advice and help to this point. I got some noid lights and determined that the fuel injectors were firing so we do not have a computer or sensor problem. I also got an inline fuel pressure tester and determined that there was 0 fuel pressure. Previously the fuel pump had been pumping fuel (evidently sporadically) but now it will not pump at all. I had also removed the pump and tested it outside of the tank and it functioned normally. There was no power being supplied to the fuel pump when the key was turned on and the engine being cranked (the blue/yellow wire at the connector at the top of the fuel tank). I started tracing at the fuel pump relay, the red wire to the relay was hot under all circumstances (normal) the yellow wire was hot during cranking (normal) and the relay itself tested normal so it would appear I have a wiring issue. There is a wiring loom connector in between the fuel pump relay and the fuel pump harness. My question is where is this connector located in the car and what is the best way to get to it? This is the spot where I would like to test next to try and figure out where the short or problem is. Also where does the fuel pump loom ground to?. Thanks again for any help you can give. Greg Link to comment Share on other sites More sharing options...
Fairtax4me Posted November 24, 2015 Share Posted November 24, 2015 Easiest check is to check for 12v on both sides of the fuel pump at the pump connector. Set up your meter with clips so you can watch it at Key On and while cranking. Need something like 12v going to the pump. (It will be less, but how much less depends on condition of the wiring in the car.) Good ground will show less than 1v on the ground side of the pump. If you have a bad ground it will show high voltage or even same as supply voltage on the ground side. I don't remember where it grounds, but if you have a poor ground you can cut the ground wire where it comes in through the floor under the seat and ground to a point under the seat. I think there's a ground point already on the far right side. Link to comment Share on other sites More sharing options...
Sangerboy Posted November 30, 2015 Author Share Posted November 30, 2015 Final update, after more thorough checking it turned out to be the fuel pump itself. This was very frustrating as it was the first thing I checked and tested it twice, once in the car and once outside after I had removed it from the tank. I guess it just happened to test positive those times and wouldn't work under a load. This threw me but I finally circled back to the true problem after everything else was checking out OK. Oh well, I learned more about the car and once again have great appreciation of how easy this car is to work on. Took the car out for a 35 mile test drive including a good grade on the freeway and everything is working and running perfectly. Thanks again for everyone's help and input. On our way to 350K 1 Link to comment Share on other sites More sharing options...
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