S70 Jumping RPMs at Idle (video)

Xpletive · Post by **Xpletive** » 29 Aug 2011, 12:43

Hey guys. Replaced the crankshaft position sensor which was throwing a code yet the tachometer is still acting nuts. Code has not returned though

Check out the video - any help would be appreciated. Changed the plugs / wires / cap & rotor with all new OEM gear as well.

Xpletive · Post by **Xpletive** » 30 Aug 2011, 09:46

Anyone with any similar experiences? This is after a transmission swap - i've looked everywhere.

This is a 98 S70 GLT by the way.

Post by **jblackburn** » 30 Aug 2011, 09:47

My only thought is that perhaps the camshaft and RPM sensor wires are interfering with each other. This is usually more of a problem on the cars with the old-style ignition and distributor wires (1998 and prior), but see if you can separate/shield the two from each other and see if you get a difference.

Is this the 1999?

Xpletive · Post by **Xpletive** » 30 Aug 2011, 09:58

jblackburn wrote:My only thought is that perhaps the camshaft and RPM sensor wires are interfering with each other. This is usually more of a problem on the cars with the old-style ignition and distributor wires, but see if you can separate/shield the two from each other and see if you get a difference.

What can i use for shielding? About to go try it now.

Post by **jblackburn** » 30 Aug 2011, 10:07

Aluminum foil will serve as a good diagnostic tool to see if that is indeed a problem. Zip-tying them as far apart as possible usually does the trick, though.

Xpletive · Post by **Xpletive** » 30 Aug 2011, 18:05

Covered the crankshaft position sensor wire in foil - a lot less fluctuation but its still there.

Decided to keep the car running while disconnecting the camshaft position sensor - it fluctuated just as much with it connected. Car wouldn't start with it disconnected, but could it be that the sensor is working intermittently after the car is started?

I also noticed my positive battery cable that runs to the starter being pretty warm with the limited amount of time the car was on. Could it be the connection to the starter giving me issues? Does anyone have any starter motor diagrams for the battery cable connection?

Post by **jblackburn** » 30 Aug 2011, 18:17

Just so we're clear here - and not to make you feel like an idiot or anything - just for clarification since people mix the two up all the time, which sensor did you replace and were disconnecting?

Camshaft sensor:

Crankshaftm (RPM) sensor:

The lower one (crankshaft sensor) is responsible for the tach and firing the ignition coil. The camshaft sensor is responsible for injector timing.

The battery cables should not get hot in such a short while. That could be a potential cause - you may want to perform a voltage drop test on that cable. Just a simple google search should turn you up with some good how-to's on that scenario.

Post by **JRL** » 30 Aug 2011, 18:55

If your cable is in poor condition and is hot ... change it ASAP and I'll bet your problem will be solved

Xpletive · Post by **Xpletive** » 30 Aug 2011, 19:01

jblackburn wrote:Just so we're clear here - and not to make you feel like an idiot or anything - just for clarification since people mix the two up all the time, which sensor did you replace and were disconnecting?

No worries i know they are easily mixed up. I replaced the crankshaft/RPM sensor that sits in the top of the transmission housing, and then disconnected the camshaft sensor to test how the car would react.

jblackburn wrote: The battery cables should not get hot in such a short while. That could be a potential cause - you may want to perform a voltage drop test on that cable. Just a simple google search should turn you up with some good how-to's on that scenario.

I'll try the voltage drop test in the morning.

JRL wrote:If your cable is in poor condition and is hot ... change it ASAP and I'll bet your problem will be solved

Cable is in good condition but i noticed it was a bit warm.

Post by **Fish stick88** » 30 Aug 2011, 19:18

Its got ADD or something. hahah LMAO, sounds like voltage problem to me too, i'd think you would notice a dimming in the lights if it was, maybe im wrong with the V70