RPM source and stuck in first

[Jeff753]

This morning I started up and the tach showed 1400 RPM's at idle even though it was about 600. I drove it and it wouldn't shift out of 1st. I drove about 1/4 mile and the RPM's on the tach remained 800 high. Finally it started shifting and drove normal but the tach remains 800 high through a forty minute drive.

I checked the ohms on the two Allison 3000mh sensors and they were both 336 and 338. The motor was still a little warm. I thought I read the output sensor should be about 300 ohms??

Any ideas on why the tach is suddenly high and do the sensors ohm value sees high at 336 and 338?

[dbarton291]

Starting in January 2006, the speed sensors used in the 3000 have a spec of 340 ohms at 68 degrees F. I wonder if the transmission thought it was cold and was restricting shifting out of first. May be a thermistor issue. What was the ambient temperature at the time?



The transmission doesn't have anything to do with the readings on the engine tachometer.

[Jeff753]

Originally Posted by dbarton291

Starting in January 2006, the speed sensors used in the 3000 have a spec of 340 ohms at 68 degrees F. I wonder if the transmission thought it was cold and was restricting shifting out of first. May be a thermistor issue. What was the ambient temperature at the time?



The transmission doesn't have anything to do with the readings on the engine tachometer.

It was about 65. Since my tach was reading 1000 RPMs high (and still is) I wonder if the engine was starting to derate and not allowing the transmission to shift?
Does the tach get its data from the engine ecm which gets it from the engine speed sensor?

It finally shifted (after about a 1/4 mile and shifted normally for that short 45 minute drive.
Tach still reads high.

[dbarton291]

The engine tachometer is normally driven by an output from the engine ECM. Its input is an engine speed sensor. Some older vehicles drive the tach directly from the alternator or another system on the engine.

[Jeff753]

Originally Posted by dbarton291

The engine tachometer is normally driven by an output from the engine ECM. Its input is an engine speed sensor. Some older vehicles drive the tach directly from the alternator or another system on the engine.

I have a 2001 ISC. I thought the tach was driven by the grey wire on the alternator but that might just be the voltage regulator. That grey wire goes to a relay on the low voltage box in the rear.

I'm thinking now it must be derived from the engine speed sensor since it wouldn't shift out of first gear.

[Jeff753]

Just for kicks I replaced the alt fail relay as it appears the tach wire comes from the alternator sense wire. Tach was still high but with it running I was able to rotate the grey sensor wire on the back of the dash and bingo. Rpm normal. I had wiggled it before but it wasn't running. So I loosened the connections sprayed them good with Deoxit and tightened them. Did my speedo too because it's usually 10-15% high. Will see if that made a difference Friday when I drive it.
I don't know how a loose tach gauge would cause the Allison to stick in first though. I replaced the output speed sensor and will see if it shifts Friday.
The old sensor ohmed at 300 at 65 degrees so it's probably not bad. FYI the new speed sensors since 2006 I believe ohm at 340 at 65 degrees. The new 340 ohm design is interchangeable with the old 300 ohm design. The input and output sensors are the same part. The turbine speed sensor is different. About $90 each.
So tach issue resolved.
My silverleaf cables are enroute .