Tach Reads 1/2 Speed

[ulrace]

91 ls400 Runs perfectly when dead cold. When it warms up (just a few blocks of driving) It starts to miss of and on when at a steady speed. under acceleration it runs great.

The tach seems to read about 1/2 the actual rpm all the time. Needle jumps when the miss starts. Anbody have any ideas? I have a code 13 . New plugs and wires already.

thanks

bob

[aarman4]

91 ls400 Runs perfectly when dead cold. When it warms up (just a few blocks of driving) It starts to miss of and on when at a steady speed. under acceleration it runs great.

The tach seems to read about 1/2 the actual rpm all the time. Needle jumps when the miss starts. Anbody have any ideas? I have a code 13 . New plugs and wires already.

thanks

bob

Code 13 is RPM signal #2

Now, this is a complete guess because I am unfamiliar with the Tach system, but if it is RPM signal #2, and there is a code(12) for RPM signal #1, then there must be two RPM sensors... This is where I start stepping out on a limb... If one of them has failed, it may be possible that it is only reading the RPM with one sensor giving you half a reading.. I am SO sorry if this is bass ackwards of a thought.. I am not sure where they are located, but if they are easy to access, then I would imagine that you could possible switch them around... Clear your check engine codes, and then restart your vehicle... If you get a CEL code 12, then you need to replace the RPM sensor... If you get a code 13 again, then you probable haave a problem with whatever is trying to be read by the RPM #2 sensor.

[ulrace]

91 ls400 Runs perfectly when dead cold. When it warms up (just a few blocks of driving) It starts to miss of and on when at a steady speed. under acceleration it runs great.

The tach seems to read about 1/2 the actual rpm all the time. Needle jumps when the miss starts. Anbody have any ideas? I have a code 13 . New plugs and wires already.

thanks

bob

Code 13 is RPM signal #2

Now, this is a complete guess because I am unfamiliar with the Tach system, but if it is RPM signal #2, and there is a code(12) for RPM signal #1, then there must be two RPM sensors... This is where I start stepping out on a limb... If one of them has failed, it may be possible that it is only reading the RPM with one sensor giving you half a reading.. I am SO sorry if this is bass ackwards of a thought.. I am not sure where they are located, but if they are easy to access, then I would imagine that you could possible switch them around... Clear your check engine codes, and then restart your vehicle... If you get a CEL code 12, then you need to replace the RPM sensor... If you get a code 13 again, then you probable haave a problem with whatever is trying to be read by the RPM #2 sensor.

My best guess is that the tach reads from the cam sensors which are located behind the distributor rotors. These are not easy to access as you have to get down to the timing belt with the cover off the front of the engine to access the caps. If there is an easier way someone please help? i am going to try unplugging each sensor and checking codes then. Thansk for the idea; we are on the same track.

bob

[aarman4]

91 ls400 Runs perfectly when dead cold. When it warms up (just a few blocks of driving) It starts to miss of and on when at a steady speed. under acceleration it runs great.

The tach seems to read about 1/2 the actual rpm all the time. Needle jumps when the miss starts. Anbody have any ideas? I have a code 13 . New plugs and wires already.

thanks

bob

Code 13 is RPM signal #2

Now, this is a complete guess because I am unfamiliar with the Tach system, but if it is RPM signal #2, and there is a code(12) for RPM signal #1, then there must be two RPM sensors... This is where I start stepping out on a limb... If one of them has failed, it may be possible that it is only reading the RPM with one sensor giving you half a reading.. I am SO sorry if this is bass ackwards of a thought.. I am not sure where they are located, but if they are easy to access, then I would imagine that you could possible switch them around... Clear your check engine codes, and then restart your vehicle... If you get a CEL code 12, then you need to replace the RPM sensor... If you get a code 13 again, then you probable haave a problem with whatever is trying to be read by the RPM #2 sensor.

My best guess is that the tach reads from the cam sensors which are located behind the distributor rotors. These are not easy to access as you have to get down to the timing belt with the cover off the front of the engine to access the caps. If there is an easier way someone please help? i am going to try unplugging each sensor and checking codes then. Thansk for the idea; we are on the same track.

bob

If you switch(not sure if they will reach) but if you switch the electrical connectors you may accomplish the same thing as switching the actual sensors... Not sure if this is viable though for the length of the electrical connections... Let me know if/when you have any results!

[lexus082]

I also have an improper tach signal. What does your tachometer read at idle and at cold start. My tach reads 750 at cold start idle and then drops to like 200-400 when its really around 700 rpms. Like the question was already asked does the tach signal come from the camshaft sensors?

I looked in my parts book and there's nothing listed as a camshaft sensor. There's only crank position sensors listed.