Easy HTS question:

[WI wide body]

The Head Temperature Sensor cable goes from where it screws into #2 cylinder thru a rubber grommet and then into the engine compartment. Looking at it with the driver rear wheel removed, what are the two black lines that go thru the rubber grommet along with the HTS cable?

[Bill in OKC]

Those are the crank position sensor and flywheel speed sensor wires -> 3.2

[WI wide body]

Thanx Bill. Yeah, I should have mentioned it...but it is an '86 with the 3.2 engine.

What exactly is their purpose...does either of them feed into the DME brain under the seat? If not what do they do?

[Bill in OKC]

They do send signals to the ECU. They run up through the firewall alongside the CHT lead and connect to the bracket on the top driver's side of the engine. Most EFI engines have something similar - the crank position is a master signal that tells the ecu the start of a revolution and the speed sensor breaks the revolution into a bunch of smaller indexes. It gives the ecu an idea when to fire the injectors and plugs from counting the flywheel pulses past the index mark. There are a few threads on here about what happens if one or the other goes bad. I found out the hard way that if the speed sensor goes bad, the ecu won't enable the fuel pump.

[WI wide body]

I guess that I've just been lucky with all the older 911's that I've owned. Never had any of those two sensors go bad. Did replace the Head Temp. Sensor on an '85 Carrera.

The bracket that you mention where those other two connect...is that the same bracket where the HTS is the top one of the three?

Actually, I suspected that the O2 sensor cable was probably one of the connections below the HTS cable connection. Changed an O2 oince, but I just spliced the wires to the O2 gizmo above the Cat. converter and didn't go any further.

Just curious, but how difficult is it to replace those two? Compared to the O2 or the HTS...which are relatively easy.

[Bill in OKC]

It is harder than the o2 sensor, I've not replaced my temp sensor but it is probably along that difficulty. Removing the big grommet was the hardest part for me - then threading the wires up through the firewall took some patience. If you get the rear wheel off it is not too bad to reach - seems like I had to do a lot of 1/8 turns then repositioning the allen wrench to get the screw out. I had read about difficulty getting the sensor out so I squirted some silikroil on it and worked it out nice and slow. I just replaced the one and a month later the crank sensor died so I had to do it all over again. It has been a year now and everything is great.