MFI RPM Sensor?

Jeff Higgins · 03-21-2005, 09:50 AM

There is a small sending unit, or sensor of some kind threaded into the crank case vent cover on the top of the crank case that looks like it is specific to MFI. It has two leads to it, what looks like a ceramic body and a brass base/nut that threads into the cover. The previous owner told me it was an "RPM sensor" that controls the fuel shutoff solenoid. Looking at my manuals, however, I see there is an "RPM transducer" on the electrical panel mounted on the left side of the engine compartment. This "RPM transducer" is supposed to run off a signal from the points or the coil. The shut-off system works, having done the checks in CMA to verify that, but CMA does not show where the RPM "sensor" or "transducer" is located. Just what is this thing on the vent cover?

targa911S · 03-21-2005, 10:01 AM

I believe that is the oil pressure sending unit. that or the oil temp unit. got a pic of that pup?

69911e · 03-21-2005, 10:20 AM

That is the thermo time switch which enables the cold start injection when the oil is cold. Many have changes this to a manual system by now.

Jeff Higgins · 10:43 AM

If I understand correctly, the thermo time switch would be running off of engine temp and feeding a signal to the fuel filter console. When cold, it allows fuel pressurized by the electric fuel pump to squirt into the stacks via the rubber hoses that connect from the backs of the stacks to aid cold start, like an accelerator pump on a carb. So if the leads to it made contact with the brass nut at its base, effectively closing the circut, would it cause fuel to be delivered through this circut? The reason I ask is that the car was intermittantly "hunting" at idle for awhile, and over the weekend, it progressed to a high rpm miss-fire, popping in the exhaust. When I notice these leads had rotated on their posts enough to make contact with the base, and corrected this, the problems went away. Here is an exploded view showing the vent cover in red that this threads into; sorry I don't have any photos right now. It threads in on the side opposite the spud for the vent hose, at about the same angle going the other way.

http://www.pelicanparts.com/catalog/images/engine/1-1-1-48.jpg

Quick edit: I see Pelican lists, but unfortunately does not have an image of, a thermotime switch that fits both MFI and CIS. It says it is on a chain cover, not the vent cover. Come to think of it, Wayne's book on rebuilding 911 engines uses a 2.7 CIS engine as an example. The vent cover looks like the red one in the above diagram, with no provisions for the thermotime switch. Are you sure that is what this is?

derek murray · derek murray's Garage

Quote:

Originally posted by Jeff Higgins
Are you sure that is what this is?

I have changed the thermotime switch on my car within the last year... and it is mounted in the crankcase breather cover (the red thing shown in your link).

targa911S · 03-21-2005, 10:58 AM

does it have brass "thumb screws" that attach the wires to the unit?

targa911S · 03-21-2005, 11:00 AM

so is this the sensor that enables the solinoid on the top of the fuel filter?

Jeff Higgins · 03-21-2005, 11:05 AM

Here we go, I found an exploded view that shows the part in question. It is part number 75 in this view. Thermotime switch?

http://www.pelicanparts.com/911/911_Parts/1969/engine_big.htm

derek murray · derek murray's Garage

My wires are connected with simple faston connectors. However, I'm pretty sure your's are for the thermtime switch as well (just with a different connector)

I had this tidbit of information in my MFI notes, which should be of some help. (The writer was Pelican 'flintstone')

Thermotime switch:
This switch allows operation of the cold start valve at engine temperatures below 45 Degrees C (113 degrees F) and then only for a few seconds during starting. Power is applied to the switchs internal resistor from the starter. The internal resistor heats a bimetallic strip which interrupts the current flow on the ground side of the cold start connection. After engine warmup, engine heat is responsible for holding open the bimetallic strip, so that the cold start valve will not energize when the starter is operated on a warm engine. Also, after several unsuccessful starting tries on a cold engine, the internal resistor will heat the thermotime switch and cold start valve operation will be inhibited until the switch cools.

Failure modes: An open circuit at the internal resistor will allow the cold start valve to operate an excessive amount of time if the engine does not start soon on a cold engine. An open circuit at the bimetallic strip in the switch will prevent operation of the cold start valve.

Early_S_Man · 03-21-2005, 11:28 AM

The round brass contacts that might be described as 'thumbtacks' are for 0.25" & 0.187" Faston connectors that MUST NOT be interchanged, as one [the 'G' terminal] is connected directly to the #50 Yellow stater solenoid lead from the ignition switch ... and grounding that lead can produce smoke from the ignition switch, not to mention that the starter won't engage!!!

The 'G' terminal is the heating element, and should measure 20 - 25 Ohms to the case ground.

The 'W' terminal is the switched ground, and should be open when the sensor is above 112°F.

Device #14 in the diagram below is the ThermoTime Switch. and device #52 is the cold start relay operated by the switched ground from the ThermoTime Switch.

targa911S · 03-21-2005, 12:11 PM

How DO you guys read these things..Chinese algebra to me.

Jeff Higgins · 03-21-2005, 12:12 PM

So would #12 be the cold start solenoid on top of the fuel filter console? What would be the effect of the "W" terminal on the thermotime switch intermittantly grounding to the base? Would it cause the system to activate and squirt fuel? I'm trying to get a handle on why my idle was fluctuating and the high rpm miss started happening. When I secured these connections so that there was no way to make contact with the base, the problems dissapeared. The connectors used on this are nothing special; they look like any other prong-type connector. They do rotate way too easily on the "thumbtacks" on the sides of the switch. I'm affraid it was re-wired at one time and the wrong connectors were used. It sounds like I was darn lucky the "G" connector was not the one that made contact.