Thermo-Time Switch question

[outsider347]

79 911 3.0
I am resembling my engine, and I'd like to test the sw. I have the Bosch K Jet workshop manual, but no resistance values are given for the 79

At 59 deg F. I get a 25 ohm
At 90 deg F I get 40 ohm read


Comments please
Tks

[ahh911]

Looking at my 81 SC notes:

Room Temp:
G to GND 24 Ohms
W to Gnd 0 Ohms
G to W 24 Ohms

Hair Dryer, hot:
G to GND 61 Ohms
W to GND 100 Ohms
G to W 40 Ohms

If you measured G to W it reads like your measurements.

Phil

[outsider347]

Phil
My engine is still on the stand....
I assume that "G & W" are the colors of the wires. Correct?
I measured on the terminals of the sw.

Thank You

[ahh911]

I don't remember..

What I can add though is:
VDO
2.81
45degC
12v/40w
36/3/4
91161711700

Markings on the device.

I can check, but you should be able to figure out w and G.
Looking at Bentley, W= red/black and G=Yellow.
I guess there were markings on the sensor itself showing W and G.

Phil

[Ginger77]

I'd like to revive this thread because I'm having issues testing my Thermo time switch. And my CIS is experiencing issues that I think might be related to the switch.

My old switch has the exact same markings as Phil posted.

The Bosch manual states the following values and mine are in bold:
Below 104F
G-Ground 30-40 24
W-Ground 0 0
G-W 30-40 24

Above 122F
G-Ground 55-85 60
W-Ground 0 98
G-W 55-85 40

In other words values identical to Phil's but not in spec. Since I'm having issues I bit the bullet and purchased the $200 replacement.



However, I'm not getting any resistance from this new sensor. Is anyone familiar with it? It's a different manufacturer but I was really hoping to now have a switch that is in spec.

[cmcfaul]

put it in hot water and see if the circuit opens.

Chris
73 E

[ahh911]

Ginger77,

I thought my values were right. There were several different values for these resistances depending on exact models, but they were all ball park ok. I had the whole fuel injection system tested on it's own and I saw the cold start valve do the correct time for temperature injection. Basically stuck a endoscope somewhere and saw that there was no leaks, then hit it and saw the nice spray for the correct count.

hot
W-Ground 0 98

If you were expecting zero ohms for this one, I think that's a mistake. I drew out the simple circuit and believe close to 100 is where it should be. p.s. the car starts well, 98% of the time, over 1000 miles since the rebuild with plenty of next morning starts ranging from 5 degc to 18degc. One time a lean backfire, richened the fuel mixture as the frequency valve duty cycle was 57 and pretty sweet since.

Phil

[cmcfaul]

Can always bypass it and make it manual, like a choke. I did mine to work off the starter when I had a switch on. Worked perfectly.....just like a choke.

Chris
73 911 E

[boyt911sc]

Originally Posted by outsider347

79 911 3.0
I am resembling my engine, and I'd like to test the sw. I have the Bosch K Jet workshop manual, but no resistance values are given for the 79

At 59 deg F. I get a 25 ohm
At 90 deg F I get 40 ohm read


Comments please
Tks

Outsider,

This is how I test the thermotime switch (TTS). See pictures below.

A continuity tester or a test light is a good tool to test the operation of a thermo-time switch. Using a 12-volt battery, a test light, and a couple of wires to connect the battery terminals to complete the circuit provide a good way to see and observe the light comes on and off after a couple of seconds. Connect the positive (+) terminal of the battery to terminal G and the negative terminal to the ground, in this case to the chain housing. The test light for illumination demonstration is connected between terminals G & W.



I use two (2) different test lights, the fancy one above and a generic test light from Harbor Freight. The HB test light gives a better and brighter illumination appropriate for the demo.



A good TTS will lit up and turn off after a couple of seconds. A defective one will lit up and stay on for a long long time or will not lit up at all. I don’t bother to measure the resistances of the TTS. My interest is to determine if the TTS is good or defective.

How do you test a light bulb? Use a known good light bulb and test it. Then plug the light bulb in question. Same way to do with a THERMO-TIME SWITCH.

Tony

[Flat6pac]

The TTS is part of the cold start circuit
It’s activated off the starter to inject cold start fuel
Bruce

[gled49]

+1 what boy911 says. Forget resistance. System needs to complete a circuit for a few seconds during cold start.