Obatining a CSV Micro Switch

[AusPorscheGuy]

Hi guys

It appears that the cold start valve micro switch on my 74 911 is not working. Basically the valve works when I bypass the switch but not when the switch is in the circuit. Does anybody know where this particular part can be sourced from ? PS I'm in Aus so I am expecting this to be challenging ....

Any help greatly appreciated.

Cheers Daren

[steely]

Can you test the switch itself with an ohmmeter to make sure it is the culprit?
if it is part number 90161310600, they can be obtained from our host ( but, boy are they dear).
I have to think there is a much more reasonable substitute out there.

[AusPorscheGuy]

Thanks... Checked the switch resistance when closed. It is about 20 ohms ..Infinite when open. Not sure what the correct closed resistance should be.

D

[E Sully]

Sounds like the micro switch is working. You just need to set the gap at the throttle body. With the hand lever lever all the way down, the micro switch is open and prevents the cold start valve from operating.
Proper cold starting procedure is with the lever up, which closes the switch. Then when the starter is engaged, current will go through the micro switch and TTS to the cold start valve.

[AusPorscheGuy]

Thanks Ed

I guess that's what has got me confused. I have removed the microswitch and the bracket from the side of the throttle body and the CSV as well in order to test. When I crank the car the CSV doesn't spray with either the switch in the open or closed position. When I take the switch completely out of the circuit and jumper the terminals the CSV sprays. So I assumed the switch was bad although the resistance values would suggest otherwise.

D

[steely]

In a perfect world, the 20 ohms should be closer to zero, but I agree with Ed that the switch seems to be working. (Another) But, it obviously isn't doing it's thing in circuit, possibly from contact corrosion, so maybe the 20ohms is enough to steal some of the voltage away from the CSV. If the CSV solenoid is supposed to be 40 to 75 ohms (check me), then the 20ohms in the switch could rob the CSV of 2 or 3 volts, just low enough for the CSV not to energize when you're using just the battery to get things going.

Can you measure the voltage if this is safe (and don't feel obligated to do so):
1. across the open switch? (should be close to battery +12V (I think)
2. across switch when it is closed? ( should be 0Vdc but you might see 2 or 3V)
3. At the CSV when switch is closed?
4. At the CSV when the switch is jumpered?
5. battery Voltage

[AusPorscheGuy]

Thanks for all the help..

I know the voltage to the switch is low at 9.8V. I was expecting 12V. I'll check the voltage at the CSV and get back to you.

D

[E Sully]

Have you tested the Thermal Time Switch to make sure it is working? This could also prevent the cold start valve from working.

[AusPorscheGuy]

Not yet Ed, but I assumed that as the CSV was working with the microswitch bypassed that the Thermal Time Switch would be working.

Daren

[AusPorscheGuy]

OK guys so I had a chance to measure a few more voltages.

Battery before cranking 12.8V
Battery whilst cranking 11.4V
Voltage at Microswitch terminals when cranking 8.6V
Voltage at CSV with Microswitch closed and in circuit when cranking 5.6V
Voltage at CSV with Microswitch bypassed when cranking 8.4 Volts

So I am thinking that whilst the switch still works the internal resistance is high enough to rob the CSV of the required voltage to work. Does this sound correct ?

Cheers Daren

[Tony V]

Daren,
Something you may want to check and may be unrelated to your issues.
When I was having cold start difficulties, I finally found that the WUR wasn't getting any power from the alternator. Also a '74 911.

[AusPorscheGuy]

One thing that would really help is if anybody knows what the resistance of a working microswitch should be ?

Cheers DAren

[E Sully]

The micro switch should not have any significant resistance, it's just a basic switch.
It may have pitting or corrosion on the contacts if you are getting that much resistance.

[steely]

I agree with Ed again, the switch resistance should be low, less than 0.5 ohms.
If I understand the ckt, you have the battery, then the starter, then the switch, then maybe the thermo-timer, then the CSV.
You have a 3volt difference between the batt (11.4) and the CSV (8.4), even with the switch bypassed.
If the starter is in this path, and if this drop isn't normal (due to line age or some dirt on the starter contacts), you pull a bit more current and get some gradual problems with the switch contacts. Even though your CSV still worked (I think you said) in this 'mode'.
You could try to check the starter lug for corrosion and maybe recover some voltage, enough to get the CSV to work again even with the switch as it is, it would buy you some time while researching a replacement switch.

With the switch not by-passed, your CSV voltage dropped from 8.4 to 5.6, or 2.8, which I think is the voltage 'across' the switch itself. In a closed switch, with near zero ohms, this voltage should be less than 1 volt.

so yes, I agree with you, the sw is a problem (long way around - sorry), and I don't know if you can get some contact cleaner inside of the micro sw.

[AusPorscheGuy]

Dan, Ed

OK so I decided to try and take the switch apart. Unfortunately the case broke in half when doing it. BUT when I looked inside there was a LOT of corrosion on the terminals {see second pic). After cleaning the microswitch resistance dropped to 0.2 Ohms. I'm currently trying to epoxy the case back together with JBWeld, but I think I have found the problem !. Thanks for your help guys.

[dtw]

While you're at it, you can check the output voltage from the thermo-time switch. I couldn't get my CSV working on my '72, even though all the individual components seemed to be working. Ended up with low threshold voltage on my TTS. It was around 1/10 of a volt too low to trigger the solenoid. So close! I tried the usual suspects - cleaned the grounds and terminals, swapped out the crush washer, etc. In the end, the switch was just bad. I swapped it out with another one and my '72 has been firing up on the first crank ever since.

[steely]

Hi Daren - darn, a heckuva way to prove a theory - too bad it got sacrificed. I was thinking of you earlier and started hunting for micro switches from commercial sources and was going to report back, and now I saw your pictures. Good luck with the JB weld. maybe test it immediately before it cures to make sure none got inside.

here is just one source, but I didn't know the form factor of your switch - this is a US source, but maybe it'll be of interest. Switches - Sensors, Switches & Relays from Allied Electronics - Page 3 (there are many pages / >1300 types not sort-able by "microswitch' unfortunately.)
I think most of them would be half of the cost of an original, and would withstand up to 80C.


Do you have the part number or a picture of your sw when it was intact?

[Matt S]

I just solved a cold start problem on my '74 by replacing this switch. The old one measures 25 Ohms and a brand new equivalent is more like 0.1 Ohms. My car would crank forever without starting, but now I barely need to turn the key!

I found a generic "limit switch" that has the correct dimensions and mounting holes, so it bolts straight on. I needed to shorten the lever, which was easy to do with wire cutters and a file.

If you need an affordable replacement, or even just a second switch for troubleshooting, search for the model number Z-15GW-B.

Daren, if you still need one, they're about $25 here in Australia.