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Doug Siegel
Join Date: Oct 2007
Location: Bloomington, IN
Posts: 314
cold start valve

(88' 930) Does anyone know with absolute certainty whether the cold start valve/injector allows fuel through it at all times when the thermo-time switch is disconnected?

Old 02-25-2008, 02:50 PM
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Crotchety Old Bastard
RarlyL8's Avatar
Join Date: Jan 2000
Location: Huntsville, AL
Posts: 14,357
I pulled my Bosch manual to verify what we discussed today. According to the manual which is specific for the early US/Euro 930s, the thermo time switch and the cold start injecter do indeed run in series.
That means the thermo time switch is aptly described; it will break the electrical connection to the cold start valve at both a preset time and temperature.
Should the switch fail in the closed position (least common) the cold start valve would remain on. Should the switch fail in the open position (most common) the cold start valve will not operate.

I cannot say that all 930s are wired this way but it would make sense that they are. I see no reason for the cold start valve to have its own power supply independant of the thermol time switch (unless used as a 7th injector, but that is a whole different ballgame and does not apply to your situation).

Others please chime in.
RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 02-25-2008, 09:23 PM
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Join Date: Jan 2008
Location: Birmingham, AL
Posts: 538
No, the Cold start valve does not let fuel through it at all times unless it is faulty.

If the thermotime switch is unplugged the coldstart valve should not work at all, unless...

The red/black? (non-yellow) wire to the thermo-time switch is touching a ground on the engine somewhere or shorted to ground in the harness.

The way it works seems complicated at first but it's not too bad really.

Obviously the coldstart valve needs positive and neg. battery voltage to operate and these come from the starter and the thermotime switch.

Rarlyl8 is right about there being a connection between the two for the + side. This + power is supplied by the starter solenoid on a separate terminal than the one that triggers the starter.

Anytime the key is in the start position, this small yellow wire will receive + battery voltage from the starter. This power is then shared by the coldstart valve and the thermo-time switch.

The thermo-time switch only grounds the coldstart valve to trigger it under ideal conditions. There is a small heater circuit built into the thermo-time switch that heats up anytime the starter is turned. After just a few seconds this heater warms up and disconnects the ground from the coldstart valve

This is there to serve as a limiter so the coldstart valve won't flood the engine given the chance it is turned over and over repeatedly without cranking. hence the "time"

This little heater is also affected by the ambient temp of the engine as to not trigger when the engine is warm. hence "thermo"

I'll try to make this make more sense later, it's late I apologize...

Last edited by rsrmike; 02-25-2008 at 10:19 PM..
Old 02-25-2008, 10:16 PM
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Doug Siegel
Join Date: Oct 2007
Location: Bloomington, IN
Posts: 314
Thanks guys. I'm going to assume that the 18 degree day my car wouldn't start was not because of trouble with the WUR but the disconnected TTS. I'll also have to test the cold start valve to make sure it works. As far as the 900 rpm cold start, well, I'm gonna lean the cold pressure on the WUR (becasuse I initially richened it to compensate for failing cold start valve). Then I'll test vacuum. I'll be receiving the fuel pressure gauge soon. Thanks for the info. What would I do without the pelicanheads?!!!!

Old 02-26-2008, 08:14 AM
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