Re: <E30> Thermo-Time Switch

[Norman Boehl <nboehl@xxxxxxxxxxxxxxxx>]

I just went through this myself, so I'll offer what I learned (see below).

Thaddeus Wakefield Batt wrote:

> Hey Folks,
>      After replacing a faulty cold-start valve and performing Bentley and
> Hayes testing procedures, I have deduced that as well as having a bum
> cold-start valve, I've got electrical problems.
>
>      Grounding is supposed to occur through a black/red wire on both the
> thermo-time switch and the cold-start valve.  Problem is it doesn't.
> Bentley says this means faulty thermo-time switch - not grounding through
> the thermo time switch housing.  However, I am able to actuate the valve
> by splicing into the blck/red wire with a lead directly to ground.
>

The cold-start valve should ground through the thermo-time switch only,
as per the Bentley manual.  However, Bentley says the thermo-time switch
should activate the cold-start valve for 3-8 seconds (or thereabouts).
If you read the "fine print" this occurs with coolant temperatures below
somewhere around 90 degrees F (or therabouts).  Bear this in mind when
checking the switch.  If your coolant is above this, the thermo-time switch
will not provide the ground.

In addition, what Bentley doesn't tell you is that while the switch is activated,
the current flow is providing heat to the heating element in the TT-switch,
thus providing the activation time cycle of 3-8 seconds.  When the heating
element reaches whatever designated temperature, the switch opens, thus
deactivating the cold-start valve.  This time period that the switch remains
closed depends on how long it takes for the heating element to warm up
(thus the variable activation time).

In addition, I found that once the TT-switch/heating element had cycled
(from on (continuity-to-ground) to off (no continuity-to-ground)), the switch
would not cycle again until I allowed the TT-switch/heating element to cool
down (in my case, about 3 minutes).  Thus, once I had cycled the switch, I
could not see a ground circuit  through the TT-switch until I waited about 3
minutes.  After that time, I could once again see continuity through the TT-switch
to ground.

I performed my testing by removing the connector from the TT-switch altogether
(thus isolating it from the cold-start valve), and by applying +12V directly to
the appropriate terminal of the TT-switch.  If I remember correctly, this is the
terminal that is normally connected to the "start" signal (see your schematics).
I cannot remember the wire colors right now.  The other terminal is the one
that is supposed to provide the cround circuit to the cold-start valve.  I connected
an ohmmeter to this terminal, with the other lead of the ohmmeter connected to
the chassis of the vehicle.

The reason I did all this was because I thought I was experiencing cold-start
problems on hot afternoons in Houston a few weeks back (100+ degrees),
while having no cold-start problems in the mornings (70-80 degrees).
As it turns out, my testing was done in the garage under 90 degree conditions.
Under these conditions, the TT-switch remained closed for only about
2-3 seconds (not 3-8 seconds).  I deduced that all was fine, and that the
TT-switch remains closed for a period of time that is relative to the
ambient temperature (actually the sensor temperature) at the time at
which the start signal is first applied.  Thus, for higher ambient temps
(assuming "cold" coolant temperature), the shorter the activation cycle
of the cold-start valve.

>
> The question is, should I?  Does the time switch cut the circut
> through + or through - ? Would this (admitedly cheap) approach allow
> the cold start valve to function as designed, or would this simply actuate
> the cold start valve for as long as the ignition is actuating the starter
> regardless of the coolant temp?  And, if it did do that, would
> that be bad?

See above.   Do not short-circuit the cold-start valve "ground" signal
directly to ground (or at least do not leave it connected).  If you do,
the cold-start valve will provide fuel the entire time the valve is
grounded (i.e., during the entire start cycle).  This might provide too much
overall fuel.  I'd really have to double-check the schematics to be sure.
I know that the TT-switch heating element lead comes from the "start"
signal, but I'm not sure about the cold-start valve power lead (it almost
has to come from "start" as well).

>
>
> And finally, could the lack of Earth have led to the detruction of
> the old cold start valve?  (ie, should I disconnect the plug from
> the new valve until I  a.Get it Grounded or b.Replace the thermo
> time switch?)
>

I doubt it.  In this case, the cold-start valve circuit would simply
be "open" at all times (just like it is during normal engine running
conditions).

You might attempt the same tests as I did to see if the TT-switch is
indeed defective.

I apologize for the long-windedness, but I hope this helps.  Email me if
you have any questions, doubts, criticisms, etc.

Norm


>
> TIA,
> --thad
>
> Thaddeus Wakefield Batt
> Internet Software Engineer (Electrical Engineer ... Not Even Close)
> thaddeus@domain.elided
> BMWCCA Boston:  #1147928
> 1986 325es
> 1975 2002A for sale!

- --
***  Remove "nospam-" from address when responding via email  ***

------------------------------