Jump to content

Walk through of cold start circuit (+ status of new boards)



<TL:DR - the new board has been built and appears to work as designed! New post coming soon!>


I've learned a lot during this project. While the goal has always been to build a reproduction board, I also wanted to better understand the cold start circuit itself. Questions I wanted answered include: How do the components work together? What is the sequence of events? And eventually - is there room for modification?


I've found that the system behavior is different than my expectations. This was a fun part of the project. 


Here is a reference diagram of the test harness with labels to help with the visual.



My assumptions going into this project:

  1. Cold start relay provides, well, timing. And the timing is based on temperature of coolant.  (<-- 2.15.20 - Incorrect! The "system" provides timing, the cold start relay is just the relay. Timing comes from the thermo-time switch only).
    • Colder coolant = longer fuel spray (via cold start injector)
    • Warmer coolant = shorter fuel spray
    • Minimum fuel spray of 1 seconds, maximum fuel spray approximately 15 seconds
  2. Thermo time switch provides coolant temperature info through wire resistance (based on temperature.) (<-- 2.15.20 - Incorrect! Only timing, which is communicated through the presence of ground through TK)


After building the test harness and working with the system, the reality that I believe to be true is:

  1. Cold start relay energizes and de-energizes a relay that controls (off/on) the cold start injector
    • Relay state is based on two inputs - 12v on pin 50, Gnd on TK.
    • !! --> Huge revelation: Timer box does not provide "timing"  <-- !!!
  2. Once key is turned from "Run' to "Start", the relay is energized.
    • 12v is routed to the temperature time switch (to heat the bi-metallic strip)
    • 12v is routed to the cold start injector (to activate the cold start injector)
  3. Once the temperature time switch has reached "operating temp"
    • The cold start timer box relay opens and 12v is no longer sent to the cold start injector (stopping the flow of fuel)
  4. The temperature time switch provides all timing information.
    • The internal bi-metallic strip is normally closed, providing a ground back to the cold start timer box.
    • The bi-metallic strip heats with 12v (sent from the cold start timer box). The time it takes for the strip to bend (and breaks the ground sent to timer box) is the duration of the spray.


Cold start timer box reference diagram:



Here is the play by play of activity on the six spade connectors based on three "states" of the car electrical system. These states represent the average workflow of starting a Tii.  These are: turn key to "Run" (even briefly), turn to "Start" (until it engine engages), turn back to "Run" once it starts.


No key in ignition

  • 31 (Chassis ground) - Yes
  • 15 (12v received from ignition switch) - No
  • SV (12v to cold start injector) - No
  • 50 (12v from ignition switch during start) - No
  • TK (Ground received from temperature time switch) - Yes
  • TH (12v sent to temperature time switch) - No


Key is turned to "Run" (not "Start")

  • 31 (Chassis ground) - Yes
  • 15 (12v received from ignition switch) - Yes
  • SV (12v to cold start injector) - No
  • 50 (12v from ignition switch during start) - No
  • TK (Ground received from temperature time switch) - Yes
  • TH (12v sent to temperature time switch) - No


Key is turned to "Start"

  • 31 (Chassis ground) - Yes
  • 15 (12v received from ignition switch) - Yes
  • SV (12v to cold start injector) - Yes ("timing has begun, spray fuel to intake manifold")
  • 50 (12v from ignition switch during start) - Yes ("starter has engaged")
  • TK (Ground received from temperature time switch) - Yes
  • TH (12v sent to temperature time switch) - Yes ("bimetallic switch is heating up")
    • I assume the "H" stands for "heater"


Here is a video using an original cold start relay:


Miscellaneous observations of the circuit from the test harness:

  • Fuel is injected for ~1 second, no matter the engine temperature.
  • Cold start relay does not know when the engine has started, only that the starter has been activated. This action kicks off the timing circuit for spraying fuel to intake.
  • Cold start injector will spray for the time determined by the temperature time switch, regardless if you hold the key in "Start" for >20 seconds. In other words it won't "keep spraying".


Tii starting observations:

It is recommended (and been said before on this board) for Tii owners to pause in the "Run" stage prior to moving to "Start". This is wise in that it permits the fuel pressure to build from electric fuel pump. However, if you wish to spray fuel prior to starting, it would be necessary to "nudge" the starter and then turn the key back to "Run" and wait. No spray will happen without the starter circuit engaged.


Other behaviors:

  • Turning the key to "Start" and then turning the car over lets say for 5 seconds, then turning the key off and then back to "Start" again rapidly will reset the cold start timing circuit and again spray 15-20 seconds of fuel if the engine coolant is very cold.
  • Turning the key to "Start", letting starter engage, then "Off", then "Start", letting the starter engage, then "Off" (admit it, we have all done it ?) will continue to spray fuel into the plenum and so will likely speed up flooding. There isn't a mechanism to stop flooding from happening in the circuit.
  • One interesting thought is that if you have a well sorted Tii that starts nearly instantaneously, the cold start injector will continue to spray fuel into the plenum for possibly 20 seconds in very cold weather. Again, there isn't a mechanism for the cold start to register that the engine has started (and thus stop spraying fuel).


So, where am I with the new boards?

On track, more or less --> ? A new board has been soldered up and tested in the harness. The new board appears to have the same behavior as an OEM board. Positive results. 


<12.1.19 Edit - I will run another set of tests using a automotive battery to ensure the board works under real-world conditions, results posted here>


However, I have to adjust some of the pin locations on the circuit board schematic because I am using a modern equivalent relay vs the original (and surely NLA old school relay). Once I have finished the new board design I will send it out for printing. Printing takes about a week. My last question mark are the spade connectors, which I hope to finish while the new boards are being printed (likely to be bent by hand). Takes time to get it done (right) but my target is to have it ready by holiday time here in US. Possibly have the boards "ready to ship" by New Years.


Can then work out the rest of the details. I'm stoked that its working - been a long time in development (since August 2018).


More when I have it! 

  • Thanks 3


Recommended Comments



I've tested the board on a 12v automotive battery and no problems surfaced. Really feel good about it.


I'm also nearly done with the daughterboard design for the spade connectors. A bit more complicated upfront but should be all around better on the back end. See below.

Wanted to let you know there is progress ?






Have an idea for a custom timer box now that I have found that the thermo time switch controls all timing. Also, perhaps a 3D printer is in order to build custom housings for the new boards. 

One thing at a time - first finish the existing boards. Stay tuned!

Edited by JsnPpp
  • Like 2

Share this comment

Link to comment

This part is taking a bit longer than I had hoped but progress being made - not long and I'll send out the new PCB design to be fabricated and bob should be your uncle.


I chose to build a daughterboard using stock long male spade connectors instead of fabricating the original spade connectors individually. I like using modern hardware wherever possible and it will still retain the essence of the original. (I still want to do it however).




Measuring and layout takes the most time. Very circular process - measure, edit, print, test (and then repeat over and over).


The good thing is that the new board works, this is just the mechanics. I'm on break for the holiday week and should get it out for printing shortly.







Edited by JsnPpp
  • Like 1
  • Thanks 1

Share this comment

Link to comment
Add a comment...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.