Jump to content

Troubleshoot a Tii Cold Start Circuit

Troubleshooting your Cold Start Circuit

The Tii cold start circuit consists of several components. If you suspect the system doesn't work, a few basic troubleshooting steps with a voltmeter will help you track down the problem. And don’t be surprised if it isn’t always the cold start relay (CSR) - the biggest “aha moment” I had was realizing that the thermo-time switch (TTS) provided the timing, not the CSR. (Lesson learned - if the system isn’t spraying fuel long enough, check the thermo-time!).

Another example: My red Tii cold start circuit did not work for 18 years. After wire tracing I found that pin 50 on the CSR was not receiving 12 volts when the ignition key was turned to “Start”. On Tii’s there is a wire junction next to the fuse box and above the airbox, and on my car this junction was loose. A quick snap together and it was fixed. Of course, I already built the new board by then!

Do the basics, then you know where you stand. You may find it's a simple fix.

Step 1: Understand the Components in the Circuit

First step is to become familiar with the components. The four pieces of interest for the circuit are:

  • Ignition switch

  • Cold start relay (CSR) (27)

  • Thermo-time switch (TTS) (43)

  • Cold start valve (CSV) (30)

ra1NTdPuD7eHCKga9XdyLP0__ejVEGbWm4ZjeyBx

Image courtesy of online resources (believed BMW)

 

And here is the fuel spray timing curve, note that above 35 degrees Celsius the system will ALWAYS spray for one second is sprayed.

 

I7FnsGHiSsVEPH73fJ1nUTb7X6tx4vAlBm0RYOHy

 

Step 2: Determine the system behavior 

The first step is to verify there is a problem with the cold start circuit. If your Tii isn't starting, it could be any number of things (plugs, timing, valves, linkages, kugelfisher, etc.) Lets see if its really the cold start circuit first.

  • Build a test rig. Buy or build a simple 12v light on the end of two wires to enable you to see what is happening in the circuit when the key is turned on. Keep the wires about 12 inches long each. You can grab a simple socket and leads off of Amazon, buy a circuit tester, or dig around your parts box for an old taillight and attach a couple of wires. I built one with an led. The free ends should have wire exposed to connect to the existing circuit.

UFUfpm3G7M01uZdEQODC6OgBIVRUJ8d3iFuTSdW2

8nSeTXKnEqj_QDSBmPoc4_PWmHREyBSbXLOPl2Se

  • Disconnect the plug on the CSV (on the intake manifold) and plug this light in its place. If using an LED, make sure the Positive and Negative are oriented properly. The wires are small and so you may have to be creative. I used an old spade connector to allow the wires to hold in place while I tested.

  • Turn the ignition key from Run to Start (briefly, just kick it over, don't start the engine. You can also disconnect the coil to ensure it won't start). A properly running cold start circuit will have a light that turns on for a length of time related to the coolant temperatures (same as the fuel spray). A minimum of 1 second no matter the temperature. If it works, then you have eliminated everything but the CSV itself.

  • If the light does not turn on, time to start digging in.

 

Step 3: Troubleshooting Walkthrough

To troubleshoot, you need to systematically trace all cold start circuit wires to ensure they are attached properly and have continuity from end to end. Then you will check that each wire receives the signal it should receive. At the end of this you will know where the problem lies.

  • Use a basic volt/continuity meter to check both ends of each wire in the circuit. You are looking for wire breaks here. A voltmeter is $8 from Harbor Freight if you don’t have one. One that beeps when continuity exists is nice.

  • The continuity checks are completed with the black plug disconnected from the CSR. You must insert one voltmeter probe in the black plug harness (note the orientation of the plug so that you know which pin you are measuring) and then the other to where it is going on the engine.

  • Check continuity between:

    • The wire between Pin TH at the CSR and the TTS

    • The wire between Pin TK at the CSR and the TTS.

    • The wire between Pin 31 on CSR and the CSV

    • The wire between Pin SV on the CSR and the CSV.

    • Check that the wire connected to Pin 31 goes to Ground

    • Check that the wire to Pin 50 connects to the starter (this is the trigger wire). This should also receive 12v when the key is turned to “Start”.

 

If all ok, on to signal checking.

  • Disconnect the wiring harness from the cold start relay (remove the black plug). Use the voltmeter to check each pin on the harness (not the cold start relay side) only this time you are checking signals.

  • You will likely want a helper with this to turn the key so you can more quickly check signals. Use the table below to understand whether ground or 12v is present on each wire at all phases.

 

Harness Pin

Signal

No key

Key is turned to "Run"

Key is turned
to "Start"

31

Chassis ground

Ground

Ground

Ground

15 

12v received from ignition switch when car is running (i.e. Key is in “Run” or “Start”)

0 volts

12 volts

12 volts

SV

12v sent to cold start injector during cold start. 

 

Once key is turned to “Start”, it will send 12 volts for a time interval based on coolant temp.

0 volts

0 volts (at first) or 12 volts (after key turned to “Start”)

12 volts.

50

12v received from ignition switch when key is turned to “Start”.

0 volts

0 volts

12 volts

TK

Ground received from TTS (Ground will be removed when car is at normal operating temperature).

Ground or None

Ground or None

Ground or None

TH

12v sent from CSR to TTS during the cold start. The TTS ground signal will turn this off.

0 volts

0 volts

12 volts

 

If all of the circuit above is working as designed, you will need to explore whether the thermo-time is working properly. There are ways to test it - at a minimum check that the housing and one of the pins has continuity. Set the voltmeter to continuity check and hold one end on the metal housing and then touch one of the pins on top, then check the next pin. Only one will have continuity. There are other tests as well - search the Faq for more info (I'll add more here too).

 

More information on Cold Start Relays are in my blog --> https://www.bmw2002faq.com/blogs/entry/1980-walk-through-of-cold-start-circuit-status-of-new-boards/

 

Good luck and I hope this helps you troubleshoot your circuit!

 Share


User Feedback

Recommended Comments

There are no comments to display.



Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
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...