Thermotime wiring --- Not so simple!

[Wade Nelson]

Went out first thing this morning meter and test leads in hand, to see if I could understand why my 1980 ZX is hard to start on cold mornings. Gonna start with the Cold Start valve and Thermotime switch.

First thing I did was pull the connector off and zap the Cold Start Valve (hereafter: CSV) with 12V. Nice healthy click. No dropping resistor needed, you won't fry your CSV by applying B+ (battery positive)

Next was to put a meter on the Thermotime (Tt) leads.

Since they're in parallel with the CSV leads, should be simple, right. Just use the contacts in the CSV connector instead of wrestling the Tt connector off.

(that's a trick question....don't fall for it!)

Well sure enough, one of the leads shows 60 ohms, CLOSE ENOUGH to the spec for the little heater inside the Tt switch.

Hurrah! The heating element isn't burned out!

The other shows continuity to ground. (1-2 ohms or less). As expected, COLD.

So next I apply 12V to the lead showing 60 ohms, same as it sees when you crank the car.

The other lead should go from continuity to ground to open circuit (OC) once it warms up, right?

Ten second, thirty seconds, it never opens up. Remains grounded.

So the little relay contacts are welded shut, right? Wrong.

Who's got a guess as to what's going on? Anyone???

going on....

When I unplug the Tt leads (bullet connectors) on top of the motor and test the Tt ALONE, in ISOLATION, it operates exactly as expected. After 5-10 seconds continuity goes away, which would turn OFF the cold start valve, exactly like you'd want it to. Let it cool down a minute and continuity returns. Conclusion: Tt switch is working.

So what's going on? Bad wiring shorted to ground, so the CSV is ALWAYS seeing a ground, even after the Tt has warmed up?

Well, that might explain why my OLD CSV failed. (I'll bet it ran very rich and got truly horrible mpg's before it did.....but let's not jump to conclusions!)

So I start cutting into the wiring harness, which is baked and brittle, to see if there's a short or melted wires or anything between the Tt connector and the CSV connector.

And lo and behold, it's not all as simple as you would think.

Not only do the green/black leads (starter signal) from both Tt and CSV connectors meet in a splice, with a third going back to the switch or fuel pump relay or whatever....

So does the Yellow /white wire. (Wire which warmed-up Tt ungrounds to stop CSV operation)

In the simplified diagram in the EFI bible, the Tt provides the CSV with ground, and that's IT! The two should meet and be done.

But not on the larger, EFI harness schematic.

The side of the thermotime switch that gets grounded ALSO meets in a 3-way splice with one wire going back to the ECM. But why?

The ECM could PERHAPS use this to ground the CSV whenever it chose to, for enrichment, say under heavy load.... or perhaps for monitoring purposes.... diagnostics...

Are you with me on this? If the Tt valve heats up, and ungrounds the CSV so it can't spray anymore, the ECM could, at it's choosing, CONTINUE to ground it / command the CSV to spray.

Apparently with Key off, Engine off, (KOEO) the ECM provides a ground to this signal, which is what I was seeing on the meter. Turning key on, same thing. (At least, since I'm not cranking, the CSV isn't spraying).

Apparently the ECM could use the CSV to enrichen the mixture under heavy load or acceleration if it so chose / if the programmers designed it to.

I may add a light on the dash to TELL me when the CSV is being commanded, to figure out if the ECM is, in fact, commanding additional spray at times other than cold start.

As usual, what I thought would be ten minutes of diagnostics turns into an hour, plus another .5 writing it up for other Z-owners.

If anyone has any insight into the CSV signal going back to the ECM, now's the time to share!

Otherwise, I hope this keeps YOU from misdiagnosing your Tt switch as bad.

Here's a "more correct" diagram.....Not gonna post the full EFI schematic...

[zKars]

Rather than a way to turn it on, maybe its a way for the ECU to know the CSV is on...

[Wade Nelson]

Rather than a way to turn it on, maybe its a way for the ECU to know the CSV is on...

But with no diagnostic codes (OBD) or check engine light, where's the BENEFIT to the ECU being able to monitor the CSV?

In contrast, at WOT, above XXXX rpm's, snapping on the CSV for additional enrichment might produce performance gains!

It's hard to get injectors with enough RANGE --- small enough pulses @ idle, enough flow @ WOT --- one solution is higher PSI, another might be cheating --- adding CSV flow for full throttle operations!

I can't wait to rig up a dash light to show me if this is what the ECM is doing!

[Zed Head]

In addition to CSV enrichment,there's also another start enrichment. Probably opens the injectors longer. Maybe the ECU doesn't run the start enrichment if the CSV is not enriching either. There's a diagram in the 76 FSM, Engine Fuel chapter.

The description of how the thermotime switch works, with a drawing, is in the Engine Fuel section also, for the 76 model anyway (didn't realize it until just now). The FSMs got less descriptive over the years so the ZX FSM may not have it.