1975 280Z Cold Start Circuit Issues

[HusseinHolland]

So, now that the ambient temps have dropped, I see that the cold start system is not actually operational. When I rewired the EFI harness I found the ECT & TTS were mis-wired. I don't know if that caused an issue at the ECU.

 

In any event, I bridged the TTS connector, and have voltage output from the EFI relay 86 terminal to the cold start valve when cranking (checked with test lamp on pin 86 at relay, and 45 at CSV.)  I confirmed the CSV functions by powering & grounding the CSV to the battery with the fuel system pressurized. 

The CSV does not inject fuel however, and if I put the test light across the pins of the connector, it does not light, which would indicate to me that the ground side of the circuit is non-functional. I tested the continuity of the harness from the TTS connector (pin 46) and the CSV connector (pin 46) and both have continuity back to the harness plug I made. 

Tomorrow I'll have to check the continuity of the ground side of the harness from there back to the ECU connector. My question is, does anyone have experience with this aspect of the system that would indicate a common failure point? 

The wiring diagram is not accurate in terms of the actual wiring of this circuit - the bridge circuit for the TTS in not at the CSV, is it back in the harness on the inner fender, where 47  is T'd off into to 45 wires, and 21 is T'd into 2 46 wires. I discovered that when I did the re-wiring.

 

 

Edited October 1, 2023 by HusseinHolland

[Yarb]

If those don’t work out I believe I have a few of the other brand at the shop.

[HusseinHolland]

6 hours ago, Yarb said:

If those don’t work out I believe I have a few of the other brand at the shop.

Did you mean to post in the bulb socket thread? 🙂

[Yarb]

Sorry about that 

[HusseinHolland]

Still back & forth with the cold start circuit. I have continuity in the harness from 21 at the ECU to the CSV & TTS. The factory test is to disconnect the ECU connector & use a voltmeter on pin 21 - if there is battery voltage when cranking, the circuit has integrity.

I did that, and got the desired result. However, the CSV does not fire, even though I know the valve is functional when wired independently of the circuit.

What I'm having trouble with is the way it's all wired - looking at the diagram, it appears the TTS is in parallel, not series, so HTF it would interrupt the current to the CSV?  Makes no sense to me. The reality of the circuit wiring is thus:

FSM diagram crop

Further investigation is required.

Edited October 2, 2023 by HusseinHolland

[HusseinHolland]

A Fiat forum member pointed out what I was missing, and is not including in the FSM circuit testing - the TTS grounds the CSV, so with the TTS circuit bridged to check continuity of the wiring, the CSV cannot fire. I'll have to check the resistance of the TTS when cold & make sure it's OK, then I can reconnect it to verify CSV operation.

The operation is described in the manuals, I've read it many times - I just blew right by it, 'assuming' the ground was ECU controlled, not within the TTS

Edited October 3, 2023 by HusseinHolland

[SteveJ]

I'm glad you found your answer. I looked a little online and through a couple of service manuals yesterday. I had some understanding of how it was supposed to work, but I think the drawing you just posted explains it better than I could have. Most of the drawings I found didn't show the heater strip clearly like that one does.

[Captain Obvious]

And if you're wondering why those two devices look like they are wired in parallel, it's because they are. But not the way you originally thought.

It's confusing because the #21 connection goes to the ECU. but it's essentially a no-connect. The ECU doesn't do anything with it.

So with that in mind, you can do this:

[HusseinHolland]

20 minutes ago, Captain Obvious said:

And if you're wondering why those two devices look like they are wired in parallel, it's because they are. But not the way you originally thought.

It's confusing because the #21 connection goes to the ECU. but it's essentially a no-connect. The ECU doesn't do anything with it.

So with that in mind, you can do this:

So I have discovered. It appears the only point to the #21 connection is to allow circuit continuity check, per the FSM. I tested my TTS today, and I have ground between 46 & the t/stat housing with a cold (uncranked) motor. 

[Captain Obvious]

16 minutes ago, HusseinHolland said:

 It appears the only point to the #21 connection is to allow circuit continuity check, per the FSM.

I wasn't there when they designed it, but my read on that #21 connection was to allow the ability for the ECU to fire the cold start injector. They never implemented that feature, but it looks to me like it was wired with that ability in mind.

Would be a simple matter for the ECU to drag that side of the cold start to ground if they wanted.

[HusseinHolland]

16 minutes ago, Captain Obvious said:

I wasn't there when they designed it, but my read on that #21 connection was to allow the ability for the ECU to fire the cold start injector. They never implemented that feature, but it looks to me like it was wired with that ability in mind.

Would be a simple matter for the ECU to drag that side of the cold start to ground if they wanted.

Would have been nice if the CSV had been ECU governed. Volvo didn't get that until the adoption of Bosch LH-2.2 in the early 80's, but they never used earlier L-Jet air-flap meter configurations . They used D-Jet and K-Jet before that, which also used the TTS like the Datsun.

[Captain Obvious]

10 hours ago, HusseinHolland said:

Would have been nice if the CSV had been ECU governed.

Yeah, I agree. It's all wired for it, but they just didn't do it. I guess they just didn't want to add that complexity to the ECU's. Even through the end of the run in 83, they still used the thermotime system instead of having the ECU control the cold start. Certainly a proven brute force system and one less thing for the ECU to have to deal with.

Honestly, I've had my cold start valve removed for years and I don't miss it. Sure it takes a couple more cranks on really cold mornings, but for the tradeoffs of being able to completely remove the plumbing leak sources as well as the thermotime switch, I don't really miss it.