Another Thermotime/Cold Start Valve Issue...Please help

[RPM Resto & Custom]

Hello everyone.....I'm working on a 78 280Z 2+2 that my brother recently picked up. I just had the engine rebuilt as well as doing some patch work and other general upkeep. The car starts up and runs really well except for extended cranking in the cold weather. I started digging into the Cold Start Valve system and I'm still unable to get it working properly.

Initially I removed the cold start valve and put it into a container to see if I would get fuel when cranking it out in the cold after sitting all night. No fuel. So I jumpered the csv directly off of the battery and despite having fuel pressure it would not spray. I obtained a replacement and the same test off of the battery now yields fuel. I still won't get any fuel however when the injector is hooked up in the harness properly. After verifying that I am getting 12V to the thermotime switch I attempted to ohm the switch out and it looked to be out of spec. I picked up a new one today from my local Nissan dealer and installed it but it still won't fire the cold start valve.

The wiring all seems to be ok as I'm getting 12V to the thermotime switch and I have continuity between both sets of wires that run between the csv and thermotime switch. I also have continuity from both down to the 21 pin at the bulk connector that goes into the computer. My ground for the thermotime body to the engine seems to check out ok as well. Given all that what could be causing this to not work properly still? The most interesting thing I've found is that if I disconnect the electrical connector from the thermotime switch and jump the 45 wire to ground the injector will spray when the key is turned to the Start position.

Electricity isn't my strong suit and the diagram from EF-49 in the FSM is a bit confusing as it seems to make a circle between the thermotime switch and cold start valve. Any help would be greatly apprecited. I'd like to get it running properly for him even in the cold as it will be his daily driver. Thanks in advance.

[Zed Head]

I think that wire 46 should be the ground for the CSV. 45 is the power to the internal heating element. 47 is power supply to both. And 21 is probably the signal to the ECU to have it add enrichment through injector duration.

Have you run the test on page EF-23? That test is for the switch itself, but at the ECU connector.

Or you could test at the thermotime switch. When it's cold there should be continuity through the thermotime switch pin 46 (page EF-24) to ground (the block) and some resistance (probably 60 ohms) on pin 45 to ground. Teflon tape might screw up your ground and won't let the switch do proper grounding, since it's through the switch threads, if you've had the switch out and did too good of a job re-installing it.

[Wade Nelson]

Take an hour or so and search on cold start valve on this site. I've posted extensively on diagnosing that system, but I don't care to re-type it all. Another way to get to those posts is to review my history. Lot of stuff there.

[RPM Resto & Custom]

I think that wire 46 should be the ground for the CSV. 45 is the power to the internal heating element. 47 is power supply to both. And 21 is probably the signal to the ECU to have it add enrichment through injector duration.

Have you run the test on page EF-23? That test is for the switch itself, but at the ECU connector.

Or you could test at the thermotime switch. When it's cold there should be continuity through the thermotime switch pin 46 (page EF-24) to ground (the block) and some resistance (probably 60 ohms) on pin 45 to ground. Teflon tape might screw up your ground and won't let the switch do proper grounding, since it's through the switch threads, if you've had the switch out and did too good of a job re-installing it.

Zed I'm not sure which test on EF-23 you're referring to. I see reference to trigger signal, dropping resitor, power line circuit, and some sort or continuity test. I'm assuming it's the continuity test that flows over onto EF-24? Like I said before for some reason if I ground out wire 45 the injector will spray in the Start position. My switch does have continuity from 46 to ground when it's cold and it goes away if I warm it up...which makes me believe the new switch is operating properly. Also I did not run teflon tape on the thermotime. I checked for ground continuity from the body of the switch to some random bolts on the engine and body and seemed ok. I even went as far as pulling the thermotime out and putting it in a cup of ice water with a ground wire running off of the threads to a good engine ground and it still didn't work.

The whole circuit somehow seems to baffle me. I know the manual says when the switch is closed it operates the csv and when open obviously doesn't. What does this mean for power flowing to ground through pin 46 though? I'm assuming if 46 is flowing to ground then the csv fires? If so why doesn't it work if I just manually ground out wire 46?

Wade- I'm going to try and search for some of your info on the circuit now. I was having problems with the search feature this afternoon. Do you recall if they were just replies from you or topics you actually started?

Thanks for the help guys.

[Wade Nelson]

http://www.classiczcars.com/forums/engine-drivetrain-280zx/47643-thermotime-wiring-not-so-simple.html

[RPM Resto & Custom]

Wade I actually found that post before you threw it up in here and read through it. What was your actual initial problem? Your write up makes it sound like your CSV was always on. Was that the case? I'll have to test the continuity change of my new thermotime when 12V is applied to it and then allow it to cool...I believe I did this today but it was a long day. That only occurs on one pin correct? The other seems to be always ground as far as I can tell. Maybe I have an issue with the yellow/white wire you reference that the ground power runs through? That would explain why having the switch body grounded to the frame via an external wire isn't fixing my problem right? Because it has to have the ground through that yellow/white wire to properly activate the csv?

[RPM Resto & Custom]

Oh and did you ever figure out why the ECU receives that signal from the csv via pin 21 in the connector?

[Captain Obvious]

Wade's info is from a 1980 ZX, not a 78, and while the concepts of the cold start system are the same, I wouldn't assume that the systems are identical. For example, things that caught my eye in his thread...

a) He drew in a relay between the ignition switch "START" signal and the cold start system. You don't have that relay. You're "START" signal comes straight from the ignition switch. No relay. There might be a relay on the ZX, but you don't have one in that location on your 78.

He referenced wire colors in his text that may or may not apply to you. Looking at the 78 diagrams, seems all the cold start wiring is green or black. Both sides of your thermotime sw are black and everything else is green including both sides of your CSV. The point is not to get hung up on wire colors like the yellow/white that are referenced in Wade's ZX thread. You don't have a yellow/white.

So while the system concepts are the same, I'm not sure you can count on the specific details.

You have a yellow/black wire coming off the back of the ignition switch that goes hot in "START". That wire goes to a bunch of things including the starter and the fuel pump relay. It also goes to a connector at which point it changes color and becomes G-4 (green number four) in the EFI system. Once that wire gets into the EFI system, it splices a few times after which, G-4, G-45, G-47, and B-45 (black number 45) are all connected together.

Simply put - When you turn the key to START, you should have 12 volts on G-4, G-45, B-45, and G-47.

G-47 goes to one side of the CSV and should have 12 volts when cranking.

B-45 goes to one side of the thermotime and should also have 12 volts when cranking.

The other side of the thermotime is B-46 which connects to G-46 and G-21 (the mystery connection to the ECU).

This set of connections (B-46, G-46, and G-21) will be connected to ground when the thermotime is cold and becomes a no connect once the thermotime has warmed up and opened. This ground connection is made through the thermotime body where it is screwed into the engine.

Operation goes like this:

Key in START - 12 volts connects through the ignition switch to one side of the CSV (G-47) and the heating element side of the thermotime (B-45). If the thermotime is cold and it's internal switch is closed, it connects the other side of the CSV (G-21) to ground. If the thermotime is warm and it's internal switch is open, the other side of the CSV is connected to nothing, and the CSV will not open.

So, about that pin 21? Why does the thermotime switch feed back to the ECU? I got two possibilities.

a) First is what Wayne mentioned in his thread... That Bosch wanted the ability for the ECU to activate the CSV instead of (or in addition to) a thermotime switch. If they would switch pin 21 to ground inside the ECU, it would activate the CSV while cranking. I don't remember what, if anything, that pin is connected to inside the ECU, but it may have been or a future enhancement.

Second theory is that it is simply a test point for being able to check the function of the thermotime switch. Seems Datsun wants you to do all of the diagnostic testing right there at the big ECU connector, and they provided that wire just to allow you to have the ability to measure the thermotime switch function. Without that connection, you would have no insight into the cold start system from the ECU connector.

Remember, I wasn't there when they designed the thing, but those are my guesses.

[Zed Head]

I think that Pin 21 is used by the ECU enrichment algorithm to determine whether or not the engine is cold or has been cranking without starting for a while. The descriptions and this diagram imply that, in addition to the CSV enrichment, that there is also an injector pulse duration enrichment. Two independent Start enrichments.

[Captain Obvious]

I think that Pin 21 is used by the ECU enrichment algorithm to determine whether or not the engine is cold or has been cranking without starting for a while.

I don't believe so. I suspect the pin you are thinking of is pin 4 which goes hot in START. That would be the signal to the ECU of how long ago the engine was started. And The ECU already knows when the engine is cold due to the water temp sensor.

I'm no expert, but that's my read.

[Captain Obvious]

The most interesting thing I've found is that if I disconnect the electrical connector from the thermotime switch and jump the 45 wire to ground the injector will spray when the key is turned to the Start position.

And even after all that typing I did earlier (certainly too much), I didn't address this issue... If you did in fact connect the thermotime 45 wire to ground and then hit START, something should have gone up in smoke.

I suspect that you grounded 46, not 45. One small digit. Big functional difference.

And if in fact that's what you did, it points to a problem with the thermotime switch. I know you just replaced it, but if the CSV opens on START with 46 grounded, then the problem is on the ground side (thermotime side) somewhere.

[Zed Head]

If you read the description in the image I included you'll see that it says "irrespective of the cooling water temperature" (#3). If you look at the wiring diagram you'll see that my description is consistent with the FSM documentation. "Thermotime" has a time component from the internal resistive heater which heats the bimetal strip casuing it to twist and open the relay (Pin 45 to ground which gets power when the starter is used), and a thermal component, the bimetal strip twists from coolant heat (that's why it's mounted on the thermostat housing).

Pretty sure my reasoning is sound. Just sayin', for the record. Still doesn't help the OP get his brother's engine started when it's cold.

Edit - I think that the difficulty in looking at the wiring diagram is that it's showing mixed serial and parallel circuits, plus a "thermomechanical" device. Power is supplied to several different spots, the ECU, the relay for the CSV, the internal resistive heater in the TS. But, unlike most relay circuits, the mechanism for opening the relay, which is normally closed, is the twisting of the bimetal strip. The little blank rectangle in the switch diagram is not descriptive enough.

Edit 2 - actually, this statement "irrespective of the cooling water temperature" is double-edged and confuses things. I think that they meant the sensor readings, since the thermotime switch is affected by cooling water temperature, therefore would affect the the enrichment if Pin 21 does what I've suggested. Maybe lost in translation. Who's got an oscilloscope and some free time?

Edited February 23, 2013 by Zed Head