Wiring the thermotime switch and cold start injector?

[Adam78]

Ok, I have a megasquirt unit but I am considering using the OEM pieces for cold temp cranking fuel. I already use the AAR for fast idle. I realize this may sound like a head scratcher but I have my reasons. So my question is how would I wire this since I no longer have the stock EFI harness that I could use. What I was thinking is this: Run power from the positive battery post through a relay that closes the circuit only when the starter is engaged which then connects to the thermotime switch. I don't think polarity matters on either the thermotime or cold start injector so then run a wire from the thermotime switch to the cold start injector and from the cold start injector to the negative post on the battery. Does this sound like it would work?

thanks

 

[zKars]

Read EF-14 in the FSM to get real clear on how the Thermotime switch works and how it controls the Cold start valve. It's most important function to ensure the CSV is only used during cranking when the engine is cold (<14-22C) and to prevent the CSV from being on for too long if you do a lot of cranking before it fires.

Given all the enrichment control the MS gives you about starting and cold operation, there is no "need" to use the CSV, but it may be an interesting exercise. 

Edited January 10, 2016 by zKars

[Adam78]

Right, that's the reason for having the starter trigger the relay; the thermotime switch would only get power during cranking. I understand the relationship between the switch and the CSV and how the thermotime switch warms up from receiving current more than a certain amount of time in order to prevent the CSV from flooding the engine. I just wanted to see if my wiring logic makes sense :-)

Regarding MS, I would still need to use a little afterstart enrichment and obviously the coolant temp enrichment. It's just the cranking pulses in cold weather that is giving me some trouble.

 

[Captain Obvious]

That wiring scheme won't work. There are two things wrong.

First, you need to connect the correct side of the thermotime switch to "hot in start". One connection goes to the heating coil, and the other connection switches to ground when cold (and goes no-connect when hot). So you need to run your "hot in start" to the heating coil side of the thermotime, not the switched to ground side.

The other issue is that since the thermotime switches the cold start valve to ground, you also need to run "hot in start" to the other side of the cold start injector.

So one side of both the thermotime and CSV go to "hot in start" and the other sides of both are connected together. Just make sure you get the correct side of the thermotime.

Make sense?

[Captain Obvious]

Sorry forgot to point out a detail...

You do not need to include any additional connections to the battery ground. The ground paths are taken care of by the thermotime switch itself which connects to the block through it's mounting threads.

[Adam78]

Yes, I think that makes sense. So, just to clarify, the polarity of both the switch and the CSV matters? The reason I wasn't sure was because on the AAR and the injectors as well you can flip flop the hot and the ground and they still work so I thought maybe everything in the EFI system was designed that way. Well then my next question would be how do you know which side is which? Are the switches marked with a + and - ?

[Zed Head]

I've pondered just using a priming button.  A squirt or two per start.  Like pumping the gas pedal on a carb with accel pumps.  After a few starts you'll know what it needs.  

Seems like the same could be done with the injector circuit.  They probably used a separate valve to allow better vaporization before the cylinders.

 

I only think about it because I've recently discovered that my battery is weak.  Cold weather starts were more difficult and I have no CSV.  A battery charge helped, of course, but it got me thinking.

 

 

[Adam78]

So you would ground the CSV and then relay the hot with a switch in the cabin so you could manually energize the CSV? Interesting.

[Zed Head]

That's the thought.  I'd use a momentary switch, button or rocker, so that you couldn't leave it on accidentally.

Being down in TX though, you will probably rarely need the CSV.  And Megasquirt probably has a Start enrichment function that adds fuel during starting, by adding injector open time.  You might check the documents.

[Adam78]

Actually, it does get cold enough here to need some help. Megasquirt has a "cranking pulse width" section where you enter a PW value at -40 and 170 degrees fahrenheit and it draws a straight line between them for all the intermediate values. I'm just having a hard time getting it to start on one crank once I get below 50 degrees or so. I thought the stock CSV might be a good fallback plan, that's all.

[Captain Obvious]

Only the polarity of the thermotime matters. The CSV is just like any other injector and doesn't care. So as for figuring out which side of the thermotime is which... Hmmm. Without having the stock harness to trace?

You could put a meter on it and you should have low resistance from both pins to the case ground. But one of them would be a direct short (zero Ohms) and the other pin would go through the heater coil (low, but non-zero Ohms) before getting to ground. I don't remember offhand what the resistance of the heating element is so I don't know how easy it would be to distinguish between the heater and a dead short.

Failing that, you could verify low resistance to ground from both pins and then heat the thermotime up in a pot of hot water and test it again. The side with the heating coil wouldn't change, but the side that is supposed to switch the CSV should go open circuit.

Failing that, someone with a stock EFI harness should be able to trace the wiring and tell you which side is which. I've got one and can do that for you if you can't get the other two methods above to work.

I'm kinda surprised that the cranking pulse width feature from the megasquirt isn't enough. Did you try maxing out the pulse width to 100% at the min temp?

 

 

[Captain Obvious]

Just thinking that your thermotime switch is probably screwed into the block right now and taking it out to test it in a pot of hot water would be a PITA.

You could run the engine to warm the thermotime up instead of a pot of water. One side should go open circuit to ground when warmed up. That's the side that needs to go to the CSV. And the side that still has resistance to ground even when hot would be the one that goes to your "hot while cranking" source.