The Check Engine Light remains on the whole time. We are able to start it by cranking it for 10s, waiting for 5s, then cranking it again for 10s until it catches and runs. It idles fine and then drives well and smooth. When its warm, it starts right up.

I found the following link that documents the procedure for obtaining the flash codes from the blinker box:

http://autorepair.about.com/library/faqs/bl661g.htm

Assuming that the procedure and codes are the same for the 740 as for the 760T, then our testing tonight went as follows:

* With blinker-box pigtail in #2 socket (fuel system codes), the LED flashed a 1-2-3 code which means "1-2-3: Signal missing to/from coolant temperature sensor, possible short circuit"

That was the only code for #2 socket.

* With blinker-box pigtail in #6 socket (ignition system codes), the LED flashed a 2-2-4 code which means "2-2-4: Coolant temperature sensor faulty"

That was the only code for #6 socket.

So, if everything is lined up right in my thinking, the codes are pointing to the same thing: the coolant temp sensor. But, we just had that replaced with a new one by an experienced Volvo mechanic. He even tested the resistance on the new one compared to the old one and the old was definitely bad and the new one was definitely good.

I was able to follow the code clearing procedure documented in this link and it worked for clearing the #2 socket code but not for clearing the #6 socket code. We tried 3 times to clear the #6 socket code ... no success.

What do you think here? Puzzling.

 

Clearing codes can be tedious. Often it is necessary to hold the button down much longer than instructions state. At this point, we don't know if the codes you pulled were from the old sensor or the new one. Your tech should have cleared the codes. If he did, and they returned, then he needs to check the pig tail that connects to the sensor. The harness is quite tight there and often the "potting" or sealer in the end of the pig tail gets lose as well as the wiring. Alternately, there could be a wiring issue between the sensor and the ecu. It's always good form to check sensor output not only at the sensor but at the appropriate pins at the ecu. This will tell you if the issue lies somewhere in between or within the ecu itself. And yes, the 760T is identical under the hood to the 740T.

 

Blinker box gives error code for Engine Coolant Temp sensor ... but the hard start symptoms (see above in this thread) seem to indicate a failing cold start valve, right? Or maybe there's a connection between the two?

 

What's the ambient temperature when you try to start it? Were you ever able to clear the codes and they then came back?

 

Your problem is in fact the Coolant Temp Sensor, textbook case.

 

Ambient Temp: between 33 and 43 degrees or so.

No, I could never clear the ignition system (#6) code altho I tried like 6 times with various button hold times. Perhaps I can't clear the code if the underlying fault it represents is still happening (in this case a prob with the ECT sensor).

We'll be tracing circuits today and testing voltages to track this down. I am hoping it is not a failed ECU 'cause I am told that is pretty pricey.

 

Well, good friend and Volvo mechanic traced wires and said ECU is bad. Even tho engine coolant temp sensor sends correct signal to ecu, the ecu doesn't send signal to cold start valve. Cold start valve works when proper ground is applied. So friend/mech says "brain damage" ... ecu not properly grounding cold start valve when it should.

New ecu needed. Where should I go for that?

 

Well, after trying a new (used) ECU with identical Bosch part number with no change in symptoms, we just scratched our head, then put back in the old ECU.

So, what next? We traced every wire measuring voltage etc. Everything checked out. We figured that Engine Coolant Temp (ECT) sensor itself couldn't be bad because 1) it was new and 2) we tested the resistance beforehand and it was to spec.

But, after a conversation with another Volvo mechanic (Rick from Mark's Volvo in Chattsworth, CA ... call him he's really nice and knowledgable) he said "Hold it! What color casing was on the ECT sensor you installed: blue or black?" I said, I don't know, let me check ... so I did and it was blue. He said "that's it; the blue one is the wrong one; that one only works on 1988 and older 760s. You need the black one for 1989 and newer 760s".

Turns out the black ECT sensor signals both the ECU and the ignition computer and the blue one only feeds one of them. The electrical connection fits on either one just fine. This makes sense since the blue one is $14 from IPD and the black one (more sophisticated) is $45 from IPD.

So, my friends @ NAPA didn't catch this. I have a new BLACK ECT sensor coming tomorrow and will install and hopefully close this thread with a success story. Please stay tuned for the next exciting episode.

 

Here's the pic from IPD website showing both: http://www.ipdusa.com/Volvo-700/Engine/Fuel-Injection/Fuel-Injection-Temperature-Sender/p-70-189-344-3953/

 

WHOA! STOP!!! The cold start injector does not work until REALLY low temps (like -15c). The ECU controls normal cold starts by sending double pulses to the injectors whilst cranking, then chops back to single pulses and increases pulse width after cold start.
Please disconnect the battery overnight to reset all ECU's and try again.
I discovered a corroded plug in my CTS a few weeks ago that set code and it wouldn't go away by normal methods. If the ECU still has the code stored, it will continue to work around it.
WHat the ECU does when it gets a over/under spec signal from the CTS is substitute the CTS reading for temp at 20c and bases all it's fuelling around that.
So, cold starts can be very difficult and hot starts can be difficult too.
CTS issues usually manifest as hard cold starting, poor fuel economy and a slightly rough idle.
The large, two bullet connector sensor on the head is for coolant gauge ONLY, the rear one with the standard Bosch EFI plug is the one BOTH ECU's reference.
Your Volvo mechanic friend needs to do a Bosch EFI course, and now.

 

Black-headed Coolant Temp Sensor (CTS) installed in place of the blue-headed one. ChkEngineLite is out, codes are cleared from ECU and ICU and cold-start was immediate. What a lesson. Thanks all for help.