I have a 1984 240 with B23F engine with Bosch distributer. When I changed my wiring harness I noticed the dealer had spliced into the old harness and grounded out the temp coolant sensor. When i changed the harness I did not make this change. i contacted the dealer and they checked my records. They said there was a bulletin and the change was made because during warm up the car may put out too much emissions. The service manager read me the bulliten and while it dealt with the coolant temp sensor it was much more involved than running the coolant temp sensor to ground( bulltin called for wiring changes on ECU plug.) The car starts fine cold, but after 20-30 secs it lopes a little. Grounding out the sensor disables it right? I am wondering if I should make the harness the same as the one that was removed.

What role does the temp coolant sensor have in the system?

I'd call the dealer back and ask for the TSB number. There are enough people on this forum with AllData subscriptions that this should be looked up and investigated for future reference. (to assist yourself and the rest of us too)

Thanks & God bless,
Fitz Fitzgerald.
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'87 Blue 245, NA 214K

I have an alldata sub. and just looked at all the TSBs and didn't find the one you describe. I did find one on the coolant temp. sensor, but that one had to do with removing the sensor, suspending in a bath of water and checking the resistance vs. temp. Ask the dealer for the TSB number and post it here and I'll try again to find it.

Very interesting. I've spent the last hour trying to get around paying those "entrepreneurs", for lack of more politically correct term, to view the bulletin you mention. I think you're right, the sensor is not just grounded out; that would make it impossible to start cold! But, if I recall correctly, the sensor used in LH2.0 is a two terminal thermistor with no internal ground, unlike the one used in later (black body for LH2.4 I think) so it must have an external ground wire in the harness.

Perhaps the location of the ground was changed by the bulletin. This could have resulted from an analysis of the voltage found at the original ground point with respect to the ground reference used by the A/D converter in the ECU, so maybe the bulletin just moves the ground to one that does not add a few millivolts to the voltage seen at the temp sensor pin. Just a wild guess, while I still refuse to pay Alldata and am unable to view Volvocars.com site riddled with Macromedia Flash 6. Help me out here.

The temp sensor has a large effect on the fuel delivered. There's a curve in the manuals that relates temperature to resistance. As the temperature of the coolant gets colder the injectors are held open longer to allow more fuel. The sensor warms up very fast, but the curve has to match the correct air/fuel mix needed to support smooth running during warmup, so, it looks like somewhere down the line after building your car this was adjusted in a service bulletin.

Anyone have that bulletin?

P.S. Do you have the late 84 that came with a three wire oxygen sensor? Undoubtedly your replacement harness has that accomodation. -510 ECU?

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Art Benstein near Baltimore

Yup. Volvo's new site sucks. Please, be sure to mention this to them. The more negative feedback they get, hopefully, the more likely they will reconsider such a poor design choice.

The proper address seems to be:

vtihelp@volvotechinfo.com

But make sure to CC:

“Morse, Barry (B.D.)” <bmorse3@volvocars.com>

- alex

Art

Yes I have the late model 84 with heated O2 sensor. Specifically what was changed was a tap on the blue wire in the harness was attached to a connector on the starter motor. I am pretty sure this blue wire is the positve of the temp sensor not a seperate ground.

It almost sounds like you are looking at a mod I made once to enrich the mixture during cranking. I added a diode in series with a resistor between pin 4 and 2 on the ECU in order to fake the ECT circuit into a much colder reading while the starter was cranking. Pin 4 is the voltage (blue/yellow) at the starter from the key switch pos III. Pin 2 is the ECT (blue). The mod was a fix to hard starting while I looked for the real cause, clogged injectors.

What it really is I have no idea!
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Art Benstein near Baltimore