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Some ECU/AC wiring questions.. 200 1986

posted by wolf_walker on Sun Apr 17 23:04 CST 2011
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last visit: Sat Aug 11 19:29 CST 2012

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I was running through the Bentley section on ECU pin and AMM tests just so I had my bases covered while running down my shaky idle and I came across two things that puzzled me.

First is the pin test for ground on, if I remember correctly, pins 11 and 19, there was no ground to be had. Whichever it was, I traced it back to a two pin black plug near the ECU harness. I thought I remembered there being a jumper in there, checked my 88 and sure enough, there was. So I made one, put it in and all checked well. What is the purpose of this jumper? Is it just a jumper or are there different resistance ratings in there for something or such? Isn't there a timing plug on some euro car or another for different markets?

My next confusion, is the last test in the LH2.2 section of the Bentley, it asks for there to be no continuity to ground with the ac switch off.
I have (you guessed it), 12 ohms or so to ground with the ac switch in any position. This becomes more interesting, when I noticed I can turn the fan switch to anything BUT off, and the continuity goes away. This is still not right, as the other side of the test is that with the ac switch turned on the resistance value should vary, and it does not. I checked the switch itself and both the micro-switch and the varying resistance switch portion work correctly.

Soo.. I'm stumped. I see the ac control switch goes through the heater blower switch, but honestly I'm having a hell of a time making since of the wiring diagrams in the Bentley. Also note the AC does not work, compressor is MIA right now, but I will fix it later. I was on this track wondering if LH was being told the AC was on for some reason, as I'm not clear how this AC system is supposed to work yet. Thoughts?

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Some ECU/AC wiring questions.. 200 1986

posted by wolf_walker on Mon Apr 18 19:20 CST 2011
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No go huh. Here's another. My 86 full CCW on the heater fan knob is off, on my 88 full CCW is low. Model year change or something amiss?

My ragged, tired, motor mount gone, worn out, need tuneup, etc, etc 88 definitely runs smoother than my 86 at a basic level. It also has much less piston slap. I also notice I get a fast idle when cold of an indicated 1100 rpm ro so on the 88, less so the 86, maybe 950 before settling down to 750. The 86 I have done all the idle adjustment procedures on obviously, not so the 88.

Random info and a puzzle of an old car.

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Bentley mistaken 200 1986

posted by Art Benstein on Tue Apr 19 08:07 CST 2011
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I have no clue what that pin-19 jumper selects, nor have I heard any rumblings about model year application. I'm all ears.

In Bentley on p. 241-14 last row in table e:

"No continuity ECU-16 to ground when AC switch off"

You will read continuity (depending on one's definition of it) through the blower motor resistor and windings. Probably about 12 ohms if I were to guess.

"Resistance varies as knob turns"

Nope. With the motor off (the only way you should be checking resistances here) the microswitch completes the circuit to the AC wiring, but with no gas, the low pressure switch is open, and the AC delay relay is not energized, so no change will occur as the thermostat is adjusted. With a fully charged AC, the clutch winding would be added to what the ohmmeter sees once the microswitch engages.

The fan runs on low speed when the AC microswitch tells it to. There's no model year behavior difference between 86 and 88 in this respect -- just one of the microswitches is not fully turned off. It is a common question that comes up because the thermostat gets very sticky and hard to turn off completely. It has led many to think there was some "cabin air when key on" feature in 240s, but this is just to keep the evap from freezing up if someone turns on the AC without the blower.

Doubt this has anything to do with a precision idle. Of course, if the AC switch is stuck on, the ECU is being told the AC is on.
--
Art Benstein near Baltimore

Q. What do bulletproof vests, fire escapes, windshield wipers, and laser printers all have in common?

A. All invented by WOMEN!

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Bentley mistaken 200 1986

posted by wolf_walker on Tue Apr 19 20:38 CST 2011
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OK, back home with Bentley in hand to help me remember what I was looking at.
241-14 says no continuity between 16 and ground with AC switch off. And by no continuity for simplicity's sake call it the meter not beeping when set to that mode and one lead grounded and one lead on 16.

This is the case with the heater blower knob on the full CCW position, continuity (beep) where there ought not to be(per bentley anyway). If one turns the heater blower knob to anything other than full CCW(fan off), the continuity goes away. But at no time do I get the next thing that Bentley is asking for..

Bentley goes on to say with the AC switch ON, there should be a resistance value that varies as the knob turns. One can find this value probing the rheostat that is the AC switch itself, but not at #16 on the ECU plug(at least not on this 86).
I believe there was a steady 12 ohms of resistance when there was continuity.

That may or may not make more sense.

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Bentley mistaken 200 1986

posted by Art Benstein on Wed Apr 20 07:37 CST 2011
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That may or may not make more sense.

I've a pet theory as to why Bentley is off the mark on that line. The tech who was taking the readings saw the slow charge his ohmmeter caused in the inductor which is the compressor clutch winding, while he or she was coincidentally rotating the AC control. He or she also assumed the AC control was an electrical rheostat instead of a mechanical thermostat.

Still, I believe you are barking up the wrong tree for driveability symptoms, even if your AC switch was on.
--
Art Benstein near Baltimore

Always remember that you're unique. Just like everyone else.

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Bentley mistaken 200 1986

posted by wolf_walker on Tue Apr 19 19:02 CST 2011
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Hmm, so the 88 is faulty somewhere with the fan not turning off and the bentley is just flat wrong on the test procedure? Bummer.

This 86 is still apt to choke off if you are on the throttle too fast from idle when it's heat soaked I noticed today after some mid day town driving. What am I missing here I wonder.

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