5ktq Possessed oil pressure auto check

[John Cody Forbes]

On Oct 19, 2010, at 8:20 PM, Huw Powell <audi at humanspeakers.com> wrote:

> 
> 
> On 10/19/2010 8:02 PM, John Cody Forbes wrote:
>>  It's interesting these days after 10 years and having owned 8 (as many as 4 drivable at once and currently 3) that 5000's can still come up with new issues to bug me.
>> 
>> Patient: My "fast" '87 5ktq.
>> 
>> Symptom: Auto-check oil pressure warning comes on only at idle and about 75% of the times the engine goes to idle (ie most stop lights).
>> 
>>  I have checked the black .3 bar oil pressure switch, it seems to do it's job correctly. Oil pressure via mechanical gauge is great. Tonight I unplugged and made safe the wire for the black switch and the problem persists. I did verify via bentley wiring diagram that the correct wire is on the correct switch.
>> 
>> Any BTDT before I start tearing in to this thing? The autocheck triggers on ground to mean low pressure so it can't be an intermittent open, it's got to be an intermittent ground or a bad autocheck.
> 
> 
> Are you sure the low pressure switch is NO?  I thought it was NC so a broken wire would set it off.
> 
> You apparently have plenty of spare cars to swap the sender into and test, why not try that?
> 
> -- 
> Huw Powell

In the mechanical testing side of the Bentley (page 17.10) the testing procedure indicates that the .3 bar switch is NO and the 1.8 bar switch (white) is NC. Interestingly it also states that the white switch is active over 2100rpm. My issue occurs only below about 1200rpm. Hmmm. I would expect an issue with the black switch to set the warning off when crossing 2000rpm in that case. Another interesting note is that it says that the black switch will illuminate the light, while the white switch illuminates the light and the 3 tone audible warning is activated. My issue includes the audible warning. Hmmm x2.

I'm going to wiring diagrams now. I wonder about the tach signal to the autocheck. This is an EFI car, but the tach signal is using the stock wire in the ECU harness. Maybe the autocheck uses a secondary tach signal wire from somewhere I'm not tapped in to.