[s-cars] Fwd: Oil Pressure Warning on Auto Check System '91 200 TQ

[Huw Powell]

> Can anyone help with this?  Just picked up this great Pearl '91 200
> TQ.  It's a killer great car, but the oil pressure warning on the
> autocheck system comes on between 2k and 3k rpm.  Above about 2800,
> it's on constantly, so on the highway at least it quits beeping.

As I'm sure you know, the first thing with issues like this is a 
"reality check."  For instance, if my temp gauge pegged all of a sudden, 
the first thought should be "turn it off," not "hmmm, bad sender."

That said, it's nice that most of these cars have an oil pressure gauge 
for cross checking.

So, anyway, the system in your car consists of two senders with a total 
of three outputs.  One feeds the gauge.  The other two feed the warning 
light via a control module thing.  There is a low pressure sender, I 
think it triggers below 0.8 bar, and the control unit only worries about 
that one below about 3k rpm.  At higher rpms, attention switches over to 
the other sender, which I think trips below 1.8 or so bar.

Odds are your sender is futzed, though it might be the CU (it's tall 
relay sized).  Try swapping in a borrowed one from a car that works 
normally to diagnose.

> Also, I do
> notice the oil pressure guage tends to run a little low, e.g. mid
> range or lower when idling, and just above midrange when engine is
> working or revving.  Could this be the pressure sender? 

Do you see 5 bar when cold and revving a bit?  A 1.5-2 bar warm idle is 
normal, with pressure rising to 3.5-4.5 or so at revs.  Oil weight and 
type can affect this, too.  Running 5w30 in my '57 loader got me barely 
perceptible pressure on an Audi gauge, changing to 20w50 got me 2 bar 
cold and a solid 1 bar warm.

-- 
Huw Powell

http://www.humanspeakers.com/audi

http://www.humanthoughts.org/