Electrical glitch since 1.9 install with serpentine system

[the caveman]

So since i removed the 1.7na and replaced with a 1.9 na that had a serpentine  belt system, i've had an oil pressure/ electrical issue. I already did the tach modification to bring the rpm's back in line to  true engine speed and i have a funny feeling it's got to do with the difference in what it is to what the oil pressure control is seeing. The problem is this, once warm and usually coming off the highway, as the rpm's drop to about 2500 the oil  buzzer and light come on. If i blip the motor it goes off. I am sure the oil pressure is okay because once i get the buzzer to go off if i don't raise the rpms to more than 2000 or also i can let it idle for an hour and it will never come on. Also i never have lifter noise and i don't consume any oil at all between changes. No i haven't checked my oil pressure as i don't have a gauge i can trust. It's not impossible because it's the pressure as it takes about 2 or 4 seconds for my light to go out starting cold, but never hot. But as i said once the buzzer and light come on, if i blip the throttle it goes out. Today i replaced both switches, and it's worse. I had a brand new controller for the cluster, but of course now i can't find it.

[jtanguay]

i wonder if it could be something wrong with the charging system or the W terminal signal?  did you add a resistor to the W term wire, or adjust it from the cluster?

[the caveman]

I added an adjustable resister as per a sticky i read on here [i think] . In the end i really wonder if i needed it . I should mention that sometimes it happens at 1300 rpm.

[the caveman]

I'm resigned to having to check my pressure tomorrow.
I know oil pressure and consumption don't necessarily go hand in hand, it's just had to mention everything i could think of.
Once the problem starts, almost every time it drops to whatever rpm it's doing it at, it will happen every time-usually 2300 or so but last week it was 1300. I wish i had another cluster to try. The other thing is it never does it as engine speed goes up, or while cruising.

[Vincent Waldon]

Most likely, here's what's happening:

- smaller pulley on the serp alternator means the alt is putting out a faster tach signal than your cluster is expecting.. you saw this as the tach displaying an RPM that was too high

- you've re calibrated the actual tach itself, but the oil pressure warning system is still getting the signal that's too fast.

- this means that when the oil pressure system thinks the engine is at 2000 RPM and starts listening to the high pressure sensor your engine is actually only at 1700, 1800, or so on.

- at this lower RPM the engine doesn't have the full 1.8 bar the engine is expecting at 2000 RPM and the buzzer comes on.  This will be most noticeable after cruising... the oil is hot and thin.

- rev the engine, the oil pump brings the pressure up above 1.8 bar, and the buzzer goes off

Reverse-engineering the oil pressure warning system is probably a bit tricky, since the schematics are proprietary to VDO or Motometer.  Two other options might be:

- install one of the high pressure sensors that's less sensitive:

black (1.4 bar)    068 919 081
grey    (0.9 bar)    068 919 081 A

- disable the hi pressure side of the warning system by grounding the yellow lead that goes to the sensor

[the caveman]

Thanks Vince, i was thinking along the same lines. The problem got worse when i installed new switches, and i used the more sensitive one on the head -0.3 bar.  i will check my pressure and try a higher pressure switch and repaort back.

[the caveman]

So today i checked as per the trouble shooting guide i.e.- start with head sensor wire grounded light  on, but no buzzer, then flange switch wire disconnected and the light  and buzzer should come on soon after 2000 rpms. Well it didn't come on until i revved it past 3500. So as i thought and as Vince mentioned it's because of the "wrong" engine speed being picked up because of the smaller alternator pulley. All to say i just grounded the flange switch wire. Problem is gone and at least i still have the head switch as a warning. Stupid thing is that because i have a german 1990  diesel transporter, the bentley only shows the wiring for the 82 and 83 years because they were the only years they imported them here and those model years only had one switch . Thanks again everyone.

[jtanguay]

what if you install the resistor right on the W terminal wire?? i bought a crapload of resistors and want to try that out to get mine calibrated...

[the caveman]

Did you already try to adjust it at the screw at the back of the tach. I didn't and i have a funny feeling it may have been enough. Also the one i bought is pretty small so i'm not sure where you would mount it.

[Vincent Waldon]

what if you install the resistor right on the W terminal wire?? i bought a crapload of resistors and want to try that out to get mine calibrated...

It's a reasonable idea, but unfortunately the tach and oil pressure circuits are looking for frequency rather than voltage... and a resistor in series will not change the frequency.;-)

Dakota Digital converter box could do it... but that's a pretty spendy fix.

[the caveman]

Have you read this post ?
http://www.vwdiesel.net/phpBB/viewtopic.php?t=12589
It's what i did and the stupid thing is i don't remember if the problem was there before the fix. I do remember completely freaking out the first time it happened on a warm December day. It was soon after i changed the engines and had the issue where the dipstick doesn't read properly so  although it should have been the right amount of oil [4.5 liters] it never showed on the stick and so i was questioning if i had actually put enough in.

[Vincent Waldon]

Yuppers...

The tach circuit board takes the frequency signal from the W terminal (or ignition system, t'were this a gasser) and converts it into a varying voltage that drives the needle.  The instructions in the post change the calibration of the voltage that the tach circuit uses to drive the needle.

Unfortunately that only re-calibrates the tach and not the oil pressure circuit (as near as I can tell!).

[the caveman]

Okay so i figured out the other issue i was not completely worried about but was questioning- when started cold it takes the oil light a few seconds to go out. When i switched motors i used the same oil pump that was on the 1.7 which gave me instant pressure even cold. So i know the pump is fine. I just figured the 1.9 had a bit more wear [even thought it's one of the only used diesels where there were no cracks between the valves and never turned more than 3000 rpms] and took a bit more time to build pressure . Well the other day when i checked the pressure cold, it built to 45 lbs instantly where as the light was still on, 5 seconds after pressure was up. Well i noticed the light doesn't go out until the idle jumps from 500 to 1000 rpms and you can hear the change in engine tone. I'm now pretty sure the oil light controller won't turn the light out because of the same issue with the discrepancy between what the rpms actually are and what the controller is seeing. I'm not about to try to really explain it. It's almost the same thing [but not really] as when the alternator light not go until revved. I met and talked to a shop owner who has been doing gas to diesel conversions for a few years. i asked him what he does when he the problem i have and he told me he never has had it happen to him, but in all the trucks he's done, none had a tach !

[vanbcguy]

My Mk III will have the oil light on unless the alternator is producing a signal on the W terminal... First time I did my timing belt I fired the motor up without the accessory belts on and IMMEDIATELY freaked out because my oil light wouldn't go out.

If yours needs to be revved to get the battery light to go out then the oil light probably is behaving the same way...

[the caveman]

Since the change from 1.7 to 1.9 the battery light has never done the typical thing. i guess it's one trade off for another, although i'd prefer the battery light.