Bmachine

Hello esteemed 928 community.
Disclaimers right off the bat: I am calling from the other side of the tracks. '74 BMW 3.0cs. Apologies for that. Many in our group also own a 928 so I figured it was ok to come clean right away... ;-)
One thing that is sorely lacking in the 3.0cs is a proper oil pressure gauge. So I decided to replace my useless clock gauge with a combo oil pressure and voltmeter gauge from a 928. I purchased one used and installed it the other day. I am using a new standard VDO 2 connector sending unit with ground through body. Not the later 3 connector one.
The problem I am running into is that when I first turn battery power on, the gauge jumps to full 5 bars. When I then start the engine, it drops to 0. In other words, it works backwards.
After doing a bit of research, I found out the sending unit is designed to measure 10 ohms with engine off and climb up to 185 ohms at full pressure. I have tested that on the unit in my car and I am indeed getting those numbers. So the sender seems to be working correctly. I have triple checked the wiring of course and it is correct . The second post, which connects to the idiot light, works correctly.
So why is the gauge showing high pressure at 10 ohms and low pressure at 185 ohms? Is this normal behavior? Does the 928 gauge require low ohms for low reading and high ohms for high reading?
Interestingly, after doing some searching on this site, I found a link to a site where someone else seem to have the same issue: http://jenniskens.livedsl.nl/Technic...6/MyTip673.htm
Thank you very much for any help.
Bo

FredR

The sender you have has two channels and grounds via the body. One channel is for the alarm function and is a make or break contact the other channel is an analogue output. Have you identified the channels correctly?

Bmachine

Thank you for the reply. Yes, I mentioned it in my post "I have triple checked the wiring of course and it is correct . The second post, which connects to the idiot light, works correctly."

FredR

The terminals are marked G and WK. Measure the resistance between the body and the terminal marked G and advise what reading you get. If that is what you have done and you got 10 ohms then the 928 part is complete and the problem is with your gauge head unit that has nothing to do with any 928.

Bmachine

Thank you Fred. Yes indeed, as you pointed out, I measured 10 ohms between G and ground (body in this case) on the bench as well as installed on the motor without engine turned on.
I agree with you that, by these observations, it would seem like the problem is with the gauge. I just wanted to confirm this with people more familiar with it than I. I just wanted to be sure that this gauge was not expecting something opposite such as low ohms for low reading.
Darn. I guess I need to get a new gauge then.
I really appreciate your help.
Thank you.
Bo

ramcram

As I recall, the oil 'gauge' [not the warning light] work in the opposite direction to the other gauges.
ie, earth the gauge wire to the sender unit gives you NO deflection and open circuit the gauge gives you full scale deflection.This puzzled me for a while rewiring my ROW 1986 5.0 litre 928, with the 2 pole sender unit.

FredR

Bo,

The VDO gauges seem to do some very strange things that make no sense to ordinary mortals. The 3 wire sender my later model 928 was originally kitted with being the case at hand. I had a problem even though it was wired as per the wiring diagram . In the end I gave up with it, fitted an earlier two pin sender and hey presto it worked just fine! No idea why the 3 wire sender did not work - was just pissed that I purchased a new one that is sat on a shelf gathering dust somewhere after ragging me for $100 or whatever..

As long as you have the earth and the signal correctly hooked up it should work but...?

Bmachine

I am still trying to solve this mystery. All the testing above has been done and successfully concluded. But there are two different gauges made for the 928. One which reads clockwise and one counter clockwise. Does anyone know what makes one go one way and the other go the other way? Is it simply reversing positive and negative?



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