Follow Up On Throttle Potentiometer Test...Could Someone Possibly Lend a Hand?

[Dave M]

Starting anoter thread in order to focus on one item at a time. Here is the link to the original thread which will provide background etc. Basically, it feels as though the fuel is being shut-off or a number of cylinders are down momentarily, as the car will jerk fairly hard under acceleration. This occurs most often when warming up and when accelerating from a stop with low engine speed. It also stumbles at idle and has thrown a 1216 (Throttle Potentiometer) code. When it stumbles, the CEL and parking brake light illuminte for a half-second.

So, I would like to furter investigate the potentiometer issue and am wondering if someone with an M50 could could test theres with a multimeter and record the results as per the dialogue below. Ryan R has kindly agreed
to send me a working potentiometer, but it will be in transit for a while.

This is the Bentley instruction for testing the potentiometer



As Bill G states below, he has his doubts abouty the Bentley procedure.

Anyhow, this is what we have deduced so far.........

Bill G
If you are talking about testing the throttle position sensor as per Bentleys page 130.8 - there is something wrong with Bentleys procedure. My wifes 1992 325i M50 has same sensor which is in good working order as far as I know - I measured voltages between harness connector terminals and body earth -
pin no 3 (top) 5v
pin no.2 5v
pin no. 1 6.5mV (this pin is earthed)

Also, voltage between terminals 1 and 3 5V.

Hope this helps.

Dave M

Now, when I tested all three harness pins to the strut (ground), I recorded the following (Note, my VDC was set at 20V, so I may be limited):

pin no 3 (top) negligible
pin no.2 5v
pin no. 1 negligible (this pin is earthed)

Does this mean anything to anyone?

Bill G
Seems like there is something wrong with pin 3 value.
According to the BMW Electrical Troubleshooting Manual (ETM), for Motronic 3.1 M50 -

Pin no 3 (top) connects to pin no 59 at the DME (or ECU as commonly called) and should have 5V
Pin no 2 connects to pin no 12 at DME and should have 5V - this is the pin which receives the varying voltage signal (approx range 0.4V to 4.2V depending on throttle position) back from the TPS

Pin no 1 (bottom) is earthed

My own results were measured at the removable harness, ignition on.
It would really help if someone else with a known good system could check and report their voltage values to back this up.

Bill G

As always, thanks for reading,

Dave M

[ryan roopnarine]

pin 3: 4.96v
pin 2: 4.90v
pin 1: .2 volts

(my grounds are probably dirty, and the area aroudn the TPS plug looks greasy, which might explain why).

[ryan roopnarine]

what im thinkingis that maybe bentley meant to say .5 volts (appx) instead of 5 volts (appx), which would explain a lot.

[pingu]

I reckon it's more likely to be 5V than 0.5V (but this is mainly an educated guess). For a start, 5V is a well known voltage that is used to power digital electronic circuitry (such as in the ECU). Also, 5V means that the signal from the throttle potentiometer is relatively insensitive to noise; in contrast, if the potentiometer was supplied with just 0.5V then a 0.1V fluctuation (because of electromagnetic noise induced from a nearby cable) would cause a 20% error.

[pingu]

Since you've got 2 ticks and 1 cross (what is the significance of the rectangle around "ground in harness connector") then it currently looks like you've got an intermittent 5V supply to the pot or an intermittent ground to the pot.

[Jon K]

Hey guys maybe I can help - I have my TPS connected to my standalone. The OEM ECU is providing the 5v power signal and I have the "signal wire" tapped to my standalone ECU. I can watch the TPS voltage go from 0.4v at 0% throttle to 4.2v 100% throttle. I am taking the signal from the wire listed in the bentley for the Motronic 3.1 ECU for "TPS Signal"

[Dave M]

Since you've got 2 ticks and 1 cross (what is the significance of the rectangle around "ground in harness connector") then it currently looks like you've got an intermittent 5V supply to the pot or an intermittent ground to the pot.

The two ticks signify that the resistance tests were ‘good’. The reason for the box, is that I did not know how to distinguish ‘ground’ on the harness connector. I assumed it was the terminal connected to terminal 1 (bottom) on the potentiometer. When I tested terminal 1 on the harness while connected to terminal 1 on the potentiometer, I got nothing. In fact, when I tested all three terminals on the harness while connected to the potentiometer, I still got nothing.

Now, when I used the strut mount as ground, I got a ~5V reading from ONLY terminal 2 on the harness. Ryan R and Bill G have recorded ~5V from terminal 3 as well.

So, to re-iterate, I am testing the harness connector terminals and only have voltage on #2 (~5v). I am more and more convinced that the power supply to the potentiometer is the culprit.

The money question is…..how do I determine where the problem lies? Could it be the harness, could it be the DME, the DME relay, something else…….?

Thanks for all your help, I’ll be listening,

Dave M

[Dave M]

Hey guys maybe I can help - I have my TPS connected to my standalone. The OEM ECU is providing the 5v power signal and I have the "signal wire" tapped to my standalone ECU. I can watch the TPS voltage go from 0.4v at 0% throttle to 4.2v 100% throttle. I am taking the signal from the wire listed in the bentley for the Motronic 3.1 ECU for "TPS Signal"

Thanks Jon, my resistance readings were fine. I may, however, double check the variable one as when I apply throttle at low rpm, it occasionaly stumbles, and when I let off the throttle at low rpm, it feels as though the fuel is cut off way to fast.

Dave M

[ryan roopnarine]

now i know what you were trying to ask. that ground in "harness connector" likely means the ground that services the dme+harness connector. it doesn't matter where you ground it, though you can put your ground to the metal frame in the e-box, if you like. none of the harness connectors for the TPS can be connected together to generate a voltage. sounds like maybe a dme relay or something. i hesitate to say anything, because i know either bill r or winfred will come along and say something completely different i either keep a dme relay or FP relay in my toolbox, if i find it ill send it along as well.


ps...if it were me, i'd start by purchasing a can of contact cleaner (if you can't get a decent price from any one else, just pick up a can of radio shack's severely overpriced contact cleaner, and clean the pins on the TPS connector and the TPS pins themselves. we don't drive japanese cars here, so sometimes we have to suffer the indignity of using da kontact kleaner on stuff to get it working again.

[Dave M]

now i know what you were trying to ask. that ground in "harness connector" likely means the ground that services the dme+harness connector. it doesn't matter where you ground it, though you can put your ground to the metal frame in the e-box, if you like. none of the harness connectors for the TPS can be connected together to generate a voltage. sounds like maybe a dme relay or something. i hesitate to say anything, because i know either bill r or winfred will come along and say something completely different i either keep a dme relay or FP relay in my toolbox, if i find it ill send it along as well.


ps...if it were me, i'd start by purchasing a can of contact cleaner (if you can't get a decent price from any one else, just pick up a can of radio shack's severely overpriced contact cleaner, and clean the pins on the TPS connector and the TPS pins themselves. we don't drive japanese cars here, so sometimes we have to suffer the indignity of using da kontact kleaner on stuff to get it working again.

Will pick up some contact cleaner on the way home, clean the terminals and try to get some voltage to #3. If I can't I'm ready to order a DME relay (so don't worry about sending one) and see if that does the trick.