VOM-and TPS ohmage measuring question.....

[ryan roopnarine]

question... at 200ohm range on my multimeter, i get .4 ohms measurement when i touch the leads together for a continuity check....does this rule out the instrument for making accurate assessments?

i measured my TPS as having 1427 ohms on a 2000 ohm scale. the bentley says it should be 1k ohms in the same position. if i multiply out .4 times 1k, it gives me about 1400 ohms, which would make sense. however, when i switch my meter to 20k ohms, and do the wide open test (book says APPROXIMATELY 4K OHMS) the meter reads 3.758 k ohms.

i guess my question is, as bentley says the values are approximately 1k and 4k, is my meter accurate enough to measure the tps, since it says .4 ohms on a continutity test?

[SRR2]

The .4 ohms is an offset (add/subtract, not multiply/divide) in resistance measurement due to meter error and lead resistance. If you're measuring something with very low resistance, you'd subtract the offset from the reading to get a better idea of the resistance of the item you're testing. "TPS" = throttle position switch? If so, Bentley is FOS if they say 1k ohms. That's borderline inoperative. You should measure essentially 0 ohms (say less than 10 ohms to allow for the condition of your meter and leads) in the switch on the lowest resistance setting of your multimeter when the contacts are closed, and no continuity when they're open. What Bentley are you using? I don't remember anything like this in mine.

Does your TPS have a WOT position? I've examined both the 320 and 302 TPSs and neither has the WOT switch in them, ergo, no WOT signal to the ECU. What P/N is your switch?

[SRR2]

Oh, ok, nevermind. You have the M50/60 version where there's actually a pot in there to sense actual throttle position. It's not a simple switch like on the M20 and M30. My comments about the multimeter still apply, but disregard the stuff about the operation of the switch.

In the case of your sensor, it looks like there's a 4K pot connected between 1-3 and its wiper is on 2. Their test is OK.

Here's what you need to do to sort this out. Connect one lead to 2 and the other to 1. Turn the pot for minimum resistance. What do you get? Turn the pot for maximum resistance. What do you get? Move the lead from 1 to 3 and crank the pot to the other end for minimum resistance. What do you get? And then what maximum resistance? What do you get from 1-3? You can disregard the .4 ohms meter/ lead offset in this test.

[ryan roopnarine]

i made two posts for this question...one for people that might know about voms and one for people that have measured the tps....thought they might overlap but wasn't sure...here's the text from that post





..........anyone that knows the nuances of measuring the TPS, (m50 or other)pls. respond.....

--------------------------------------------------------------------------------

i put up a thread for anyone knowledgable about vom's. long story short, vom is measuring .4 ohms for closed continuity when touching probes together. i did the closed position test, non-moving test for the TPS, it reads 3780 ohms on a 20k ohm scale....the book says the value is 4k ohms APPROXIMATE. i guess i was hoping somebody could clarify the approximate part...is it like +/- 10 ohms, 100 ohms (&c. &c). i did the variable test for closed to open test, the closed reads 1325 ohms on 2k ohms, if i multiply out .4 ohms times 1000 it would be like 1400 ohms but i have a hard time believing that the meter would pick up that error on that large of a measurement. the wide open test however makes no sense. book says 4k ohms wide open, meter is bringing back 3.785k ohms on a 20k ohm scale. i guess this might mean its kaput, but i would really like some confirmation help, please.

[SRR2]

I can help you with this, but you're gonna have to answer my questions. This other post isn't useful. based on the sketchy information you have there, it looks like your TPS is probably ok, but without the other information, I wouldn't bet a lot of money on it.

Get that notion of "multiplying out the .4 ohms" out of your head. That's not the way it works. That .4 ohms is just the residual resistance in the meter and leads. For this test you can ignore it. And even if you can't ignore it on very low resistances, you don't multiply it, you subtract it.

[ryan roopnarine]

Oh, ok, nevermind. You have the M50/60 version where there's actually a pot in there to sense actual throttle position. It's not a simple switch like on the M20 and M30. My comments about the multimeter still apply, but disregard the stuff about the operation of the switch.
In the case of your sensor, it looks like there's a 4K pot connected between 1-3 and its wiper is on 2. Their test is OK.

Here's what you need to do to sort this out. Connect one lead to 2 and the other to 1. Turn the pot for minimum resistance. What do you get?
1325 ohms on a 2000 ohm scale

Turn the pot for maximum resistance. What do you get?

3.725 k ohms on a 20k ohm scale


Move the lead from 1 to 3 and crank the pot to the other end for minimum resistance. What do you get?

when its shut like i thought bentley says it should be, its reading about 1330 ohms on 2k scale.

And then what maximum resistance? What do you get from 1-3? You can disregard the .4 ohms meter/ lead offset in this test

i haven't measured the 1+3 terminals when the throttle was open all the way, i don't think that bentley gives a specification for that so i didn't. i can certainly do that when i go home though.

[ryan roopnarine]

bump, please

[Javier]

TPS in M50 1992 should feed a voltage signal from 0.4 to 4.2 volts to the ECU (DME M3.1).

It is a pot with offset resistors at both ends, so following SSR2 advise, fill in this table:

A= Resistance from 1 to 2 at closed throttle = ??? ohms
B= Resistance from 2 to 3 at closed throttle = ??? ohms
C= Resistance from 1 to 2 at full open throttle = ??? ohms
D= Resistance from 2 to 3 at full open throttle = ??? ohms

then A + B should equals C + D
5 * A/(A+B) should equals about 0.4, and
5 * C/(C+D) should equals about 4.2.

Significant variations would be suspicious, I have no M50 TPS, so cannot give you reference values from my car, may be someone else can. M30 has only contact switches for idle and wide open.

Personally, I find the 1472 value a little bit high, related to 3758, if ECU does not receive top low value, may not realize you are looking for no gas (idle) adjustment through ICV.

You should also verify that you have 5 Vdc at the TPS connector pin 3 when key is at run.

If makes you feel better, yes you should ignore the 0.4 ohm minimum reading of your meter, it is of no significance when you are taking readings in the order of 4K (four orders of magnitude difference).

Javier

[SRR2]

I think he has most of that already, based on his last post with data. It looks somewhere in the ballpark according to your description, but if your .4 and 4.2 ratios are right, maybe not. His low end resistance would be too high I think. The nasty thing is that this is tough to troubleshoot with resistance alone. You really need to impose a voltage on the pot and read what comes off the wiper. That takes series resistance in the wiper out of it as well as CRV. Same thing with that kluge of an AFM on the M20/30 too. I hate pots. Anything that uses a pot as a transducer is a kluge and the engineer who cooked it up should be hung by his toes.

BTW, I'm pretty sure you don't have a WOT switch in your TPS. If you have a chance, please check it and tell me the part number if you in fact have one. (I know, I know, Bentley etc. says it's there. Thing is, I've looked at a few of these and have yet to find a switch with WOT installed.) I'd like to know the P/N of the switch that has it -- if there really is such a thing.

[Javier]

I'm curious on the resistance values, so to check the BMW drawings are acurate on the 0.4 - 4.2 range.

Did you solve the SRS issue?

Javier