Checking Your AFM and TPS


Okay I have read multiple threads about checking these items and the procedure being in the Bentley. Well I have a Bentley, and other than making sure the opening on the AFM moves smoothly, I can’t find an electrical test to perform on it. Is this correct or how do people know they have a bad AFM and should replace it? Now on to the TPS, there is an electrical procedure for this, so here it is:

Test the switch by checking for continuity at the switch terminals. Connect an ohmmeter between terminals 2 and 18 (front and middle). Open the throttle part way by hand. Slowly let the throttle return to its idle stop. There should be continuity at the terminals when the throttle lever is approximately .20 - .60 mm (.008 - .024 in) from its stop. Then connect between terminals 3 and 18 (back and middle). Open the throttle slowly. There should be continuity when the throttle switch is within 10 + or - 2 degrees of the full-throttle position.

It then says that you can adjust the sensor position by loosening the screws and check it again. This surprises me since it sounds like most guys just replace them instead of adjust them. Are you guys trying to adjust them and no matter what you never get continuity at the terminals?


The AFM electrical test is on 6-22.
But be careful - I checked mine - found some “flat spots” tried bending the arm so the contacts didn’t ride in the grooves and now I have more “flat spots”. :S


King Tut wrote:

[quote]Okay I have read
It then says that you can adjust the sensor position by loosening the screws and check it again. This surprises me since it sounds like most guys just replace them instead of adjust them. Are you guys trying to adjust them and no matter what you never get continuity at the terminals?[/quote]

The TPS tends to get soaked with oil and fail over time. Apparently there is a fix where you drill a small hole in it’s underside so oil won’t stay in it. I’ve pulled 3 TPS’s from motors that I’ve bought and each one was bad. Either it had continuity where it shouldn’t or vice versa. The problem has never been one of adjustment. They did the T and they did the P. But the S was right out.

I still don’t have a spare TPS.


So I decided to go out and test my TPS real fast, and I got results I definitely wasn’t expecting. The connection between 2 and 18 had continuity all the time on my meter while the 3 and 18 worked as expected only getting continuity right before WOT. Can anyone explain this or do I just chalk it up as another bad TPS sensor and buy a new one. I also noticed there was some fluid in the sensor and connector which was probably oil.


rsott wrote:

[quote]The AFM electrical test is on 6-22.
But be careful - I checked mine - found some “flat spots” tried bending the arm so the contacts didn’t ride in the grooves and now I have more “flat spots”. :S[/quote]

Thanks, I completely missed that somehow. I guess I stopped reading right before that paragraph yesterday. Here are the instructions for the electrical test for the AFM:

Disconnect the harness connector from the sensor and connect an ohmmeter between terminals 7 and 8 (middle/right and right with 2 humps on top of sensor) of the sensor. The resistance between the terminals should increase steadily without any flat spots as the sensor flap is moved to the full open position. If any faults are found, the air flow sensor is faulty and should be replaced.

So it sounds like bending the arm is not an approved Bentley method. :silly:


King Tut wrote:

I’m not reading my Bentley right now, just reading KingTut’s post. But IIRC pins 2 and 18 are open with the throttle closed. Then at just a little bit of throttle 2 and 18 close and you have continuity. So what you are missing is the “open” that tells the DME that you are at idle. Maybe you just need to rotate the TPS a little. Try backing off your idle screw so you can turn your throttle a little more closed and see if you get pins 2 and 18 to open.

If that doesn’t work, than your DME will have problems understanding that you are at idle. Which may not be a crisis.


Actually it is the other way around because pins 2 and 18 are always closed so the TPS is always saying the throttle is closed which is what really confused me. I checked it multiple times and I always had around 30 something ohms (forgot to check the scale) no matter the position of the throttle plate.


King Tut wrote:

Either pin 2 or 3 is supposed to change state a small distance from idle. Check Bentley to identify which. If it’s not changing, rotate the TPS a little and test again.


I wrote the Bentley instructions word for word up above. I know what pin 2 and 3 are supposed to do. Pin 2 definitely doesn’t work as stated in the Bentley manual as it never changes. It always showed some resistance regardless of throttle position which is the opposite of what it should be doing, so I don’t think adjusting will do anything. I might check it again tomorrow and put my multimeter into continuity mode instead of just reading ohms. The car seems to run fine, and I am ready for Road Atlanta, so I might just forget about this till I get back next week. I really just wanted a better understanding of what others consider failure for a TPS sensor. I know my WOT is working, so I am happy.


King Tut wrote:

Don’t be a hard head. I’m going to say this one more time and then I’m done. In order to test your TPS you have to allow the TPS to rotate a little more and see if pin 2 and 18 will close at idle. Back off your idle screw or rotate your TPS to do this. Your TPS may be fine. You won’t know until you let it rotate a little more.


I’m not trying to be a hard head. There must be a miscommunitcation because you say to see if pin 2 and 18 will close at idle while I am trying to tell you they are already closed at idle along with any other throttle position currently. They appear to remain closed (showing resistance) regardless of throttle position. If they were open all the time I could see rotating the TPS allowing them to close at idle like they should with a properly working TPS. I’m hoping I just messed up the test somehow at this point.


When switches are open they have infinite resistance. When they are closed they have no resistance. Pins 2 and 18 should show no resistance at idle, and then infinite (or at least a whole lot of) resistance when the throttle rottates off of idle. Turn the TPS a little so the throttle will rotate it a little more. Then see if pins 2 and 18 go from lots of resistance to almost no resistance when the TPS is turned that additional bit.

There should be only two states for pins 2 and 18. Open (infinite resistance) and closed (no resistance). A lot of resistance and not much resistance will be good enough for open and closed. “Remain closed (showing resistance)” is hard to understand.


Okay so I was confused by what you meant. I was just talking about my ohmmeter reading resistance. When I turn the meter on it showed OL for overload when it wasn’t connected to 2 and 18. Then when I connected to 2 and 18 it always showed some ohms of resistance no matter the throttle position. When I connected to 3 and 18 it showed OL right until I got near WOT and then it showed some ohms of resistance which is what I expected to happen. I’m gonna try it again and just use the continuity mode instead of trying to read resistance.


King Tut wrote:

The resistance mode is fine but we’re approaching this clumsily. “Some ohms of resistance” is too ambiguous. There are 2 states here…“Open” meaning infinite resistance or at least a helova lot of resistance. And “Closed” meaning no resistance or at least not much resistance. It’s fine to read the amount of resistance and determine “open” or “closed”, but you have to decide “am I looking at a helova lot of resistance, or am I looking at not much”.

Screw around with the TPS and see if you can make pins 2 & 18 get continuity (close) at idle.


King Tut wrote:

Yes, but the SpecE30 powers-that-be say it is OK… The stock air flow meter plastic lid may be opened and a slight adjustment may be made to the internal components to allow proper electrical contact, away from a worn groove.


deleted after I read more of the thread


is the TPS disconnected when you’re doing this? Measuring resistance requires an open circuit.

King Tut wrote:


It was definitely disconnected as you can’t access the pins on the sensor without the connector disconnected. I should have just used continuity mode instead of getting into resistance since there is no range of values you are supposed to look for. I will investigate this again once I get the car back home.


dumb new guy bumping this up. checked my tps and no continuity on the switch tests. i hadn’t read this thread so I was shocked when I jimmied the cap off and found it full of oil. I guess this is common. Cleaned it and now it works. re: drilling these to let oil drain–you guys really doing this? anyone running cap off or just siliconing the dog shit out of it? does this really have a drastic impact on hp? my a/f on the dyno seemed ok. does it affect timing maybe?


My TPS has the hole drilled in it. Can’t say how prevalent this is.

How exactly the DME responds to various situations is a bit of a black art. There isn’t consensus on this and I’m no tuner. Best I can do is state what I believe to be true.

The DME has a variety of 3d “maps” that determine injector duration and ignition timing. The 3rd axis is volume of air coming in. The values for injector duration and ignition are also modified by coolant temp (big variable), ambient temp (small variable), O2 sensor info (adjusts fuel trim), and probably another thing or two that I’m forgetting.

Rob Eskew’s boss, name escapes me, has good insights on this sort of thing. He and I aren’t in complete agreement, but his take might be more accurate than mine.

Under partial throttle conditions the DME sets injector duration and ignition timing based on all of it’s inputs. Remember those 3d maps that control the engine management. This is called “closed loop” operation. Under some conditions tho, the DME quits listening to a lot of it’s inputs, and kind of blindly tries to operate as best it can. Think of it as suddenly becoming dumber. This is called “open loop” operation.

During open loop operation, the only variable that the DME listens to (IIRC) is coolant temp. That said, the DME also uses the latest fuel trim info, so if your fuel trim is goofed during partial throttle, it will remain goofed for WOT.

A couple things can trigger open loop. 1) AFM door completely open. I’m told this happens around 4500rpm. 2) TPS signaling WOT. Seems like there’s a 3rd thing but it’s not popping into my head.

So the TPS is one of a couple ways that triggers the DME to go to closed loop operation.

TPS also tells the DME that the engine is idling or is at partial throttle, but for us the important thing is that the TPS tells the DME that you’ve just gone to WOT. This assumes that the DME hasn’t already figured it out by the fully open AFM door.

Impact of a broke-dick TPS. If the above is correct then the only way that the DME is going to know that you are at WOT between 3500-(~)4500 is the TPS. Therefore I would expect a broke TPS to affect midrange performance, but not top end.