quick ... temperature sensor fooling

[grave77]

Hi guys, I remember once there was a thread here and MarkD told about adding a resistor to the temperature sensor to change the fuel mixture. I need a quick tip of how to fool this sensor to pinpoint an emission problem in my engine. thanks in advance

[shogun]

I quote:
Finally spliced & hardwired the 100 resistor under the boot of the DME temp sensor connector. I am pleased with the smoother idle the +100 ohms gives it. I tried 150 ohms & the idle was smoother yet but it had flat spots under acceleration at higher rpm's. The +100 ohms gives a noticable performance increase and along with the .011", idle is quite nice, no Lexus mind you but still very smooth with minimal dropping of a cyl now & then.

DME temp sensor resistor, 100 ohm x 1 watt, metal film type resistor (less sensitive to temp variations than the old carbon resistors):

The boot conceals the resistor completely:

Temporary test resistor setup:


Lately, I revisited the DME temp sensor and found that it reads out of range when hot, to the point that the fuel mixture is excessively leaned out causing it to stumble. It reads 178 ohms at operating temp (approx. 180F). Bentley says it should measure 270-400 ohms at operating temp, so I put a 100 ohm resistor in series with the sensor connector to put it at 278 ohms. It made a noticeable improvement in idle quality and helped thru-out the rpm range to make the engine rev more smoothly and feel a tiny bit extra power. I tried a 150 ohm resistor & it helped the idle even more but caused flat spots under hard acceleration & felt like it reduced power, so I'm going to splice & hardwire the 100 ohm resistor under the boot after it's due for the bi-annual emmissions test in November.

[grave77]

that sounds cool ... I will go test it now ... I will tell you what happens. thanks allot for this precious information.

[Interceptor]

I quote:
Finally spliced & hardwired the 100 resistor under the boot of the DME temp sensor connector. I am pleased with the smoother idle the +100 ohms gives it. I tried 150 ohms & the idle was smoother yet but it had flat spots under acceleration at higher rpm's. The +100 ohms gives a noticable performance increase and along with the .011", idle is quite nice, no Lexus mind you but still very smooth with minimal dropping of a cyl now & then.

I presume this works only with M30 engines with AFM? How about M50 with MAF?

[Bellicose Right Winger]

Conceptually this will work with any engine, whether there is any improvement in an M50 remains to be seen. It can be done with intake air temp sensor sensor also, at least on cars with AFM. I've read different opinions on whether MFM equipped engines use intake air temp sensor for cold start enrichment or just for knock control. In the early '80's we would wire 2K ohm potentiometers in series with intake air temp sensor to make mixture "adjustable" from drivers seat. Both techniques fool the DME into thinking engine or outside air temp is cooler then actual and richens mixture. This was a dealer installed fix for cold start driveability complaints in 60-70F weather in early motronic BMW's, using a 270 ohm resistor IIRC. I'm certain there's a SIB on it. It's also possible that these modifications are temporary until the adaption algorthims in DME figure out something's changed.

Paul Shovestul

I presume this works only with M30 engines with AFM? How about M50 with MAF?

[Bill R.]

faulty dme temp sensor, he was trying to put it within the range of a good one.... and by doing this to a good sensor it looks like your fooling it by what 25%? This may just make it stay in open loop continually which would be a not so good idea. If you're trying to correct a rough idle why not just adjust the idle air bypass on the afm to make it as rich as possible and still stay within the allowable limits of the dme? And the resistor fix that Paul is talking about was to fix a known problem, bmw corrected this on the later models of e28's but you still see people on ebay selling these things...

I quote:
Finally spliced & hardwired the 100 resistor under the boot of the DME temp sensor connector. I am pleased with the smoother idle the +100 ohms gives it. I tried 150 ohms & the idle was smoother yet but it had flat spots under acceleration at higher rpm's. The +100 ohms gives a noticable performance increase and along with the .011", idle is quite nice, no Lexus mind you but still very smooth with minimal dropping of a cyl now & then.

DME temp sensor resistor, 100 ohm x 1 watt, metal film type resistor (less sensitive to temp variations than the old carbon resistors):

The boot conceals the resistor completely:

Temporary test resistor setup:


Lately, I revisited the DME temp sensor and found that it reads out of range when hot, to the point that the fuel mixture is excessively leaned out causing it to stumble. It reads 178 ohms at operating temp (approx. 180F). Bentley says it should measure 270-400 ohms at operating temp, so I put a 100 ohm resistor in series with the sensor connector to put it at 278 ohms. It made a noticeable improvement in idle quality and helped thru-out the rpm range to make the engine rev more smoothly and feel a tiny bit extra power. I tried a 150 ohm resistor & it helped the idle even more but caused flat spots under hard acceleration & felt like it reduced power, so I'm going to splice & hardwire the 100 ohm resistor under the boot after it's due for the bi-annual emmissions test in November.

[Bill R.]

temp /resistance numbers for it, I'm going to assume that its linear so you can draw out your own graph and get the numbers at realistic operating temps and determine how much you want to change it.. Keep in mind that the dme uses this temp info to help protect the engine, when the temps get extreme besides turning the aux fan on they also richen the mixture and retard the timing typically to help protect the motor.. by altering this you may fool this feature.

faulty dme temp sensor, he was trying to put it within the range of a good one.... and by doing this to a good sensor it looks like your fooling it by what 25%? This may just make it stay in open loop continually which would be a not so good idea. If you're trying to correct a rough idle why not just adjust the idle air bypass on the afm to make it as rich as possible and still stay within the allowable limits of the dme? And the resistor fix that Paul is talking about was to fix a known problem, bmw corrected this on the later models of e28's but you still see people on ebay selling these things...

[grave77]

I have something here ... I measured the resistance on the temp. sensor when the engine was a bit warm it read 680 ohms. is that within the range? I'm trying here to decrease the mixture concentration by adj. the temp. sensor. I also need to know if the AFM voltage should be around 1.25v at idle? I already added the 100 ohms for test this week to see how much carbon will be emitted.
one last things I measured the spark plugs wires and it read 6K ohms some were 5.8 some were 6.2 ... any ideas here?