mats

Hi. 1998 502 Mag MPI. According to scan tool the ECM only registers 73% opening at WOT. What impact does this have on performance if any? What should the Throttle Position sensor resistance be at idle and WOT? My TPS measures 1.63 kohms and 8.53 kohms ( C to A, C to B) at the idle end and same values but reversed at WOT end. The TPS is the 805226A1 which I believe is used on all newer 502 MPIs.

mjb

I'm not sure about the resistance rating....but the TPS has a 5 volt input signal from the ECM. At idle, the TPS output voltage should be somewhere around .45-.70 volts (maybe someone in the know can get more specific on this number) or 0% possition on the scan tool. At WOT the merc manual says output voltage should be around 4.5 volts or 100% possition on the scan tool. At a steady RPM in the midrange, the TPS output voltage is not being used by the ECM for fuel or spark adjustment but at WOT the TPS output voltage is very important so the ECM knows you are at WOT and can adjust fuel and spark accordingly. If your TPS is only indicating 73% then I would think that the ECM doen not know that you are at full throttle and that would certainly hurt your performance.

mats

Thanks for the input. I have just been in contact with CTS who is the manufacturer of the TPS and I do now understand how to adjust it. The easiest way is to take a file, extend the bolt holes and twist it until you get the correct voltage at idle and WOT. WeŽll see what happens.....

Raylar

You should not have to adjust or file the mountings on your TPS. If the scan tool is only reading 73% and you have verified that reading when you visually see the throttl open 100%, then something is either wrong internally with the TPS sensor or the wiring or voltage to the ecm. If its about 4.5 volts when the throttl is fully open then scan tool should see a signal from the ecm of about 98-100% throttle and the throttle should be open that corresponding amount. Yes if this is the real signal from the ECM of 73% at WOT then the performance of the motor will be greatly affected, since the ecm thinks the throttle is open only 73%. Check all the connections to and make sure they are not wet, corroded or damaged, this can send a low signal to the ecm from the TPS.

Ray @ Raylar

Dave_N

Hey mats,
I tested two TPS's I have, one take out and one new one.

Take out
"C" to" A" 7.24 kohms to 1.38 kohms
"C" to "B" 1.38 kohms to 7.24 kohms

New
"C" to" A" 7.63 kohms to 1.41 kohms
"C" to "B" 1.42 kohms to 7.63 kohms

Dave

mats

Thanks for the input. My conclusion is that there is nothing wrong with the TP sensor. It appears that there are two things needed to make the ECM realize the whole swing 0-100%. One is good connections of course. I will check those. The other is to mount the TP sensor at the right angle. The throttle plate shaft that also operates the TP sensor on a 502 only travels less than 90 degrees from idle to WOT. The TP sensor on the other hand has a swing of 100 degrees. It must be mounted almost exactly at the right angle to be able to deliver 0.7 to 4.5 V. I will need to borrow a scan tool to verify that it works. Thanks. The next question is what the ECM does with the information. It definitely looks at rapid changes to see if you hammer or decelerate. I am not sure however, how the absolute value is used. Made a three hour test run. The plugs look even better now!?

Dave_N

mats,
More info for you. The above data was with the sensors off the throttle body. As you pointed out, the range of movement is different on the throttle body vs off.

So I tested the above TPS's on a 502MPI throttle body. First resistance and then voltage with power supplied by a sensor simulator at exactly 5.00 volts.

This is what I got;

Take Out
Resistance - "C" to "A" - 6.53 kohms to 2.00 kohms
Voltage - .75 v to 4.47 v

New
Resistance - "C" to "A" - 6.71 kohms to 2.04 kohms
Voltage - .73 v to 4.49 v

Dave

mats

Thanks Dave! It shows that it is quite a squeeze , if not impossible sometimes, to get the TPS to cover the whole 0.7-4.5V range.

Dave_N

mats,
what does yours show (resistance) on the throttle body?

Dave

mats

The resistance at the throttlebody now shows 2.58 low and 7.87 high. The TPS alone has a resistance C to A of 8.53 and 1.63 C to B. The resistance A to B is hence 6.90 (8.53-1.63). That should theoretically mean a swing from appr 0.685 to 4.525 V applying 5.00 V to it and assuming an infinite resistance at the ECM. WeŽll see what the scan tool says.......

As the CTS guys said the variation between the components is quite wide but it does not matter in this type of application as long as the resistance progression is linear. It is the mechanical travel and the input voltage that in the end defines the swing.

Boating sure is a learning experience.........