Sensors

Throttle Position Sensor - TPS

The throttle position sensor is a variable resistor which converts the angle of the throttle plate into a dc voltage between approximately 0v and 5v. It is used mainly to fine tune the fuel delivery for driveability and fuel economy, a faulty sensor could cause big problems though. The sensor indicates when the throttle is shut so that the ecu can take control of the engine speed using the idle speed control valve and also on deceleration, when the revs are high enough, the fuel can be shut off which helps mpg. It also indicates when a lot of throttle is being used, so the mixture can be made richer for maximum power. There are two types of sensor. The PF01 used on the level 1 and level 6 ecu and the PF09 used on the level 8 ecu.

 

PF01 - this is a variable resistor but with a switch in series with the wiper so that when the throttle is closed the output goes open circuit. The idea behind this is that the closed position is clearly defined. The sensor must be set up so the slightest movement will close the switch and the voltage will change from 5v to 4.4v.(the voltage decreases to 0.35v when the throttle is fully open). The disadvantage is that the most likely fault is an open circuit due to wear or a wiring problem, which cannot be identified by the ecu and so will not flag up a fault. A faulty output could cut the fuel during normal driving causing misfires or an engine which won't rev. The setting up of the switch position is critical as the throttle does not have to open very far for the revs to rise and the ecu must realize that it is the throttle controlling the revs and not the idle speed control valve, the result may be surges or stalling.

PF09 - This sensor is just a variable resistor and the extra programming capacity in the level 8 ecu is able to automatically calibrate the closed throttle position. The calibration takes place during normal driving by detecting the lowest stable voltage from the tps (which should be the closed position) and it then sets the threshold to that value plus a few millivolts. Because the voltage should never be 0v or 5v, unlike the PF01, then an error can be flagged up if they do to show a tps error and the program modified to ignore it. The tps is wired with the supplies reversed compared with the level1 / level6 so the voltage increases with throttle angle.

 

 

 

Manifold Absolute Pressure Sensor - MAP

The MAP sensor is the primary load input for the ECU. It creates a voltage directly proportional to the vacuum (or boost pressure) in the inlet manifold after the throttle plate. If you know the engine speed and the air pressure, then you know how much air is entering the cylinders, so you can then calculate how much fuel to inject. The standard MAP sensor is a 2 bar device which means it can measure 1 bar below atmospheric pressure and 1 bar above atmospheric pressure (-15psi to +15psi). If boost pressures above 1 bar are going to be encountered then a 3 bar MAP sensor will be required. This also measures 1 bar below atmospheric but goes up to 2 bars above atmospheric (-15psi to +30psi). You cannot just swap a 3 bar sensor for a 2 bar without changing the engine mapping because, although the voltage range is the same, the voltage to pressure ratio is different. The 2 bar sensor outputs 2.35 volts at atmospheric pressure whereas the 3 bar outputs 1.68 volts.

The map sensor voltage is converted inside the ecu into two types of value, an 8 bit value and a 16 bit value. The 16 bit value is used by the fuel and ignition look up tables because it is corrected for air temperature. The calculations are fairly complex, but the resulting values for different air temperatures are shown in the table below. As the air temperature increases, and the air becomes less dense, then the look up value moves down the look up table so that less fuel being injected. The following tables are for the L1 - L8.

MAP Volts	8 bit MAP	0'C ACT 16 bit MAP	20'C ACT 16 bit MAP	40'C ACT 16 bit MAP	60'C ACT 16 bit MAP	80'C ACT 16 bit MAP
4.75	242	3111	3030	2933	2843	2741
4.5	229	2952	2875	2783	2697	2601
4.25	216	2793	2720	2633	2552	2460
4.0	204	2634	2565	2483	2406	2320
3.75	191	2475	2410	2333	2261	2180
3.5	178	2316	2255	2183	2115	2040
3.25	165	2156	2100	2033	1970	1900
3.0	153	1997	1945	1883	1824	1759
2.75	140	1838	1790	1733	1679	1619
2.5	127	1679	1635	1582	1534	1479
2.25	114	1520	1480	1432	1388	1339
2.0	102	1360	1325	1282	1243	1198
1.75	89	1201	1170	1132	1097	1058
1.5	76	1042	1015	982	952	918
1.25	63	883	860	832	806	778
1.0	51	724	705	682	661	637
0.75	38	564	550	532	516	497
0.5	25	405	395	382	370	357
0.25	12	246	240	232	225	217

 

If the map sensor is upgraded to a 3 bar from a 2 bar then the pressure headers will need to be adjusted to compensate. The table below shows the result of changing the sensor.

16 bit MAP	2 bar pressure	3 bar pressure
901h = 2305	1.67	2.20
804h = 2052	1.49	1.96
712h = 1810	1.31	1.73
642h = 1602	1.16	1.53
573h = 1395	1.01	1.33
4AFh = 1199	0.87	1.14
3DFh = 991	0.72	0.94
33Eh = 830	0.60	0.79
2AEh = 686	0.50	0.65
224h = 548	0.40	0.52
193h = 403	0.29	0.38
15Ah = 346	0.25	0.33
120h = 288	0.21	0.27

As can be seen from the above table, if you don't change the headers after upgrading the sensor then the fuel and ignition values will no longer correspond to the pressure they were mapped to. I.e. The fuel quantity which was correct for 1.01 bar will now only be supplied at 1.33 bar. Basically all the pressures have increased by a factor of 1.315. Therefore to return the headers back to the correct value you need to divide all the 16 bit map values by 1.315 as below.

old 2 bar 16 bit MAP	3 bar pressure	new 3 bar 16 bit MAP
901h = 2305	1.67	6D9h = 1753
804h = 2052	1.49	618h = 1560
712h = 1810	1.31	560h = 1376
642h = 1602	1.16	4C2h = 1218
573h = 1395	1.01	425h = 1061
4AFh = 1199	0.87	390h = 912
3DFh = 991	0.72	2F1h = 753
33Eh = 830	0.60	277h = 631
2AEh = 686	0.50	20Ah = 522
224h = 548	0.40	1A1h = 417
193h = 403	0.29	132h = 306
15Ah = 346	0.25	107h = 263
120h = 288	0.21	0DBh = 219

Obviously just changing the pressures to be the same for a 2 bar sensor is a bit pointless as you are still limiting yourself to1 bar of boost pressure. What normally happens is that the lower values are moved so that the area where the car is normally driven (from 0.2 bar to 0.8 bar) are kept as they were for maximum driveability. The remaining points can then be stretched out to cover the increased boost pressures required. The on boost points don't need to be as closely spaced as the ones around idle.

The wiring for the map sensor should be screened, and the screen connected to the chassis, to reduce interference getting into the signal to the ecu. The ecu is sensitive to changes of a few millivolts which on other sensors are not too critical as they are filtered electrically and in software to slow them down. The map signal meanwhile is sampled every 90 degrees and stored. A trend for each cylinder is created and if the pressure suddenly rises then it is assumed the throttle has been opened and a fuel correction factor is added to prevent a lean mixture causing a flat spot. The amount of fuel added depends on the map sensor voltage variation so it is important that the signal is a true representation of the manifold vacuum and not a noise spike.

The extra fuel is injected as a separate pulse to the main pulse and can be seen in the trace above after a snap throttle opening. In the above example, after 4 engine revs (2 injection cycles), it reverts back to normal operation as the map value settles. The red trace shows the 90 degree sync pulses. The reason for the early injection pulse is to richen up the over weak mixture which has just entered the cylinder before the inlet valve has time to close. This correction is only required at lower revs so it stops above 3000rpm.

 

 

 

Engine Coolant Temperature Sensor - ECT

The ECT sensor is a negative temperature coefficient thermistor which is placed in the water jacket of the engines cylinder head. It is basically a resistor which changes its value depending on temperature and is one of the most important inputs for the ecu. The ecu uses the sensor as a potential divider to create a voltage which varies with temperature over about a 4 volt range. A faulty sensor can create massive under or over fueling, which will not be flagged up by the ecu as a fault unless it goes fully open or short circuit. The resistance of the sensor is nonlinear and is shown in the table below.

Temp 'C	Resistance
-40	100950
-30	53100
-20	29120
-10	16600
0	9750
10	5970
20	3750
25	3000
30	2420
40	1596
50	1080
60	746
70	526
80	377
90	275
100	204
110	153
125	102

 

 

Air Charge Temperature Sensor - ACT

The ACT sensor is a negative temperature coefficient thermistor which is placed in the intake plenum to measure the temperature of the air entering the cylinders. It is the same sensing element as the ECT sensor but, instead of being in a brass tube, it is exposed. This gives it a faster response to temperature changes. The ecu uses the sensor as a potential divider to create a voltage which varies with temperature over about a 4 volt range. The ACT input does not make the same degree of fueling correction as the ECT sensor but is still an important input, especially on a turbo engine, when it is used to reduce boost pressure if the air temperature is too hot or cold. A faulty sensor will not be flagged up by the ecu as a fault unless it goes fully open or short circuit. The resistance of the sensor is the same as the ECT sensor and is shown below.

Temp 'C	Resistance
-40	100950
-30	53100
-20	29120
-10	16600
0	9750
10	5970
20	3750
25	3000
30	2420
40	1596
50	1080
60	746
70	526
80	377
90	275
100	204
110	153
125	102