TPI SENSOR IDENTIFICATION and LOCATION

The pictures shown below  are the standard sensors and modules typical to a tuned port injection set up.


Picture above is the egr temperature switch, also known as a diagnostic switch. Its operation, along with other engine sensing parameters inform the ecm on the status of the egr valve and/or solenoid.  This is used only on the 1986 TBI engines and the 1986-89 TPI engines. The right hand picture shows the location of this sensor.  The 1990-92 TPI uses the O2 sensor for feedback to determined whether or not the EGR valve is functioning correctly and will not have the diagnostic switch. 


The O2 sensor is a very important sensor as it is used to monitor the oxygen content in the exhaust stream. It is basically a miniature battery that sends a millivolt signal to the ecm. It is important to keep high voltage wiring such as spark plug wires at least a couple of inches  away from the O2 wire. Along with other sensors such as coolant, air, throttle position and either maf or map, it commands the ecm to adjust the fuel injector on time to maintain as close as a 14.7 to 1 air/fuel ratio as possible.

The single wire O2 sensor was used where the sensor was installed very close to the exhaust ports on the engine, such as stock exhaust manifold. If you are using headers, you will need to have a sensor bung installed in the header collector to install a heated O2 sensor and have the appropriate wiring to make it work. the second picture above a commonly used 4-wire sensor used on many 1996 and later 6 and 8 cylinder engines. The heated O2 sensor is needed on a header system, because the exhaust gases cool down too much at idle and will not activate the O2 sensor, the ecm then "sees" this and goes into "open loop" and causes the engine to run rich and idle rough, even though the engine may be fully warmed up.

The diagram above shows how simple the installation is. The tan wire (ecm ground) is connected to the O2 sensor low wire for the 1227727/730 speed density engines. This is ecm terminal E15. On the1226870- 1227165 MAF ecm it is ecm terminal D6. The ecm sensor (purple) input goes to the appropriate input to the ecm. The black wire goes to the chassis ground. The last wire (pink) is the power wire for the heated element. This taps into the ignition power wire which is hot only when the ignition switch is in the "on" or "run" position. Be sure to use a inline 10A fuse to protect the wiring in case of a shorted condition.

The knock sensor is a special electronic microphone specially designed to detect  knock or pre-detonation in the engine cylinders. The signal from the sensor is then sent to a esc module (1985-88 models) or directly to the ecm (1989 and later) and tells it to retard the timing until the knock is eliminated. Take note that the '85-88 sensors will not interchange with 1990-92 speed density TPI and later and vice versa. If you have a esc module you will need the earlier sensor.

Location for the knock sensor is on the passenger side of the block as shown here.

Another thing to remember that the knock sensor module is matches to engine size, whether it is a 305 or 350, on the TBI and MAF TPI engines. The knock sensor is matched to the prom on the speed density TPI since the esc knock control module is embedded on it, and it assumes you have the proper chip for the engine size, it is possible to use a 305 knock sensor, with a 305 prom re-burned for a 350 that has the embedded knock sensor and it would work, but it is not recommended because each engine displacement has its own specific knock signature frequency that is match to it.

The knock sensors are also different for the LT1 engines, although the physically look the same as the earlier knock sensors, except the connector end is different, so they cannot be intentionally  installed on the earlier engines.


ESC module used on 1985-89 model TPI's. Later years (1990-92) will not have one as the spark control is embedded on the prom.


Picture above shows knock sensor module on the speed density cars. Although the "730" was also used on the V6 cars with the identical looking prom, the knock module is different enough that a V6 prom, even with the main chip re-burned with a V8 calibration will not work and cause the engine to either "limp home" or not detect knock and eventually damage the engine. The 1992-93 LT1 prom uses the same system.

The knock sensor (above) used on the 1994-96 LT1/LT4 engines. There are two different types depending on if the engine has one or two knock sensors, see the LT1 electrical page for more information.


The first picture is the TPS and IAC used on the 1985-92 TPI engines. The second picture shows the TPS and IAC used on the 1992-96 LT1 engines. The late model style TPS was also used on the 1991 and later TBI engines.

The throttle position sensor (TPS) is a variable resistor that changes resistance as the throttle angle is changes, this sends a voltage signal back to the ecm, along with the other sensors to calculate fuel injector on time. Normal voltage at idle is around .55V, while at WOT it is closer to 4.5V.

NOTE: The Throttle body TPS sensors from the TPI engines will not swap from the TBI although the physically look the same, internally, the operate the voltage output in the opposite direction unless you care to swap the power (gray) and sensor ground (black) wires on the TPS connector.

The location of the throttle position sensor (TPS) blue arrow, and the Idle air control (IAC) red arrow. The LT1 sensors are physically different but are in the same location as the 1985-92 TPI engines.


The idle air control motor (IAC) is a device controlled by the ecm to adjust the idle speed. There is a air bleed in the throttle body and the pintle in IAC acts as a restriction, thus a controlled vacuum leak. The ecm controls the position of the IAC based on start up, warm up, and closed loop operation as well as when ever loads are imposed on the engine such as A/C. Working properly it is extremely effective in cold weather drivability.


The mass air flow sensor (MAF) in the first picture is used on 1985 through 1989 models only. It is extremely accurate in measuring the weight of the incoming air based on temperature, humidity, barometric pressure and velocity, with this information with other sensors allows the ecm to fine tune fuel calculations. The second picture is the MAF sensor used on the 1994 and later LT1/LT4 engines.


The coolant temperature sensor (CST) senses the coolant temperature, while the manifold/intake air temperature sensor (MAT or IAT) monitors in the incoming air temperature. Both of the sensors are called thermistor as they change resistance based on temperature. The sensors are used by the ecm also with the other sensors to control fuel injectors. The cst is also used by the ecm to control the cooling fan. The picture on the right shows the location of coolant sensor (blue arrow). the cold start injector thermo-time switch (red arrow) is used only on the 1985-88 TPI engines. In 1989 the cold start injector and switch was eliminated and cold start enrichment is done through the main injectors.

  There were two different types of MAT sensors used. The early style on the left was an shielded sensor used on the 1985 and some 1986 models, the sensor on the right is the same as the coolant sensor. The respected plug ins are needed   to match the sensor used. The right hand picture shows the location of the MAT sensor (under the rear of the intake plenum).

The map sensor used on 1990-92 TPI systems, replacing the MAF system of earlier years. It is not as accurate as the maf in calculating air mass, only as detecting the load of the engine through manifold vacuum. The speed density as it is called is very sensitive to anything that is modified that will upset the preset calibrations in the prom, engine performance suffers as the engine wears and systems degrade. But it is still popular for swaps as the wiring is less involved, air intake plumbing is usually simplified, and the map sensor (around $30) is considerable cheaper than a maf sensor (typically $200).  The MAP was mounted on a boss cast into the TPI top plenum for the 199-92 cars. If you are swapping a pre 1990 intake without the mounting bosses to a speed density system, you will need to mount the MAP on the firewall or similar location.


The cold start injector used on the 1985-88 TPI engines. The fuel pipe is plumbed into the back of the left hand (drivers) side fuel rail. This is also used in conjunction with the thermo-time switch mounted in the front of the manifold. If you are eliminating this on a MAF TPI set up, you will need the prom reprogrammed for the 1989 calibration. If you are using a 1985-88 TPI intake with a speed density system, you just leave this on place or use a cold start injection delete kit, which plugs the opening for the eliminated injector and fuel pipe (right hand picture).



The fuel pump back up switch (left hand side sensor) is used for the protection of the motor if the oil pressure falls below 5 psi. This is mandatory to use and bypassing it is not a wise idea. It also serves as a fuel pump switch in case you have a bad relay, once oil pressure is sensed, it will allow the fuel pump to run. The sensor on the right is for the oil pressure warning light or gauge.

There is two types of fuel pump back up switches. The one on the left is the early style with a 1/8" NPT that uses a two terminal connector. Some vehicles such as Cadillac 4.1L V8 has a three prong, which the center prong is for a warning light. The sensor on the right is for the later TPI engines, if equipped and can be either 1/8" or 1/4" NPT and can be either of the two or three pin.


The EGR solenoid switch mounts on the rear right hand side of intake manifold. The vacuum lines go to a full vacuum source from the throttle body and from the solenoid to the egr valve itself.

This is a picture of the Air Select (green connector) and Air Divert (white connector). The air supply comes from a pipe that mounts to the back of the A.I.R (smog)  pump mounted on the engine. 

The AIR system includes a pump, control valves -air select or (port solenoid) and air divert or (converter solenoid) a check valve and plumbing to exhaust manifolds, air cleaner assembly (if used) and catalytic converter.

Air flows from the smog pump through the control solenoids and to a check valve. The check valve prevents the back flow of exhaust into the pump in case of an exhaust backfire or pump failure. The air control valve provides air direct to the air cleaner (if equipped) for converter protection or directs air to the switching valve.

The air switching valve directs air to the exhaust ports during cold engine operation or anytime the system is in open loop operation. It directs air to the converter beds during closed loop operation. When divert is needed, the ECM turns off the converter solenoid and causing air to be diverted to air cleaner or to atmosphere. Air is directed as such when:

Both the control valve and the switching solenoid valve are ecm controlled.

If no oxygen flow enters the exhaust stream at the exhaust ports or converter pipe, HC an CO emissions will be too high. 

Air flowing to the exhaust ports all the time could increase temperature of converter.

Electrical failure of the control valve will divert air flow to the air cleaner at all times. Air flow to the converter all the time if an open circuit occurs in the divert valve. Mechanical failures in the valves could cause air flow incorrectly to the exhaust ports or converter.

Using a scanner will test the function of the air management system. The scanner can activate the air switch solenoid, routing the air into the exhaust ports which then can be monitored by watching the O2 sensor voltage. Extra air in the system ahead of the O2 sensor will drive the sensor voltage low and the integrator up.

Air management is controlled by Air Select (Port) and Air Divert (Converter) solenoid valves which are ECM activated as needed. When the solenoid is commanded by ECM, manifold vacuum will activate valve and allow air to be directed as follows:



The canister purge valve is also an important part of the emission system and was in use even before the fuel injection era. It works by trapping the fuel fumes in a carbon filter canister and purges these fumes into the intake manifold behind the plates of the throttle body. The ecm controls the pulse width modulation (turned ON and OFF several times a second), this duty cycle is determined by air flow and engine load. The duty cycle id calculated by the ECM and the output determined when the following conditions have been met:





Shown above are the typical tpi ecm's used. Starting left, going clockwise: ecm #1226870 with the separate prom and calpak chips. This ecm was only used in 1985, not shown is the "piggyback" maf control module. This picture shows an aftermarket replacement ecm. Top: ecm #1227165 used from 1986 to 1989, shown with memcal that combines both the prom and calpak in one package. Right: ecm#1227730, speed density, also memcal, but note the different connectors as compared to earlier ecm's. Bottom: ecm#1227727, used exclusively on the 1990-91 Corvettes, speed density, memcal, notice the four connector plug ins.


This is the maf controller exclusively used on the 1985 TPI's, Camaro's, Corvette's and Firebird's.


The ecm's shown above have to be used with it's particular harness, be especially careful when getting a 1986 through 1989 harness, look at the connection ends to be sure you are not getting a 1985 harness, which will have a extra plug in that is square and has a black, red, blue, two orange wires, and pink/black wires. This is for the maf controller module, so if you are buying a 1985 harness be sure it has this controller and the proper ecm(1226870) to go with it. This ecm will not interchange with the 1986-89 harness although the connectors to the ecm are the same!

The same caution applies if you are wanting either a maf or speed density harness. For maf systems (1985-89) look for the group of connectors that go towards the throttle body. There will be a 4 pin square connector with four wires (IAC), a flat 3 pin/3 wire connector (TPS), and 2pin/2 wire connector with black and yellow wire (CTS) and if equipped with cold start a square 2pin/2 wire, (pink and purple) connector, for 1985-88 harnesses. In this group should be a flat 5 pin/5 wire connector with the following wire colors: red, drk. blue, drk. green, black and black with white stripe. This connector is for your mass air flow sensor. This identifies your mass air flow harness. Also be aware of the differences between the Camaro and Firebird harnesses, in relations to what side the vapor canister purge control sits on. Look for a  connector with two wires, usually a black/pink and dark green/yellow wires that is a long length by itself. [The ecm connectors look like this].

If you are wanting a speed density harness, look for the map sensor connector on the main trunk line of the harness, close to the right hand bank of injector connectors. This location is close to the right hand rear of the top air plenum where there is a map sensor threaded boss. The map connector is a 3pin/3 wire with a light green, gray and black wires. This connector is for your map sensor. Also look at the ecm connectors, there should be three separate, two black, one yellow. This identifies your speed density harness. Also be aware of the differences in the location of the canister purge solenoid. [The ecm connectors  look like this].

The 1990-92 Corvette harness is unique in itself. The ecm is mounted in the engine compartment and is identified by the ecm service #1227727. The ecm has four separate connectors that plug into the ecm, that are four different colors: yellow or gray, red, green and brown. It is a speed density system. [ The ecm connectors look like this].

When you have decided what harness to use, and have checked to see if its what someone has described it as, look it over very carefully. Look for broken connectors, missing connectors, frayed or broken wiring. If you are handy at doing electrical repairs and use the chart above on connector interchanges, you may be able to strike a better bargaining price, if the seller knows you may walk away. It is a lot easier to examine a harness when it is out of the vehicle. The problem with a orphaned harness (removed from vehicle) it is likely most of the sensors will be missing. The person's may have be impatient or ignorant on knowing how to remove the connectors from the injectors, cold start switch and cold start injector. Ditto for the starter and alternator connections. This is not a problem since the battery wire from alternator to the starter is one piece that is found on many GM RWD vehicles. The same goes for the purple wire to the starter solenoid, it is an easy to fix item. If the relay plug in junctions have been cut, you will find that many GM vehicles from 1981 to 1986 use the same relays that also with some effort can be replaced. Some relays particular to a certain function may be keyed only to a specific connector, but with some careful work these can be figured out.

If you decide to buy a aftermarket harness, they are hassle free, come with installation instructions, parts listing numbers for the relays and sensors. And come with a guarantee and usually a toll free number for help. This is the way to go if you have neither time or skills to repair or modify a damaged harness. The aftermarket harness will have fewer wires to hassle with tapping into your vehicles electrical system, lessening the chances of mistakes.

The harnesses, both stock and aftermarket will have provisions for the TCC in a overdrive transmission and VSS hookup so the converter lock up and other computer functions such as egr function and closed loop fuel calibration that depend on the speed sensor. Some aftermarket harnesses may or may not have provisions for the smog pump control circuits. Most will work with you if you need to have full compliance with your local emission laws, so be sure to check to see if this applies to you. 

VSS (Vehicle Speed Sensor)

If you are not using a overdrive transmission such as a 2004R or 700R4, and using just a THM350 or 400, you'll still need to have a provision for some kind of speed sensor. Although there is kits to allow the OD transmissions to function as if they were computer controlled, the speed sensor is also used for other functions. If the computer does not see the VSS functioning, it will set a trouble code in the computer and the engine will not perform to its potential. The only way around this is possibly to have the prom chip in the ecm reprogrammed  to ignore the speed sensor.

1985-1989 TBI/TPI engines:

Camaro: Uses the optical VSS buffer that attaches to the back of the speedometer head. The speedometer head has  two reflective blades 180 degrees apart that functions as a 2,000 pulse per mile (2K)  sensor output to the ecm.  Some of these sensors have 2 outputs, the brown wire goes to the ecm, the other wire (usually red) is the output to the cruise control module. 

Most conversion projects wont use this set up unless the vehicle they are doing a TPI transplant has been produced into the computer controlled era starting in 1981, such as the 1981 Camaro, Firebird, Malibu, El Camino and others.


Picture above is the 2K pulse optical speed sensor

Firebird: Most (not all) Firebirds seemed to be equipped with permanent magnet VSS that attaches to the transmission in place of the speedometer cable. These vehicles use an electronic speedometer. The PM VSS has two wires (purple and yellow) which goes to a VSS buffer module. The VSS is a 4000 pulse per mile (4K), the buffer takes this signal and splits it into two outputs. The green wire is a 4K output for the electronic speedometer, the red wire is a 2K output to the cruise control module. The automatics and manual transmissions VSS sensors do not interchange.


Pictured above is the 4K PM VSS uses on the 1985-92 700R4 and 4L60 automatic transmissions. The manual trans sensor is physically different. It uses the VSS buffer module to condition and split the signal. The gear on the end of the shaft comes in many different gear teeth numbers to match gear ratio and tire size for speedometer-ecm-cruise control accuracy.

If you have an 700R4 with the electronic speedometer and you are putting this is a vehicle with a mechanical speedometer, sells the replacement gear module with a mechanical speedometer attachment. There are two different styles, one is for transmissions with 34-39 tooth driven gears, and one for 40-45 tooth driven gears, and has the 4K pulse per mile pulse signal  needed for the 1990-92 SD TPI. 


This picture and diagram shows the wiring set up necessary if you decide to go this route. All speed density TPI and most TBI engines from 1990 up to 1992 use this VSS system.

VSS Sensors 1993 and later

The 4L60E used from 1993 and later uses a different type of PM VSS sensor. The magnet is exposed on the end of the sensor and is set up to use a 40 tooth reluctor installed on the output shaft of the transmission. Using a 4L60E on an earlier TPI conversion will require the addition of a separate stand alone controller, which is very expensive (typically $500+)


The 4L60E VSS speed sensor and the transmission mounting.

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