Will a bad throttle position sensor keep an 89 L98 from starting?

 

no it will start but it will throw a bad response.

 

Yes

If TPS is faulty and indicating that the throttle is open (>25% open ) when the engine is cranked over ,
the computer will shut off the fuel ( injectors ) because it thinks you have the pedal on the floor to clear a flooded engine
(same you would on a carb engine )

 

Yep, it happened to my old Z28. Installed a new one and started right away.

 

Thanks for the advice...

 

Is there a simple test that can be done on the throttle position sensor to tell if it is bad or ok?

 

You test a TPS by measuring the voltages on the 3 wires. You have to get some kind of connection to the pins inside the connector to do this. Most people use a paper clip. You have to wiggle it around and work it into the connector from the side the wires come out of until it makes a connection with the contacts.

Turn on the ignition without starting the car (unless you like to deal with flying belts and such). Connect the negative lead of the voltmeter to some metal on the engine. Probe all three wires:

Black - ground - zero volts (or very close to it)
Gray - 5 volt reference voltage from ECM - should be fairly close to 5 volts
Blue - TPS output voltage - should be near 0.54 volts with the throttle closed. You can adjust it by loosening the screws and turning it. Check it after tightening the screws because sometimes it will move while the screws are being tightened.

You can test the TPS operation by probing the blue wire and SLOWLY opening the throttle. The voltage should increase smoothly with no big changes or jumps until it reaches about 5 volts. It probably won't get all the way to 5 volts. 4.5 volts or so is OK. My recollection is that the ECM considers anything over 70% (3.5 volts) to be WOT.

 

unplug it to see if it runs ok. ecm will use a default, or map sensor. but that does not verify the harness wires/terminals. follow Cliff's directions to be sure.

 

Thanks for the help guys.

 

Ok...the grey wire is reading good at 4.87 volts (5 volts is normal). The blue wire is reading 16.7 volts (should be close to .54 volts). Does this mean the TPS is bad or can I adjust it back to .54 volts? So if I'm hearing you right, the ECM thinks the gas pedal is on the floor and the fuel injectors are shut down causing the car not to receive the gas.

 

16.7 volts makes no sense. It can't be higher than the 5 volt reference from the ECM.

That voltage has to come from SOMEWHERE and I don't know of any sources that high (unless the engine is running and the alternator regulator is messed up).

Try unplugging the TPS connector and probing the middle contact (blue wire) (ignition on). If you have anything higher than (pretty close to) zero volts on the blue wire then it's shorted to something else. It's connected to ground through a "pull down" resistor inside the ECM.

I suspect you should start pricing TPS sensors...

 

I know...the voltage makes no sense. The volt meter is set on 12 volts. The grey wire reads right though. Will it hurt anything to see if it will start with the TPS unplugged? This car went from running great to not starting. Never had any trouble since I replaced the fuel injectors about 2 years ago. It's crazy...

 

I often wonder if people with problems read these suggestions!

do you see the "unplug it to see if it runs" above?

as for the 16.7v, I have no idea what you are using to measure it.

 

Yes Joe I saw the post about unplugging the TPS and see if the car will start. I wanted to be sure that it wasn't going to tear something else up by starting the car with it unplugged. And, since the car does run with the TPS unplugged, the TPS must have a short in it. I appreciate the help I received here on the forum. All the troubleshooting tips helped to diagnose my problem. Thanks...

 

I did use a digital volt meter set on 12 volts dc to obtain these readings. I wonder if people really do read these posts.

 

as you said, can't trust everything you read. there was only 5v coming to the sensor, 16.7 coming out? most of us knew right then that the OP didn't understand what he was doing. "it's crazy."

when you get a strange reading like that, you question the meter or your use of it. I set my pocket meter on vdc, and it calculates from zero to wherever.

when you figure it out, let us know how dumb we are.

 

Ok... replaced the TPS with a new one. On the adjustment, the best I could get the reading was .9 on the volt meter. The engine starts up but is running rough. It has an odd increasing idle speed. When you try to give it some gas, it chokes down. To sum it up, it's not running well. More help is needed...thanks.

 

Now that you have a new TPS it sounds like you need a new voltmeter.

Measure 12 volts on the 20 volt range. Measure the TPS blue wire on the 2 volt range. TPS connector must be plugged in. You need a good ground connection for the reference, normally the black probe lead. It really doesn't matter which polarity you use. The meter will read -volts if the probes are reversed, but so what? It's the NUMBER that's important. As I like to say, "The electrons don't care".

You ARE on "VOLTS DC", right? These meters will measure something if you have them on AC VOLTS because of the way the meters work internally. AC voltage is "normalized" to DC voltage by circuitry in the meter, so 12 VDC makes the same power as 12 VAC. There are several ways to measure AC voltage: Peak (from zero to the maximum positive or negative swing), Peak-to-peak (from the maximum negative to the maximum positive) and RMS (root mean square), which is basically the area under the sine wave curve. RMS AC volts = DC volts.

 

Ok...finally got the .54 reading on the new TPS!!! My meter has a mega volts dc setting which allowed me to get the correct setting. I think the reading ended up being like .543. The car cranked right up and my service engine soon light is off. Car is idling good and accelerating good as well. Guys I appreciate all the help I got with this issue. Thanks...