I have a smog check coming. my 81 in California has that ccc computer . Code comes up # 21..... tps. I replaced the sensor but it still comes when I put it in drive. Rpm drops to 600.......and the lite comes on. And...if this helps...the lite go's out when rpm go's up. But the code is still there when I test it. What next? I thought I knew these systems pretty well.......but this ones got me. New computer time ?

 

check tps voltage, reset it and reset the light

 

I forgot what the voltage was........you can watch it go up and down when I prob the green wire and tip in the throttle. Didnt go past 4.5v. removed the bat cable to reset......still comes up only when I put it in drive. Like low rpm sets it off. I would think it would throw a 23 map code......but not. Know any experts?....I was thinking the c-4 group? Thanks for the quick reply too.........

 

I would post in C4, they run the same basic setup on the 84's and there's a lot of experience there, good luck

 

You can't just install it you have to set it. You did set it right?

Jim

 

Nuts..........................no. I see the adjusting hole thats plugged so you can't mess with it..........so I didn't try to remove it. So hows adjustment done?..............

 

I don't know how to do an 81. On the 82 it's set by the angle you fasten it at. Some one will come by that knows eventually.

From searching I think it should read .56 at idle with a range to 5 volts

 

hurry...................

 

Here are a couple of threads.

http://forums.corvetteforum.com/c3-t...nsor-81-a.html

http://forums.corvetteforum.com/c3-t...ease-81-a.html

It doesn't really sound like it should be adjusted. Just installed correctly.

I think you need to see if the voltage at a stop position is .560. If it is then the TPS is likely not the problem. You may have a connection issue. Check the wires. Make sure the plug on the ECM is good and clean.

 

perfect info............Ive gotta take measurements this morning and let you know........Didnt even know there was a plugin near the fire wall.......More in a min........

 

I'll be darned.............It's a bad computer. I traced the signal from the tps to the weather pack connectors to the plug ins at the ecm. found the terminals corroded somewhat. so cleaned them up and ran a test again.......and no engine lite working during the cycle. Hummmm......well good, I ordered a reman computer from advanced auto for $69. Be here in 3-4 days. Cant beat the price. I was kinda hoping it was that and not the harness stuff breaking down. Heck it's 32 years old now. Can't be leave I still have to smog check a car thats 32. ...........more later....

 

That's good news. The biggest mistake people make is buying those batteries that are not sealed. Acid leaks out and corrodes everything.

Hopefully that ECM will solve the problem. Do you have to transfer the chip in an 81? If so be careful not to damage the prong.

 

Yea......i have to transfer the chip. Always wondered about those performance chips and how they do. This is ecm #3 for this car. Last one was $115 bucks, so prices are on the right course. Our 85 iroc with a quadrajet........you can really tell when the ecm advances the timing in 3rd.......just before the lockup. Bet another chip starts right away. $169 was a little steep for me to try one..........Getting dark here in california.........better put the 81 back in the barn.......

 

Would like to chime in this thread. If you don't mind could you answer a question that just popped into my head? Can you adjust the idle RPM by changing the value of the voltage at the TCP? For example, if I set the idle at .60 would the RPM rise, fall, or stay the same?

 

hmmmmmmmmmmmmm......well.........I...... .... I just dont' Know what one of those are. But I bet it's expensive....

 

No. The TPS is set at .56 for the factory setting of the RPM's.

The TPS tells the ECM what position the throttle plates in the carb are set at. Lets assume the idle is 600 RPM at .56. At .60 the idle is probably 720 RPM. The TPS tells the ECM what the idle is. You can't have the TPS set at what a higher idle value but have the carb throttle plates set for the lower RPM. They both have to be in sync.

If you want to adjust the idle do it with the idle screw. Example, .56 for 600 RPM (assumption). Open the throttle plates by adjusting the idle screw to 700 will cause the TPS to sit at a higher number eg .60. so your start point will be 700 RPM and .60 on the TPS. If you open the plates to much you will confuse the ECM. You always set the TPS to be in sync with the factory RPMS.

Its one of the biggest mistakes those that own Crossfires make (thats the 82 Corvette). they increase the idle from the base of 550 RPM to 700 RPM and set the the TPS to .527. so the TPS is telling the computer to send air and fuel for 550 RPM but the throttle bodies (carbs) are set at 700 RPM. The two are not in sync. Make sense? Its all about sync.

 

jdp6000,

Thank you for the information: I checked and found the setting to be at the factory setting of .560 volts. It just seems the idle is a bit low to idle smoothly (and surge at this low RPM). Anything above idle seems fine and running smoothly. I felt the RPM at idle needed a slight bump, especially when I re-charge the A/C and put it into operation. I anticipate, at current setting, trouble with idle at red lights, etc. with A/C running. Could the surging be something else? MAP or TPC?

 

Is the change to the tps setting proportional to the change in the rpm. For example, if crossfire idles better at 650 rather than 550, should the tps setting be raised proportionately? .525 v is the setting for 550 standard idling rpms. Then 650 would want the tps setting to be .620 (650/550 * .525 = .620)?

 

It's a chain of settings. Everything in the shop manual is based on factory settings. When they say to set the TPS at .525 it's based on the factory idle off 550 ( I believe that's the right number). Picture it, rear and front TBI's are balanced, idle is 550 rpm, TPS is .527. That's your start point, if you adjust the idle screw you change the angle of the throttle plates on both TBI's. you are increasing the idle. The arm the TPS rests against changes with the opening of the throttle plates. So the TPS will show a higher voltage.

You can't set the idle first and the TPS last. At a higher RPM more air is passing through the TBI's but the TPS is telling the ECM the air passing is for 550 RPM because its set to .525( the 550 rpm value). So your air flow is for say 700 rpm but your fuel flow is for 550 rpm, no balance.

You can play with it. Get someone to sit in the car, started in gear, use a spark plug gapper and place it between the TPS arm and resting point on the TBI. See if it makes a difference, at some point it will surge and stall.

A big problem is small vacuum leaks over time your idle will creep up from 550 but the TPS is still set for .527. Your MAP is no longer reading 20 psi.We start playing with other settings rather then addressing the small vacuum leak. So your TPS tells the ECM idle is 550, MAP says its 450 because of manifold pressure loss, RPM are 750 or what ever, ECM thinks you are doing 18 mph and sends more fuel, IAC's are going squirrelly. Nothing works right any more.

So back to the beginning, everything must be set taking factory specs into account. Set everything at factory and increase from there. If the car doesn't like the factory RPM something else is wrong. No amount of adjusting will make it right.

 

I'm not up on the 81. The TPS is set to work at the factory idle. If you adjust the idle up the arm the TPS rests on should automatically increase the voltage.

How is load sensed on the 81? I don't know. I don't know how the ECM would compensate for load, you are not fuel injected. I don't think you have an idle air control valve either.

I'm not sure how you would get the idle up other then adjusting the idle screw.

Do you have problems with the vacuum system? Any leaks? Do you hear a hiss in the cabin?