code 31, 24 and tps help??

[infamous7]

b4 i get flamed...the search button is working for shit right now and i wasnt planning on posting but i need help from this point....

the short and sweet:

just finished a full teardown rebuild on an 87 7MGTE...everything is hooked up correctly, the car fires and idles pretty smooth considering that as soon as i hit the throttle it just dies or bogs and pops and struggles bad to make rpms. im seeing a code 31 and 24 which i fully understand to be afm/iat codes

i have checked ohms at 'tha' to be w/in spec, and all reference voltages at my ecu are about 4.9 which i know to be a good ref voltage but the tsrm says that at idle it should only be 2-4 and 4-6 at IGSWon so im a little unsure of those results

i als went as far as testing my b+ terminals to be good and my tps which checks good for ohms but i am getting about 35mv at the ecu IDL pin when it says i should have 0v

this being said...is it a bad TPS or do i need to consider that it may be a fuel issue....i know its going lean when its popping and shit but it fires and idles fine?

WTF????

 

[Nick M]

I don't have the schematic in front of me, but since you have both airflow meter codes, I would think about a loose or bad connection. I say that because I am pretty sure it is common power.

THA=thermal air. Good job on actually looking up your problem.

 

[jetjock]

Your problem is AFM related. The TPS is fine. Not to mention it won't cause that symtom even if it was bad.

What "reference" voltages are you referring to? Vc or the standard test voltages in the TRSM? If Vc than 4.9 is good. If standard voltages 4.9 is bad, especially if you're referring to the airflow signal. If fact that's your problem right there...

 

[infamous7]

i checked vc and ks to the ecu and both were giving me 4.9 at idle...the vc is good from what i gather but the ks is supposed to fluctuate right?? so would that mean bad afm?? because this is the 2nd one and they arent easy to come by!

i meant reference as in a 5v sig to the ecu, sorry for not clarifying.

 

[Nick M]

The airflow meter signal is hertz based. I dont' think the book lists a spec. My old school books might say, but I don't have them around right now. Somebody that has data logged would know what it looks like.

But since your ECU is showing a code for both parts, you likely have a wiring problem, not a meter problem. Which is why I said a bad connection, and JJ told you exactly what the iproblem was.

 

[infamous7]

does anybody have those freq specs?? that is if i can even source a meter for that.

so if its a wiring issue but im seeing the same voltage as "vc" then would it be safe to assume that maybe vc and ks are shorting together...a "short to power" situation?? im so excited that i get to trace my entire harness now that i could shit myself :3d_frown:

 

[jetjock]

Ks can't be accurately measured with a DMM because as Nick said it's a pulse with varying duty cycle. The voltage spec given in the TRSM reflects what a typical DMM will display but you really need a meter that does frequency. Either that or a counter.

A stock and untampered with AFM at idle should output around 25-30 hertz and at 2500 about 70-80 hertz. A lex AFM will be around 18-22 hertz at idle.

Since you have both AFM codes and since THA and Ks are independent devices co-located within the same housing *and* since both are above 4.8 volts a wiring problem is indicated, likely an open circuit. You should be taking measurements at the ECU connector btw. That way the wiring can be checked at the same time. And no, your entire harness doesn't need to be checked. Only the 5 wires involved with the AFM.

 

[infamous7]

familiar with hz sigs but just dont have the metering equip, shouldve jacked a scope when i was property of the us navy!...and all my checks have been done at the ecu. i understand that its only those 5 wires, its just that when i installed the motor i wrapped all the wiring nice and tight and now i have to undo it all to find out where the break is...

thanks for those freqs...hopefully i can get some time to actually work on my own car instead of customers'

 

[jetjock]

Well, just to be sure, ring them out one at a time with the AFM and ECU connectors unplugged before you go unwrapping the harness. Might as well prove the problem before going that far.

 

[infamous7]

so i checked ohms on all 3 sig lines from the afm-ecu (tha, ks,and vc) and all are w/in spec(.001-.002) but my understanding of electricity is that even one small strand will provide continuity for an ohms check while that line might still be the problem.

if i were to jumper the lines directly to the ecu i could at least narrow down what wire i should focus on and then just find the break right?

 

[Nick M]

so i checked ohms on all 3 sig lines from the afm-ecu (tha, ks,and vc) and all are w/in spec(.001-.002) but my understanding of electricity is that even one small strand will provide continuity for an ohms check while that line might still be the problem.
if i were to jumper the lines directly to the ecu i could at least narrow down what wire i should focus on and then just find the break right?

Your teacher was correct in teaching you this. It is why voltage drop tests are done on circuits with devices like motors that work, but not how they are supposed to work.


That is one area where the factory manual fails. Too many ohm specs. And we aren't really looking for voltage from the meter, but frequency. Voltage does matter though, especially with the source.