ak47

I have a 305 motor from an '86 T/A that is giving me lots of headaches. It just cranks all day long when it's been sitting for more than 4 or 5 hours, and won't start until it warms up. Also, when its running, it is very sluggish at low RPMs. I need to check the MAF, and would also like to check any other trouble codes, but I don't have an ALDL connector, wiring harness doesn't seem to be all there. When I get it running, it will run fine down the highway, but it also seems to run rich, which is why I want to check the MAF. Is there any way to check the MAF directly, like with a volt meter? I tried unplugging it and starting the engine, and it actually didn't seem to change anything. Also, how can I get trouble codes without an ALDL? does anybody know how what wire on the ECM to ground to put it in diagnostics mode, and what wire goes to the check engine light? (I don't have a check engine light, but I'm guessing it gets power from a pin on the ECM?).

Thanks.

Blue_Thunder

iTrader: (1)

Your sure your aldl connector has been removed. Mine was tucked into the lower dash on the driverside on the right side of the steering column in a little plastic cover. The best way to test a MAF when you can't scan the car is to hook up a scope to your mafts power wire and sensor signal wire. When you hook that up do a wot snap in park and check your peak voltage. then look up the spec for your maf top see what its peak voltage output is. Your maf may read in hrtz. you can also check to see if your car is running lean or rich by using a gas analyzer. but the best way to check if the maf is reading correctly is to hook up a scantool that can graph and put the tps, rpm, maf, and short term fuel trims on the graph. Then do a long throttle up and hold for a minute and then back off slowly down to idle. Or take it for a basic test drive this is the best. about 4 to 6 miles is good. the tps , rpm, and maf should follow eachother. But if the short term fuel trims follow the maf on the graph then you have diagnosed a airflow measurement problem and it is most likely your maf. If the short term fuel trims don't follow your maf on the graph then you have most likely rulled out an airflow measurement problem and the problem is the engine is not breathing right. Hope this helped

ak47

Well, I may have the ALDL connector somewhere, but I wasn't able to find it where it was supposed to be, based on pictures and wiring diagrams. This engine isn't in the T/A anymore, I bought it and put it in my truck, that's why I'm having trouble diagnosing these problems. Can you post a picture of the ALDL and the wires coming off of it, or do you know of a link to something like that? If I saw one, I might be able to find mine. As for the MAF, I'll get out my volt meter and give it a try, I think this style is supposed to be a constant 0 to 5 volts based on airflow, I'll try to get a reading from it and let you know what I come up with.

Also, are you familiar with the coolant temp sensor that is used for the cold start injector? Here is a forum describing it: https://www.thirdgen.org/forums/tpi/...at-sensor.html
Is it possible to test this sensor to see if it is still good? I'm wondering if that has anything to do with my cold-start problems?

Thanks for the info!

Blue_Thunder

iTrader: (1)

Ah, So the TA engine is in a different vehicle. That could be a whole different issue. And the aldl connector should be wherever the car manual says it is for the truck you have. On all domestic cars from the 80's the diagnostic connector was located in the cabin. My car has had an engine swap and the aldl connector is where it is suposed to be acourding to my 86 iroc hanes manual. Now when you swaped engines did you change computers or PROMs. What was the original engine that was in the truck? And what is your truck? If you didn't your truck will be trying to run the engine with the parameters of the stock motor that was in the truck. Thats the issue I have with my car right now. It runs rough and very rich because I don't have the right Prom for it. I did a 305 to a 350 swap and the engine is still trying to run the engine with the 305 specs in the ecm. So if you have the wrong computer for the engine than thats your biggest problem. Id look at that first before any sensors.

ak47

My truck wasn't so advanced as to have an ALDL plug or a computer before this. It is an 85 GMC that started out as a diesel, but had junkyard 350 with a quadrajunk in it when I got it, nothing special there. I used all the electronics from the T/A when I swapped motors. If I can just find out where the wires on the ALDL go back to, I may be able to pull the codes without it, but I'm not sure if a wiring diagram would even show that? I should at least be able to test the MAF directly with a voltmeter to see if it's sending a signal back to the ECM, but I'd like to know what other trouble codes are stored.

ak47

Good news! I got some feedback from a 3GO post on the ALDL, wiring, etc.:
http://www.3rdgen.org/bb/showthread.php?t=41413
I should be able to get the trouble codes without an ALDL then, I'm going to try it tonight and post results back here.

Thanks.

ljnowell

To check for peak signal voltage you do not need a lab scope, you simly need a DVOM. Really, the labscope is more for in depth diagnosis of an analog maf, such as looking for dropouts, etc, in the signal. You want to ensure that the maf is operating within it specified range of .5-4.5v. If the maf is a digital maf, then a lab scope would be of more use, but a good DVOM can still be used if it will measure Hz. I am curious what you mean by the short term fuel trim following the MAF? The short term fuel trim will always vary on acceleration. In fact, if the short term fuel trim is increasing fuel, then that would be an acceptable reaction to more air going the the engine. Short term fuel trim is going to be based on O2 sensor readings, not the MAF. If the maf reads more air through, the o2 will momentarily go lean, and the ECM will change the PWM of the injector. This will cause the STFT to vary.

ak47

That is what I planned on doing, I don't have money for anything fancy , hopefully my voltmeter will be able to measure the MAF output. Also, what you said about the fuel trim, etc. makes sense, and if the O2 sensor is bad, that could cause fuel/air mix problems as well. I'm going to replace the O2 sensor and check the MAF for sure, I really don't know what else could cause the rough idle and sluggish starts. I've already checked fuel pressure and vacuum and replaced the coolant temp sensor. I'll let you know how it goes tonight with the sensors and trouble codes...

Blue_Thunder

iTrader: (1)

it doesn't matter if the maf is reading at the spec range I have seen Mafs go bad that read in the spec that it calls for. They can fail at high or low voltage. in other words they can fail any any rpm range. the only reason that spec is there is if when you check the maf at idle if it is way off. Another thing to look at when graphing the voltage signal of the maf is if the signal is choppy. This indicates that air is bouncing back to the maf. This is caused by an exhaust restriction. if the fuel trims go up when the maf voltage signal goes up then then you know the maf is reading wrong.

ak47

Ok, so I was going to test the MAF and get the trouble codes but I didn't get that far. The MAF power and burnoff relays both seem to be wired wrong?! The MAF isn't even getting power right now. It looks like the wires going to both relays were "modified" and I basically need to re-wire both. I should have guessed that might be the problem, but I know I tested the relays and it seemed they were working previously? While I was getting the engine ready to go I noticed a lot of "mods" in what was supposed to be a factory wiring harness, I guess the previous owner hacked it apart. Can anyone say "painless performance"? I should have When I get to the point where I can test the MAF I'll keep in mind what you said about the voltage, that is good to know. Thanks for the info, I'll keep you posted on how it goes.

ak47

The good news is that I found the diagnostic terminal, and the wires where they go directly to the ECM. The bad news is that when I ground the test pin and hook up a light to the Check Engine wire, I don't get any codes, not even a code 12, the light doesn't come on at all. I tried leaving the light hooked up in normal driving mode and still nothing. I am at a loss as to why I couldn't get any trouble codes, obviously there is something wrong with it if I can't even get a code 12 (yes, the light I hooked up does work). Any ideas what would cause this?

Oh yes, and I found that the MAF wasn't getting power. I fixed that and now the MAF seems to be working.

Blue_Thunder

iTrader: (1)

If I read your post corectly you stated that you drove the car while you had the diagnostic pin grounded. BAD BAD BAD. You will damage your ecm. Never Crank over the engine when the diagnostic pin is grounded. Hopefully you didn't damage anything while doing it.

ak47

No, I didn't leave the test pin grounded while running the engine, I only left the light hooked up to wire for the check engine light (pin A5 on the ECM), but still didn't get a light. The engine doesn't seem to want to start at all when its been sitting for a while, it fires a couple of times, but that is about all. I can't get the codes out of the computer so I'm not sure where to start anymore, I've checked the TPS and the MAF, and replaced the O2 sensor, all seem to be working correctly.

... Oh, and for some odd reason, my cooling fans come on when in diagnostic mode with the key in run. Is that supposed to happen?

ljnowell

Thats not exactly true either, GM in its early OBD I days did use the terminal grounded with the engine running to flash out a sequence regarding rich/lean operation.

Choppiness on a scope pattern for a MAF does not necesarily mean that there is an exhaust restriction. In fact it is more likely that the MAF is dirty, or has faulty internal circuitry. If STFT varies less than 10%, its not anything to worry about, more than that and it is a problem. A certain amount of fluctuation upon acceleration is normal, as stated before.

Also, 80's domestic cars rarely had an ALDL in the cabin of the vehicle, Ford and Chrysler both kept them under the hood throughout the decade. GM is the one that kept them in the cabin.

All of the scope talk is worthless though, because he does not have a scope. Looking at the recent posts, it seems as though a thorough examination of all the wiring is needed prior to any more troubleshooting.

ak47

All very good, and you are correct that I do not have a scope to analyze the engine. I think I'll have to get it to a point where I can drive it to the local shop and let the pros look it over. I've checked just about everything I can and am at a loss on the starting problems. I'll keep you updated on how that goes.

Thanks.

Blue_Thunder

iTrader: (1)

Thats not what I meant about stft. What I meant was if you graph the voltage of the maf and the stft. Not scope them but graph them. Lots of scantools can do this. Some dvoms can too. The stft graph should not follow the peaks and valleys of the maf voltage signal. An example of this is a maf reading a airflow at 20 grams per second and the stft is at 26, then when the maf reads low airflow like at 5 grams per second the stft is at 0. I forgot to mention that the stft would already be reading way out of spec if the maf was bad.I just finished a seminar with John Thornton presented by autotrain. And what i meant by a choppy look of the maf while graphing it was that the maf reads fine but the voltage is choppy all through the graph. That can indicate that someting is restricting airflow because the maf may be reading whats called bounceback or air that is has already measured bouncing back across the sensor. New mafs have been designed to prefent bounceback but not the older styles found in 3rd gen cars.

ak47

ljnowell was correct about the wiring, it seems that my cold-start injector was not getting power, a post in this forum by Vader described the cold-start circuit and I realized that it was not getting power. I am going to fix that wiring and hope that solves the starting problems. I'll post an update when that's done.

ak47

Ok, I hooked up power to my cold-start injector, it gets power while cranking now and that definitely fixed the starting problem. But the engine did not want to stay running, and barely had an idle. I unplugged the MAF and tried running it again, now it holds an idle and throttle response is much better. I'm guessing this is a dead giveaway that the MAF is bad? I am going to try replacing it within the next few days to see if that makes a difference.

ak47

Final Update: The cold-start injector is working, I also replaced the MAF and adjusted the idle and TPS, it runs great now! That seems to be the end of the issues.

Thanks to all who helped out!