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  #16 (permalink)  
Old 09-08-2008, 08:31 PM
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bogie

bogie if you enlighten me please take it easy and don't be tooo technical so mabey i'll have a chance to understand it. computers really are confusers

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  #17 (permalink)  
Old 09-08-2008, 08:41 PM
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Originally Posted by eatonde
I think you were talkin about the computer when u said the confuser, but i can go with that cuz its def. good at confusing me. What is it that would change and not allow it to get info from the computer just because your using a carb?? If the computer is still hooked up and the coil is still hot shouldn't the dist still fire properly?? I've never really understood all this computer stuff so please feel free to enlighten me on how it works.
There's some relays on the upper firewall. When the key is first turned on a relay drops to on, switching the pump to run for 10-15 seconds. When the engine fires the computer reads it's running from the ignition pulses. At that point it either holds that relay in or switches in a run relay and lets the pump start cycle relay drop out. There's a couple ways they built these. you really need a wire diagram to play with this. If the computer doesn't see the engine running, then it lets the start cycle pump relay drop out.

If the start pump relay is out you can jumper around it as a test to see if the pump (s) come on, if they do then the relay is most likely shot, but remember there's a lot of wire and connectors and some other sensod that affect this circuit that can also make trouble for ya. Some look to oil pressure, if that isn't there the relay shuts the pump off. All have an upset sensor, if the truck gets tipped beyond a certain angle the pump gets shut off. These are there to shut off fuel in case of an accident.

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Old 09-08-2008, 09:14 PM
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Quote:
Originally Posted by eatonde
bogie if you enlighten me please take it easy and don't be tooo technical so mabey i'll have a chance to understand it. computers really are confusers
I think when you get into this you'll see the external logic is fairly straight forward. What's going on in the computer is fairly complex, but it's sensing of the outside world is mostly one of reading resistances from certain places or seeing if switches are on or off.

The throttle position sensor is just a variable resistor turned by the throttle shaft, just like the knob volume, bass, and treble controls on old analog stereo.

Engine vacuum is read as a resistance thru a strain gauge that varies its resistance by how much force is put on or taken away from it.

Engine coolant and inlet air temperature sensors, and EGR functional, or not, sensor are what's called thermistors which is just stuff that changes resistance with temperature. The O2 sensor generates a voltage in the presence of oxygen. Of itself, it doesn't do anything, the computer compares its voltage to a map that was made in the dyno room that says for this power setting (a combination of throttle position, manifold pressure (or mass flow), operating temperature, transmission gear, and vehicle speed there should be X amount of oxygen in the exhaust stream. If there isn't enough oxygen the computer leans the mixture by reducing injector on time, if there's too much the computer holds the injectors on for more time to richen the mixture. This is basically all that's going on.

The detonation sensor is just a noise activated switch, quiet it's on, a sound like a ping it's off. When off, early computers just stopped the ignition. Detroit found this to be a shock to the driveline and changed it so the signal gets processed to either partially shut off some cylinders or to pull the timing back a few degrees for a short while.

The maps are just look up tables from the dyno room. They don't look much different than a bunch of Excel spread sheets, worst case complication they would look and function something like an Access data base. There really isn't anything going on in the computer, that you couldn't do with a lap top plugged into the diagnostic port under the dash. In fact there's a lot of folks selling programs that let you do that.

All of this is learn-able, there's a lot of books out there in everyday language that will help the average rodder figure these things out.

Bogie
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Old 09-09-2008, 10:58 AM
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The relay you are looking for is on the passenger side firewall. It is a large, oval shaped unit. There is also a fuel pump fuse, 20 amp, in the same area. It is plugged into a black "weather" cap, so you can't see the fuse until you pull the fuse and holder out of the cap. The fuel pump is also switched through the oil pressure sensor, so that if the vehicle rolls the engine will not continue to run. So no oil pressure signal, no pump either. It was also common on these for the harness to the pump (in the tank) to go bad, the new pumps quite often come with a new set of wires. The ground for the pump only "clips to the frame, so it could also be bad. I have seen the wiring get pinched along the frame and corrode with green death and not supply enough voltage to the pumps as well. Also, you can look by the d/s exaughst where the pipe connects to the manifold, I have seen more than one with the wires burnt off in that area, especially if the harness hold down clips have deteriorated. That allows the wiring to come closer to the hot manifold, couple that with an exaughst leak and wires will burn right off.

The knock sensors used are actually an AC voltage generator, using piezioelectric principals. Basically they have a crystal in them, and when pressure is applied to it, it generates voltage. When detonation occurs, the sound wave travels through the coolant, creating a pressure shock, and the vibration in the coolat is enough to cause the crystal to generate voltage. In '87, it would pull the timing back to compensate. MAP sensor can work in a similar manner, or use what is called a wheatstone bridge ( bunch of resistors, and other stuff, kinda complex). Newer stuff uses a mass air flow sensor, and it uses a resistor, or a heated wire (not so much anymore). The PCM monitors the current required to maintain a calibrated voltage drop across the resistor. As more airflow passes the resistor, it takes more current to maintain the drop, as the resistor is cooled off by the airflow. The PCM uses this input, as well as the O2 sensor info, coolant temps, ect., to map out how long to turn the injectors on. On longer, more fuel. on less, less fuel.
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Old 09-09-2008, 11:21 AM
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pump

Ok guys, He is past the pump problem. He decided that he wanted to go with an in-line electric pump, so i aint gonna argue with him. I personally would preferre to fix it the right way but he is one of those guys who feels the need to RIG everything he does. It ain't my truck so I don't care, he can do as much rigging as he wants.
However, I am still confused on why the stock dist. wont work just because your running a carb. Will it be constantly trying to adjust itself to avoid detonation and control the flow of fuel or what? i dont understand what will make it quit working. Its not that i have a problem using an HEI, i have 4 good ones in my garage, that one of which i could give him to use. Thats not an issue. I just wanna understand what will prohibit the stock dist. from working all of a sudden.

Thanks for the info about the Oil pressure shutting the fuel off, i'll hafta have him check that. It def. makes sence for GM to do it that way.
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Old 09-09-2008, 12:01 PM
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If you use the factory dizzy, it will run without the computer, but there will be very limited advance capabilities. You will only have what is built into the module, which isn't very much. Works well for a stationary engine with a constant load, but in an automotive application, you will have either detonation or retarded timing issues. The only way to get enough total in with the stock unit and no PCM control is to set the base way up, and then it usually pings off idle. Back it off to get rid of the ping and it becomes an overheating, gutless dog.
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