FBodyBros

Can someone explain to me why the PCM's grounds are ran exactly the way they are? In this diagram from ShoeBox, http://shbox.com/1/1995_pcm1.jpg, you can clearly see the difference in the way ground 451 (pins a2 and C32) and ground 551 (pins a18 and d1) are diagramed. Why does it matter and what difference will it make if it isn't routed this same way?

And just for conversation's sake, what effect would tying ALL the PCM's grounds into one central ground make versus grounding out everything in thier own location vs. the way it's routed from the factory? Anything?

Also, know any good online resources for studing electrical diagrams There's a lot that I don't understand. Thanks.

Wild Willy

I don't know specifically in your question, but I know that GM has a habit of switching grounds on and off for some applications- like the delay on the turn lights, or how they blink the DLR/running lights for turn signals- maybe that is part of the reason for using different grounding locations-

Kevin97ss

I would say they want the PCM to see a certin amout of resistance for each ground. Put them all on one lug and it may be too much resistance. This will cause sensors to be interpereted incorrectly because they base all the input on a known ground resistance.

Steve in Seattle

In this case the diagram you're looking at really isn't a bare-bones schematic.

What you're looking at is an electrical diagram written to imply the physical wiring/routing/splitting you'd see in the actual car of that year.

In 1997 GM changed their schematic design a bit, but basically, yes, you are right. As long as you are grounding loops of the same voltage or same power-source (i.e. varying voltages are created by stepping down to a common ground) you "should" be fine. Audio guys have fits over RF ground loops which cause noise due to varying distances of grounding in the system and a whole mess of crap I really tried to block out.

In this case though, the 4 grounds are likely grounded to the same spot (haven't checked, so don't hold me to that ), but the diagram shows you which pin is which wire if you were looking to test a faulty ground (i.e. say you thought the circuit that grounds through A3 was finicky, you could identify which wire went to A3 by noting its the only one that connects to another conducter before getting grounded.

functionally you could just as easily draw all 4 grounding to one ground symbol/post... but there way shows a little more "physical" representation.

I've noticed this in many electrical diagrams in Helms manuals... kind of a nice feature when tracking down a post, wire, sensor, etc...

Steve in Seattle

Yes, the PCM and ABS units usually ground for a signal (like traction control activation), but in the diagram he lists I don't see any other inputs... grounding to a conductor that's grounded, and a grounding post wouldn't affect this operation though.

FBodyBros

Okay, so let me get this straight with you guys.. Originally, there's the four grounds that ground out to 2 different locations on the engine for the PCM. 451 (Pins A2 and C32) and 551 (Pins A18 and D1). Then there's the Battery to Engine Ground running to the passenger side motor mount and also the Ground Strap running from the Driver's side head to the driver's side frame rail.

Currently on our car, we have a '95 Iron Headed LT1 from a B-Body with a '95 F-body T56 in a '94 Z/28 Body. The engine harness was a '94 A4 that's been converted for the T-56. At the same time, we pull the entire A4 Connector and removed some of the A/C wiring from the harness. Lastly, I tried all of the PCM grounds into one central point, made a ground running from that connection to the driver's side head, and another 10 ga. ground running to the junction on the passenger fenderwell. We still have the ground running from the battery to the engine and also the ground from the driver's side head to the frame.

Also VATS is disabled on the car and a push-button start has been installed. The push button start is ran from the main power wire on the starter to inside the car for the switch back to the accessory post on the starter using 10 ga wire and a 60 Amp push button.

We have an A/C Delete Pulley and TunerCATS for the codes on the A/C System. Most of the fittings have been removed but the blower housing is still in the car until we build a new housing designed only for heat. None of the pigtails for the A/C are plugged in into the actual sensors and A/C Clutch Request wire has been completly removed including the pin at the PCM.

Currently the car is throwing a Hi-Res Failure as soon as we start it. Also, there's a hard start issue. I'm assuming that will go away as soon as we figure out the current problem. The weird part about this problem is that we only have the Hi-Res Failure code since I loaded a new tune that turned off the MAF Sensor. We had to tune out the MAF Sensor because a CANNOT get the PCM to send a 5V Reference to the MAF. We've tried 3 PCM's, 2 MAF Sensor's, and perfrmed all of the electrical work mentioned earlier in this post trying to solve this car's problems.

Here is a link to a post we made about a week or so ago and got no where with in Lt1 Tech. https://www.camaroz28.com/forums/sho...d.php?t=648092

Honestly I think all of these problems are a direct result of something we've done to the wiring harness but have no idea about where to start to try and correct it. Anybody have some suggestions or thoughts about what could be going on here?

97FormulaWS-6

Grounds are typically split because they want to keep the "digital" side separate from the "analog" side of the PCM.

If you join them closely it can allow for noise to transfer from one side to the other and cause issues.

This is very typical practice in electronics to split these grounds in a system with both analog and digital systems.

Most likely this is what is causing your problems; there is a direct path between the Analog and Digital sides and one is disrupting the other. The grounds may both be hooked to the body, but the body adds some resistance between them; just enough to eliminate the noise issue.

FBodyBros

Okay, that does make sense. But now onto my next set of questions...

What systems use a digital signal for the PCM to interpet and what systems are analog.

For the digital systems, does anyone know which ones are "interconnected" (I.E. what circuts must be operational in order for another circut(s) to function properly) and the same for the analog, if at all?

Has anyone taken apart a PCM and mapped out how the circuts function inside it? Shoebox, Injuneer, MadWolf?

And my last question... I know the PCM uses a 5V reference signal for several sensors. MAF, MAP, A/C, VATS and whatever else. Are any of these systems dependant on any other circuts functioning properly?

Steve in Seattle

Great point. This one issue people usually have with tapping into a Tach line for more than one device. Even though tapping your stock tach-gauge signal line for a shift-light reference single works alright, every time you tap that line for another signal the 'pulses' lose definition. Some devices may still read alright, but other more sensitive ones won't. Nothing new there, common Q/Z on this board.

The crazy part though is that hooking up a voltage meter to any of the inputs still shows the same voltage.

I thought for the longest time that the increased tapping was due to an increased load on the PCM output (I couldn't get a shiftlight and WB unit to read the same line properly without killing the signal), sacrificing voltage, but testing with a voltmeter nixed that idea. Even though I tapped both wires with the same joint -- wrapped, crimped, and soldered the connections -- I still got the "signal" loss without a large voltage drop (maybe 0.1-0.3v drop at most).

Mind you, this was a live signal wire using frequency (or "pulse-width") for data though I guess it makes sense that the various such devices the PCM uses pulse-width (i.e. MAF frequency in, tach out, Traction-control timing demand, etc...) would need separate grounds from the voltage-based signals (temp sensors, etc...).

Steve in Seattle

Thermisistors would all be analog (Coolant temps, oil temp, IAC), as would pressure sensors (MAP, AC pressure). Most any sensor that has a sliding "0-5v" or "0-12v" signal.

"digital" or "Frequency based" signals are considered on/off style and are normally labeled: 0,5v instead of 0-5v. This would include the MAF sensor, the tach output. This ALSO includes the low and high res signals from the Optispark.

Technically the TCS-spark retard input is also pulse-width modulated (but in practice this can be wired like an on-off switch where frequency is pointless and the presence of any ground is turns on the spark retard, then the PCM can be programed using only the max retard table/column). Unless you have TCS/ASR this is a non-issue.

I know there are more of each, but really, just take a look at the Helms manual and the sensor value column will tell you if data is conveyed as "voltage" or as "frequency/pulse-width"... that's what you're looking for. Merging gronud wires shouldn't affect voltage-driven sensors, which means if you have problems with those, you have a different problem.

Steve in Seattle

If they're voltage-driven, grounding to the same conductor is fine (as long as it has the carrying capacity to do so... don't ground ten 20-gauge wires to one 20-gage wire for example.). If you're worried about it, you can measure voltage or current though the sensor to see if the change in grounds dropped the values any, but I doubt any 0-5v sensors would be affected as long as the ground was solid.

FBodyBros

Steve in Seattle,

Hey man, thanks for all the good information. Finally, I think I'm going to be able to make some actual progess on this car. Unfortunatly for tonight, it looks like all I'm going to be able to really get done is speculate. Finally, I have a couple ideas about what I need to do to finish this car and actually get some good times next spring.