CarAteMyMoney

I've had this problem for some time now and i'm hell bent on fixing it.

When I plug my scanner in, it goes back and forth between all Zero's and the right data. Then when I step on the gas I lose connection alltogether with the ECM.

All the grounds seem to be connected, and the serial wire from the ECM to the ALDL branches out and also goes to the gauge cluster, if that helps.

The car is an L98 corvette, yeah I know it's not an f-body but I know there are some knowledgable people here so I figured i'd give it a shot.

Vader

Can we presume that you have a digital instrument display? The display may be shunting the signal below the necessary input level of your data port. Are you using a direct coupled input or optoisolated?

CarAteMyMoney

Umm, it's a plain old 86 with digital display. The schematic says that the serial wire runs from the ECM to the Gauges to the ALDL port.
It's not showing any voltage at the ALDL, and it's supposed to show between 2-5 volts. Every now and again it jumps to like 3 volts, but mostly zero.

Vader

8-$£¥¢,

When I asked about the input, I was referring to the input of the scanning device. If you have an optoisloated input converter for the scanner, it shouldn't be a problem, but if it is a TTL device, it might be dragging the signal down to an unusable level.

CarAteMyMoney

I've had the same problem with a few scanners.. OTC Monitor 86, monitor 2000, and I tried a monitor 4000 once.

I use the 2000 most often, which worked perfect on my friends iroc the other day

The scanners use a rubber fitting with some pins that plugs into the ALDL, and a cable goes directly to the handheld scanner. On the 2000, there is a connector in the cable so you can use it with other types of cars, that's about it

But my point is that there's nothing showing up at the serial wire on the ALDL

Vader

Right. I was concerned that you might be using a laptop-type scanner with one of the homemade TTL signal conditioners. A marginal connection somewhere could potentially cause enough signal loss to fall below the recognition level of the scanner input. I believe the later OTCs are optoisolated, so that shouldn't be an issue.

Obviously, connections are still a concern. When you connect your scanner to the ALDL, does the instrument panel still display data?

CarAteMyMoney

Yes, the dash does display everything while the scanner is connected but showing all zero's.. shows speed, rpm, gas mileage, oil pressure, voltage, trip odo, etc...

I also noticed today that the serial wire DOES have voltage, 3.5-4.9 volts, only while I jump the pins (diagnostic and ground) as if I were pulling codes.

Since it's sending voltage, then wouldnt it stand to reason that the computer is sending all the zero's?
That's basically what im trying to figure out, why i'm not getting any sort of consistent readings

any ideas what to check next? also it's done the exact same thing with two different computers.. and it also does it if i splice in a new serial wire directly from the ECM to ALDL

oh, i also noticed that when im cruising the tach will fluctuate a couple of hundred RPM's

CarAteMyMoney

Oh, wow look at that

I put a paper clip in to get codes, and I left it in and plugged the scanner in with the paper clip still in the ALDL terminal....

...and the scanner is working fine! showing all the values, and they all seem to be within tolerance

any idea WHY it works like this????

Vader

As I mentioned, connections may be an issue. While the paper clip is a dead-short connection (zero ohms), the scanner likely uses a pull-down transistor or other "soft" switch. It there is a marginal connection somewhere, the pulldown may not drop the voltage at the 'B' terminal below the threshold for Field Service. Recall that 3.9K sets up Backup Fuel and Spark mode, and 10K sets up 160b ALDL mode. 0 ohms sets up Field Service mode.

The actual values for these ECM modes are as follows:

Over 20K ohms - Normal Mode;
9,500 - 10,500 ohms - Factory ALDL test, serial data, full memory dump;
3,800 - 4,200 ohms - ECM Factory Test, Backup Fuel mode;
Less than 500 ohms - Field Service mode, serial data output at 160 or 8192 baud.

Your scanners may have had poor connections to the ECM logic ground 'A' terminal, or there may be marginal connections at the ECM which won't pass enough current to drop the 'B' terminal signal below the threshold when added to the internal resistance of the soft switch in the scanner(s).

It's just a guess, but having a little experience in TTL logic circuits, it's plausible. I'd still be looking at connections.