I recently picked up a tech 1 scantool for a song and I decided to take it for a run with my 87 gt. at low rpms everything runs fine but when I have the scan tool plugged in the car begins to give strange (false) readings at high rpms. The engine also acquires a severe miss when the scantool is monitoring it at high speed. What could be the cause of this?

The engine will run differant when the tech 1 is plugged in. I do not remember what mode it is called. It is not good to drive with it plugged in for an extended period.

thanks for the reply. I have not driven more than a few minutes with it plugged in. I am hoping to get some normal operating numbers for future comparison if necessary. If i turn off the engine while the scan tool is plugged in the display begins to blink. The display also blinks when the bizarre readings occur as if it is having trouble communicating with the ecm.

It is supposed to blink when the key is off.

You can drive it for a while with the scanner plugged in, just can not leave it for extended periods. Not really sure what you mean about the bizarre readings. That part is likely not normal, depending on what you mean by bizarre.

heres a more detailed description.

I am sitting at a gas station and I plug the tech 1 in, select pont. fiero engine and got to diagnostics and read through all the sensors.

I pull out and drive to the onramp. while driving to the onramp I notice a large delay the car reads 25 mph and it takes several seconds for the tech 1 to read this. then when I stop at the light to pull on tot he hwy, it continues reading 25 for a while.

I pull out on the hwy and bring the car up to 70. the tech 1 has a few readings like 30 mph or 53 mph before it stops and begins flashing.

After the tech one starts flashing the car starts missing, and i pull over to turn the car off and check my connections.

I try again and the same thing happens, so I pull over again and disconnect it completely.

by the way, I have an 89 grand am which I did the same thing to and did not have any problems, so I assume the tech 1 is working correctly.

ALDL mode is a direct short Data (talk) mode is 10k. It shouldn't cause anything strange to happen when using a 10k mode. Check pins 1 &2 on the connector for 10K on the scan tool to make sure. Also if the SES light comes on it's in ALDL mode which means you are running BASE TIMING with no computer control over advancement. So the spark fires at exactly what it's set at during idle. The timing advances when you throttle up so in ALDL it isn't doing this and will cause the fuel to still be burning when the exhaust valve opens causing the popping. Don't run the car like this it's BAD.

My Turbolink had to be modified to allow it to talk to the ECM with a 10k across the pins.

yeah, the scantool says 10k mode, and the engine light is not on.... and my service manual refers to 10k mode as aldl mode so whatever its called... its definately 10k mode and the car appears to be in 10k mode. I will test with an ohm meter however just to make sure.

OBD1 ECMs are not designed to handle streaming data well. They do it but not very well and nearly all show a timing change when scanning. In many cases it's bad enough to cause the problems listed here and other things. The only solution is don't drive the cars like that. There's nothing actually wrong with the scanner, ECM, or engine.

Some are more picky than others. It can be affected by the specific PROM load and other things besides just the version of ECM. Even the later P4 ECMs used on the DIS engines have some problems like this. (P4 I think has both faster CPU clock and data steam (baud) rate.)

The CPUs and programs aren't designed to be doing two things at once, especially while driving. When you stream data, you take CPU cycles away from running the engine. The ECM CPU can't switch jobs anything like even the oldest PC could. When the CPU is managing a data block, it's pretty much ignoring the engine.

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Hello,


this is not quite true. The V6 Fiero ECMs handle the task of data output quite well. The processor load is not a problem, and if the ECM is not outputting diagnostic data in 10k mode, at least the 86-88 ECMs output mileage information instead, and the CPU load is identical.

The problem with 10k mode is that it is for diagnostics, and certain ECM characteristics change, which may be useful for diagnostics, but not for driving, which is why GM recommends against it.

But CPU load is no problem at that data rate.

Best regards,

Oliver

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Visit my website: www.fieros.de for lots of technical and other Fiero related information

Denodster, I am over near Jackson, MI. about an hour away from you. If you want to stop by you are more than welcome to use my AutoXray unit and see what happens.

Jack

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