OBD1 Datalogging

/ / Tuning Information and Common Questions

Obtaining a Datalog Contents:

OBD1 Specific
Rules for Submitting to PCM of NC
Simple Diagnostics
Sensor Parameters
Idle Setup


OBD1 Specific
While there are many solutions on the web to scan your own car, we have grown especially fond of those sold at moates.net.  Their products have offered reliable service and the product is very well packaged and professional.  We have used Moated hardware for years here in our own shop with no major failures or problems to speak of.   Another good company to try  is OBD1.com

* For all OBD1 GM vehicles our favorite is the ALDU1 & CABL1


We are a bit picky when we have datalogs sent to us.  Just like Windows users don’t like to switch to Mac and vice versa, we like to stick to TTS Datamaster for our 86-93 Car and 93-95 Truck datalogs. TTS Datamaster provides a nice reliable platform for scanning which is easy to use and easy to on the eyes.

Datamaster is available for download here. Please notice that there are many versions available so be sure to pick the correct one for your application.



To initiate a datalog with TTS Datamaster

1. Open the correct version of software for your platform

86-89 TPI use version 32
90-92 TPI use version 8D
92-93 LT1 use DA
94-95 LT1 use EE

2. Click File -> Record ALDL Data

3. A new window appears, click Setup -> Com Port and choose the appropriate com port for your cable. It should be one of the selectable items in the list.

4. Click File -> Open, Navigate to a easy to find location (like your desktop) and type a new name for a log. Click Open.

5. Click Start in the lower left corner of the Serial Data Acquisition window. Drive the car for 15-20 minutes if possible.  To end the log click Stop and then File -> Save Now.  Note that Save Now might not be selectable as the software will automatically save itself periodically.

6. Send the log to us.


Rules for Submitting to PCM of NC

Before sending a datalog to PCM of NC please take note of these following rules.

1. Use common sense and include your full name. We cannot look up orders by email alias as easy as a full name.
2. Please use the scanning software we recommend. For most OBD1 Vehicles this is TTS Datamaster.
3. Please capture plenty of data in one single file. Multiple small files are cumbersome and inconsistent for us to work with. Typically 15 minutes worth is the minimum.  Drive it like you normally would.  Wide open throttle samples are also handy but be sure to do them in a safe place like a sanctioned track.
4. Correct issues noted by us before sending us a new log. If we noted that you have a sensor that is bad it doesn’t help us to send us another log with the same bad sensor.

OBD1 Datalog Check list

() File type correct TTS Datamaster *.uni unless prior arrangements made
() Name log file with coherent description such as  “log 10.7.2011 using tune 4”
() A SINGLE Log file no shorter than 15 minutes. Multiple files not accepted!
() TPS and IAC adjusted properly. Click HERE for idle setup help
() Wide open throttle data. You must drive through at least one gear at 100% throttle. Do not roll in the throttle, or get data in anything other than 100% throttle
() Trouble Codes present – Fix before proceeding

More often than not we receive files with incomplete, wrong, or bad data.  In this case we will send back the following with X marks noting the problem. Please take time to re-read this document if necessary and correct the issue.  Thanks!
Failed OBD1 Datalog Return list

() Incorrect file type correct TTS Datamaster *.uni unless prior arrangements made
() Name log file with coherent description such as “log 10.7.2011 using tune 4″”
() A SINGLE Log file no shorter than 15 minutes.
() Multiple files not accepted!
() TPS and IAC adjusted properly. Click HERE for idle setup help
() No wide open throttle data. You must drive through at least one gear at 100% throttle. Do not roll in the throttle, or get data in anything other than 100% throttle
() Trouble Codes present – Fix before proceeding
() Sensor problems – Fix before proceeding


Simple Diagnostics

It is always a good idea to see if there are any DTC’s (Diagnostic Trouble Code).  With Datamaster if you have a code there will be a red box illuminated in the top right corner of the main Datamaster window.  Double click the box.  Inside the box you’ll see a red box (or many). These indicate trouble codes.  If you click on the little red box it will tell you what the code is.

Datamaster showing DTC’s

Sensor Parameters

You do not need to be a professional technician to catch most problems.  A simple glance over your sensor perimeters will reveal most problems.  For instance, if your engine coolant temperature sensor reads -40F and you do not live at the north pole chances are the sensor is bad OR the wiring/connection to that sensor is faulty.  Remember that these computers are only as smart as the people who programmed them and they may not give you a DTC.  Here are some examples

Notice the TPS %. It is 100% nearly all the time. Obviously this is not the actual case and indicates a problem with the TPS sensor.  Also note that there are no trouble codes present.  This customer would otherwise think he may have a tuning issue if he did not have a datalog.

This one may not jump out at most of you.  This car is equipped with 2 oxygen sensors both of which are supposed to jump between 300 and 800 in a fast heart beat motion once the car is in closed loop.  This car has one dead o2 sensor and one lazy o2 sensor. You can see clearly the purple line has insufficient activity especially when compared to the opposite o2 sensor highlighted in blue.

We are here to help and with a datalog you are equipped to tell us exactly what is wrong with your car.  If you are unable to diagnose a problem yourself let one of us know and we will be glad to assist.

Idle Setup

Let me start off by saying, no aftermarket throttle body should be expected to work out of the box. Also let me say that if you have a large cam and/or more displacement you should pay close attention.  All GM vehicles in our focus use what is called a IAC motor.  This motor controls a air bypass in the throttle body which basically controls your idle. For example, if your idle is low and needs to be raised the motor will open a valve and give the engine more air which will allow the idle to come up.  The IAC motor has a certain range it may operate in, officially its 0 – 160 IAC counts.  If the valve is closed completely it will be at 0 counts, open is 160.

What you want to do is set your throttle body up so that you put your IAC motor in a good effective range so that it may catch any idle dips and trim high idles just like it does in a factory vehicle.   To do this open your scanner and look for IAC counts.  With a fully warmed engine at idle in P/N we recommend 30-50 counts.  The closer to 30 the better but don’t drive yourself crazy finding it, after all that is what the IAC is for anyway. If your IAC counts are not in this area adjust your throttle stop until it does.  If your IAC counts are high add throttle stop (make blades open).  If your IAC counts are too low remove throttle stop.  All aftermarket LT1/TPI throttle bodies are adjustable. Stock throttle bodies are adjustable as well after you knock out the plug. The Computer will see that the TPS voltage is increasing as you adjust up. This may lead the computer to believe that you are intentionally holding the gas open so at this point it will kick the idle up under throttle follower and throttle cracker routines.  BE SURE that your throttle position (%) is zero before noting IAC position and making a subsequent change. You can reset your throttle position % by resetting the ECM, unplug battery, pull ECM fuse, etc.

After you set your throttle stop look to see if your throttle position sensor is still within range.  We recommend about .65 volts or so. An acceptable range is .5v-.69v. If you are outside this range elongate or slot the TPS mounting holes so that you can “clock” it until you get the appropriate voltage.

Here is a snapshot of a vehicle with a improperly setup throttle body.  I have added red stars next to important data to help  you find them.


IAC Pos = 160  throttle stop too low, IAC valve max open trying to keep idle
TPS Volts = .65v   This value is good for the time being
TPS % = 0  Critical, be sure your at idle before making adjustments. Your foot should not be on the gas pedal
Idle RPM = 1100 Not highlighted, but important.  This is a big cam car that requires a higher idle. This value is the idle in which the car was tuned to idle at, or its desired idle.
RPM = 625 The actual idle RPM which you can see is far less than the Idle RPM or desired idle. Here in lies the problem!

From what you can see this car has the throttle stop set way too low.  The car has a desired idle of 1100 RPM which is required for a cam of this size. It is currently at 625 RPM and it wants to be 1100 RPM so the IAC valve is wide open. At this point in time the IAC is at its max adjustment and it is still not able to reach the desired RPM.  The car is not running good, it is chugging, and stalling.  A couple of turns in the throttle stop will put the IAC in a place where it can adjust and maintain the idle required for this car.

Copyright 2008 PCM of NC INC.