Diagnostic Data Connector '87 R1500

Have a modern code reader that functions nicely on most of my vehicles. However, what I think is an OBD1 connector on my 87 GMC R1500 is different. But not to worry, there are numerous offers for a short adapter harness with the appropriate connectors. I've had two such harnesses. The connectors that mate to truck on both adapter cables are identical and illustrated in the attached photo. Indeed, the pins on these connectors appear as if they would mate with pins on the truck harness. However, the shape of the connector shell cavity is not a mirror image of the truck connector. A slight variation in grooves/ridges prevents the adapter harness from mating with the truck's data connector.

Are there more than one style of OBD1 connector? Has anyone purchased an OBD1/OBD1 adapter harness that works with their 'square'?

Yes. There's 16 and 12 pin obd1


I own this from 1320 electronics:


And I purchased ALDLDroid from Google play, and I can communicate and read live data from my 89' suburbans ECM. It's a slow baud rate, something to the tune of about 400ish so the frames are stupid slow, but it works, and has helped me diagnose engine trouble that didn't set an SES light or the infamous "code 42"

there is OBD1 and OBD2. i forget the change over date, sometime in the mid 1990s. they are not the same and require different readers.

I have the same setup mentioned above that I use on my 1990 suburban. it is very helpful for diagnostics

Nuckollsr said:

Have a modern code reader that functions nicely on most of my vehicles. However, what I think is an OBD1 connector on my 87 GMC R1500 is different. But not to worry, there are numerous offers for a short adapter harness with the appropriate connectors. I've had two such harnesses. The connectors that mate to truck on both adapter cables are identical and illustrated in the attached photo. Indeed, the pins on these connectors appear as if they would mate with pins on the truck harness. However, the shape of the connector shell cavity is not a mirror image of the truck connector. A slight variation in grooves/ridges prevents the adapter harness from mating with the truck's data connector.

Are there more than one style of OBD1 connector? Has anyone purchased an OBD1/OBD1 adapter harness that works with their 'square'?

Click to expand...

Yes, and what you are looking at on your truck is called an ALDL connector. Before OBD II came out there was no such thing as OBD anything so each manufacture had it's own diagnostic system and own name for it. So your truck has an ALDL connector.

SirRobyn0 said:

Yes, and what you are looking at on your truck is called an ALDL connector. Before OBD II came out there was no such thing as OBD anything so each manufacture had it's own diagnostic system and own name for it. So your truck has an ALDL connector.

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Before OBD1 diagnostics were horrible between manufacturers. A Nissan required getting way under the dash turning a screw on the side of the ECM and count the flashes. Tranny was even worse to pull codes. Can't remember the exact procedure but it was a combination of hitting the brake, putting trans in various gears and cycling the overdrive. Glad most are gone now.

fast 99 said:

Before OBD1 diagnostics were horrible between manufacturers. A Nissan required getting way under the dash turning a screw on the side of the ECM and count the flashes. Tranny was even worse to pull codes. Can't remember the exact procedure but it was a combination of hitting the brake, putting trans in various gears and cycling the overdrive. Glad most are gone now.

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I agree and the some of the systems parameters were so wide that the computer would not detect certain failed sensors, so the truck might run like chit but the check engine light isn't lit up. Then it's knowing what your looking at in the data stream if there is one or taking stabs at which sensor might be bad and testing with a muli-meter. No I don't miss that at all. GM had one of the better systems though. The system could be read by a scanner and there was a data stream and code memory. I remember some early Ford's had no data stream and no memory so unless the failed sensor was outside parameters the exact moment codes were pulled it would come back fine. Also on the GM system you could jump two terminals to flash codes, and you could also jump two terminals and the check light would flash O2 readings in code, but I no longer remember the specifics on that. I just remember doing it back in the day.

Yes, you can stick a piece of wire and jump the terminals at the top right corner labeled A and B and get it to flash codes with key on

It's Morse code, and there's a pattern with an explanation of the SES light.


You also set the idle of a TBI by putting it into "diagnostic mode" with A and B jumped.

Agree, GM was the easiest to get flash outs. Jumping the ALDL would pulse the SES light 3 times for each code starting with the lowest number code. Such as 4 long flashes and 3 short would be code 43. When the cycle was complete the ECM would flash 1 long and 2 short for 12. Real basic stuff, too bad we will never go back there.

Vbb199 said:

Yes. There's 16 and 12 pin obd1

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Snoots said:

You must be registered for see images attach

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Found it!

fast 99 said:

Agree, GM was the easiest to get flash outs. Jumping the ALDL would pulse the SES light 3 times for each code starting with the lowest number code. Such as 4 long flashes and 3 short would be code 43. When the cycle was complete the ECM would flash 1 long and 2 short for 12. Real basic stuff, too bad we will never go back there.

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I don't know if they were easiest. Chrysler you just rolled the key on and off three times and it would flash codes. I think they were easiest, but I think GM's system was better, a little more refined. Could you image trying to diagnose one of todays cars with an ALDL connector and paper clip....

Most of today's vehicles are beyond DIY. Having access to Mitchell or another diagnostic site is almost a must. Many control modules are VIN specific. Some can be re-flashed others need to start from new. So very clean data transmission is also required. Plus, access to the manufacturers site, more cost. I know there are companies that will do it, but the diagnosis better be right.

Example, something as mundane as a directional signal not working. Chrysler is known for fuse block failures causing all sorts of issues that's one of them. It is called a smart junction block. Of course, it's vehicle specific. Some can be re-flashed from used others not. Cost is almost what some of these vehicles are worth.

Glad I will be retired soon. Problem is there are too few interested in this area of employment. Really doesn't pay enough considering the investment in tools and constant education.

Nuckollsr said:

Have a modern code reader that functions nicely on most of my vehicles. However, what I think is an OBD1 connector on my 87 GMC R1500 is different. But not to worry, there are numerous offers for a short adapter harness with the appropriate connectors. I've had two such harnesses. The connectors that mate to truck on both adapter cables are identical and illustrated in the attached photo. Indeed, the pins on these connectors appear as if they would mate with pins on the truck harness. However, the shape of the connector shell cavity is not a mirror image of the truck connector. A slight variation in grooves/ridges prevents the adapter harness from mating with the truck's data connector.

Are there more than one style of OBD1 connector? Has anyone purchased an OBD1/OBD1 adapter harness that works with their 'square'?

Click to expand...

unless your code reader goes back to 87 gm trucks (cars are often stated to be compat. differently) ?? if not your adapter wont help

fast 99 said:

Most of today's vehicles are beyond DIY. Having access to Mitchell or another diagnostic site is almost a must. Many control modules are VIN specific. Some can be re-flashed others need to start from new. So very clean data transmission is also required. Plus, access to the manufacturers site, more cost. I know there are companies that will do it, but the diagnosis better be right.

Example, something as mundane as a directional signal not working. Chrysler is known for fuse block failures causing all sorts of issues that's one of them. It is called a smart junction block. Of course, it's vehicle specific. Some can be re-flashed from used others not. Cost is almost what some of these vehicles are worth.

Glad I will be retired soon. Problem is there are too few interested in this area of employment. Really doesn't pay enough considering the investment in tools and constant education.

Click to expand...

Damnn you and I shoud sit around and talk sometime. Man you just nailed it all on the head. I'm 54 so I have around a decade before retirement, but I don't think I'll make it to retirement in this business. As I've said before when I got into the business in the early 90's it was carburetors and simple injection. Now it's IDK, so far from what it was. Between that and never having enough help I'm just done. Stick a fork in me lol. And no the pay isn't there. I would have been better off if I'd of gone into plumbing, but who saw that coming in 92?

Some of the older cars those junction boxes are out of production. I just had to send one out of a.... Sebring I think was to be rebuilt took like 3 weeks.