Won't go into diagnostic mode

[Tyleranderson138]

Hey everyone new to the forum hopefully I'm in the right spot. Working on a 1992 k2500 that I can not get to go in diagnostic mode to read the codes. The service engine soon light comes on but when I jump the A/B terminal on the obd1 it will not flash any codes.

The truck is running like crap pig rich and will not advance the timing I checked the body/engine/ chassis grounds. I found a loose ground by the obd port and tightened it up and was able to get it to flash out code 12 in go into diagnostics one time and that was the only time the truck would run right advance the timing ect. Both temp sensors are new, tried plugging in another distributor for giggles. Any ideas???

 

[Tyleranderson138]

Also I checked and do have 5 volts at the b pin on the obd port

 

[thinger2]

Did it at any time have a dead battery or did you disconect the battery?
If that happened, all codes are gone and you need to start over.
Im not entirely sure about this but I also wonder if that bad ground at the OBD1 plug will wipe the codes.
In other words, you may have lost the codes.
If it had any to begin with.
And all of the feedback and stored data that is uses to adjust things.
The 12 flashing is a good sign.
That means the computer is actually doing something.
But I also dont remember if the "service engine light" is the same as the "check engine light" on a 92.
I dont think it is.
Pig rich on a tbi can be either completely blown out injectors or injector seals.
You can check that out with a timing light.
But is most likely a base timing problem.
On a 92 there is a plastic cover on the passenger side of the firewall that is held inplace by three odd plastic nuts.
Take it off.
You will see some bulkhead mounted connections.
The tan wire with a black stripe that is closest to the center of the engine is the distributor wire.
It is not under the dash.
That didnt happen untill mid 94.
Make sure the distributor is set at top dead center on the compression stroke.
Triple check it.
Clean all the crud off of the timing marks so you can see them
The marks on the front cover and the mark on the crank balencer.
Go to the dollar store and get some toenail polish and put a dab on each timing mark so you can see it.
"passion persimon" worked great for me but you do you.

Start it with the ecm unplugged and the distributer loose enough to turn it and keep turning the dist as warms while checking the timing.
It has to be warm and the thermostat has to be open..
When you get used to it.
These are pretty simple bud.
You just have to start from scratch anytime they loose power.
And,
You can do it.

 

[Schurkey]

1. Nothing wrong with "checking codes" using the jumper wire/paper clip. There's information to be had, and more than just codes (but it's been so long that I don't really remember how to verify open vs. closed loop, or whether the engine is rich vs. lean. One of them is how fast the MIL flashes when the engine is running, the other is the relative time it stays on vs. off. It'll be in the service manual.)

Having said that...Jumping A and B is a poor way to get info from the computer. Get a REAL scan tool. Access the codes AND the data stream. The codes can be useful. The data stream is ESSENTIAL. "Checking codes" is a minor part of diagnosis.

2. If you're not getting diagnostic mode, either you're not making good connection, your process is incorrect, or there's something wrong with the system.

A. Key ON, engine not running?
B. Good, low-resistance electrical connection between A and B?
C. Recheck your ground for pin A. Verify with ohmmeter.
D. Are ALL the fuses good?
E. Pig-rich and no timing advance is limp-home mode. SOMETHING is very unhappy. Could be a PROM failure, computer failure...but more likely a fuse or wire-harness problem.
F. IF ALL ELSE FAILS, Disconnect the battery to give the computer amnesia, see what codes come back, and see how it runs.​

 

[Tyleranderson138]

We've tried disconnecting the battery and wiping codes. All fuses are good, cig lighter has power, b terminal has 5 volts. It's only went into diagnostic mode once which is the only time it ran right. Other then that it will never go into diagnostic. We just grabbed a new ecu going to swap in the prom from the original computer and see if that fixes our problem!

 

[Tyleranderson138]

Good news a new ecu fixed the problem!

 

[Schurkey]

Excellent. Your catalyst is much happier now. (assuming it isn't dead from excess HC due to running rich.)

 

[Supercharged111]

Excellent. Your catalyst is much happier now. (assuming it isn't dead from excess HC due to running rich.)

They're pretty tough. My dually threw me a couple curve balls where I rolled coal due to fuel in one scenario and oil in another

 

[MANIAC]

So out of curiosity what would it mean if the b terminal on the obd1 port shows almost 10v also I have a 95 k2500 suburban with the 5.7 myself I can't get diag mode even after a replacement ECM using the same prom from the original still can't get codes