I have a customers car that we have upgraded from Ls1 to a brand new Ls3 485hp from GM.
The car is from -01 (Corvette).
The car did run just fine..... Then on the way for the tuning (Rototest och HP tuner) the car stopped.

No Comm to PCM.....
TCS C1276 HC
TCS U1255 HC
BCM U1255 HC
SDM U1000HC
SDM U1016HC
RFA U1016HC

What the ****! So Im now stuck with this car and I dont have acces to a Tech 2 or HP tuner now because my "guy" is out traveling...

Any idees?
I have checked for cable damage and grounds... but nothing.
Is the PCM toast? or maybe the TRG-001A 58x to 24x Conversion Module is broken?

 

can you clear the codes and restart it? Does it even start?

 

The No Comm with the PCM is probably causing the EBCM to show the C1276 code but I am wondering why you also get C1255 which indicates an internal fault in the EBCM. It might be worthwhile to check the serial data link wiring for problems.

Bill

 

i did this conversion....did you do the cam and crank sensor?

 

also the knock sensors on the outside of the block That new wire that connects it all.

 

Start with the basics. You need power to those devices. I would be looking with a meter in hand and see if there is power to the fuse blocks and so on. I'm assuming you checked all the fuses. There is a ground connection on the frame right between the battery and the fuse box. You might want to make sure its tight.

 

Do you mean the data connections at the passenger feet?

 

I can clear the codes and it wont start now.
I have driven the car several times in and out of the garage with no problem. And now No comm PCM...

 

All the systems communicate between themselves and the PCM & BCM through the “serial buss”. The serial buss is actually a wire that is common to all the computerized systems and these wires are all tied together in essence making ONE continuous wire. If at ANY POINT along this wire a short occurs, all types of odd things can happen, including complete shutdown of all systems.

Next to the BCM in the passenger foot-well, there are two gray gang connectors that connect the terminals of the serial buss wires together into one. The connector that contains the most wires is for critical systems. The other connector with less wires is for the non-critical systems such as the door modules, seat module, etc. If you unplug the connector with the less wires, you can isolate and eliminate those non-critical systems from the rest of the car.

IF the serial buss short is in one of the (non-critical) systems, unplugging the star connector for these systems will allow the rest of the car to function normally.

The connectors in the door hinge areas (left and right doors) are a common problem area for shorting of the serial buss.

Unplugging the gray star connector with the less wires is the quickest way to eliminate the doors, etc.

Most likely your issue with the serial buss is under hood and any other areas of the car that have been accessed while working on the car.

You must also re-examine all wiring harnesses and the grounds, starting with the areas that were involved with the engine swap.

It wouldn’t hurt to look at the ground wires at the rear of the head, next to the battery, and etc.

 

Are the 01 subject to the dreaded battery leak which would mess up the PCM or was that the earlier models?

 

At some point C5s came with an AGM battery to prevent leaks. AND in 2004, GM switched to a top post battery. BUT, most batteries in C5s have been replaced by now, and any non-AGM side post battery can leak.

Check the computer under the passenger side battery in the engine compartment for signs of battery leakage (generally white deposits).

 

I have tested to disconnect the Data connections one at the time without getting any smarter... It only says "No comm" on every unit..

Then I measered the ground supplie to the PCM and it was good.
And the ignition supply and battery supply to the PCM is good.

So what is next?

What I have found is a ground between the frame and engine that was a little lose (Left side). Can the lose ground have damaged the PCM?
Because the car worked just fine before.


Check the computer under the passenger side battery in the engine compartment for signs of battery leakage (generally white deposits).

The PCM unit was stuck in some white stuff...like corrosion.....looks like it have been under water for a loooonnngg time. Old Battery leakage? Its a new battery now.

A have never heard of a PCM failer here in Sweden...is it commen in the US?

 

I would check all of the grounds I am guessing everything was fine until the LS3 swap. I frend of mine did a LS3 swap and the car ran good for a few days them would not start. I trouble shot it down to a ground connection that was loose the tech that did the swap was sloppy with the wire connections.

It was a real pain to figure out but it runs fine now. Check and double check all the connections, look for pinched, cut or burned wires if all is good you could have a PCM failure.

Good luck and keep us posted.

 

There were known problems with leaking batteries messing up the PCM, but I don't know if it was the connections or the PCM itself. If you found a lot of white stuff, like corrosion, that could be the culprit.

A loose ground shouldn't destroy the PCM, but will make it act funny. The corrosion would be my biggest concern since it seems you found some. Posting a pic of it may help. Do you have a known good PCM you can swap in for giggles?

Have you tried cleaning up the corrosion and checking to see if that helps?

 

I have checked the grounds but i will do that again.... All 5 that are lose when the engine are out

 

I can take a pic today, And check the connections... I have a PCM that works but wrong year(-98) and dont think it fits.

 

Update.....after checking the star connetions och PCM connetions that looked good.
Now all the sudden it says No Comm on ALL units....
Everyone...

 

Have you tried looking at your fuse ebox under the passenger footwell ?

With all of those errors that is where I would start. Check cable from battery box to passenger footwell for power and connection being tight.

 

Now its working, It was the PCM

Thanks everyone

 

Well, give us more detail if it was the PCM. Did you have to replace it or was a connector not seated all the way?