steve1ph2

hi, while jumping the aldl to pull codes im getting c-41 "loss of ecm serial data communication". the ccm is still working. iv got interior lighting, gas gauge ect... but i have no service engine light which led me to checking for codes. the car is a `90. have any of you ran across this problem?

Jerris

Is the code a current code or a history code.
If it is a history code you may just have an intermittant loss of communication from a loose connection on the CCM or ECM

steve1ph2

its current like every time i drive it. no mph on the lcd it stays on 0 with the sys light flashing next to it. thats what led me to check for codes when i discovered the c-41 code. i dont think a bad vss sensor would cause loss of comunication between the ecm and ccm. im not even getting the self diagonstic check engine light when turning the key to start the motor.

JeffM1243

Steve there is a wire from ECM to CCM called a "serial data link"...Check the FSM for color and location..It is the communication link between the two...If it is missing a code 41 will set...

I noticed in another thread you just did a carb conversion...Did you disconnect this wire? VSS inputs to the ECM, if the "serial link" is missing the ECM cannot communicate speed to the CCM, this may be why your speedo says 0.....

bogus

the Serial Data Link is actually 2 wires, running off a common pin.

The bigger concern is that the ECM is DOA.

Can you get ECM codes via the SES light?

JeffM1243

Took a look at the 90 FSM, Bogus is correct the link is two wires, but the diagnostic checks show either wire being open will set the 41 code..

VSS inputs from ECM to the CCM separately from serial link through circuit 817, which then reports speed to the speedo, you also have loop circuit 800 (serial link) open...So, appears you have two different problems..

FSM page 8D-13 shows the CCM circuits in question, 6EA-A-8 shows the ECM circuits...I know you've already replaced the ECM but all 3 connections are in ECM "red" connector J2, I would check this connector is seated correctly, I've had this problem with my 90....

Only other common is all three wires connect on the CCM end to the "diagnostic" terminials which would indicate a partial CCM failure...

steve1ph2

yea the first thing i did was change the ecm. i was reading my service books page 8d-46 and 8d-47 for code 41. it is a miracle the i got the ccm out far enuf to unplug it and check some wires per the book. in short the book instructed me to check for continuity at the ccm plug and at the ecm plug in this case wc13 at the ccm and r11 at the ecm. check a couple wires did a ground check and every thing is pointing to ckt 800 being open to wc13 and wd12. not sure what ckt 800 is at this point. major success just getting the ccm out.

steve1ph2

ok after reading the book ckt 800 is the tan wire from the ccm to the ecm. either it its burned or along its route what ever componet it also connects to is fried. really the only other thing i can see that uses the ckt 800 wire between the ecm and ccm is the electronic hvac programmer. but that is working fine. i mean the heating and a/c head unit is displaying all the correct info temp, fan speed, on, off ect.. i was hoping to just string another wire from the ccm to the ecm but if it makes a few stops along the way thats a dead deal.

Jerris

Putting the jumper wire between the ECM and the CCM willl confirm that the problen is the open circuit and everything should work if this is the problem.
Bypassing everything should be fine for testing, make sure to use a wire of the same size as the oroginal.

steve1ph2

exactly what i was thinking and fixing to do. ill be back in a few. thanks

steve1ph2

tried the jumper wire didnt work. jumped the aldl again to check for codes. here are the new codes 1- c41, 1-h41, ---, 4 err.

rick lambert

One"s a current 41, the other is a history code 41. Meaning its happened before and is happening now. I would think a lose or bad connection may be the cause.

VR'92

Possibly the CCM is failing?

steve1ph2

if and when i get tired of tracking down the speedo problem are there after market options? the car is a `90.

steve1ph2

did all the test per the book again for code 41. this time iv come up with " repair short to ground in ckt 800 ". ckt 800 is the tan wire coming from both serial ports of the ccm to the ecm, electronic hvac programmer,aldl and anti locking brake controller. im having a hard time finding which ground wire it is and where its located. any ideas? also i did check the 12v into the ecm and it is hot.

steve1ph2

instead of tracking all the grounds down i cut the ecm ground and the vss ground at the proper ecm plug and used another wire to a known good ground location. did not work. i cut the serial data wires at both locations at the ecm plug and ran straight wires to the ccm serial pin outs. did not work. i dont know. thats obvious

steve1ph2

i was just rechecking the fuses and the ecm fuse will not light the test light with the key on or off. rechecked the hot wires going to the ecm c1,d17 and b10. the two battery feeds are lighting up the test light but the ignition feed is not with or with out the key on. im hoping somebody can jump in and help. seems im getting close.

steve1ph2

i found it. the cluster fuse is part of the circuit. ignition on for the ecm. i checked all the fuses per the service books many times. they all lit up except the ecm which i must of over looked which led me to check them again. it was the cluster fuse that was blown. one side would light the test light which is why i didnt suspect it. i must of blown it a couple of weeks ago looking for a 12v source for my fan relay set up. thanks for your help and hope this helps some one.

Gej7979

I know this is an old post hope you can help. I am having the same issues you were having with my 1991 corvette code c41. 0 reading on speedometer and odometer inop. I replace the ecm didn’t fix. Did the cluster fuse fix yours? My fuse is good but I was thinking maybe it could be the cluster. Don’t see how to get to the bcm without taking the dash apart. Took the radio out, can see the bcm but can not get to it. From what I have read the bcm has very low percentage of being bad.

Gej7979

I am having same issues with my 91 corvette. 0 reading on odometer, c41. I replaced pcm did not correct. Can you give me any advice on this. How do you get to the cmm ? Where is the instrument cluster fuse. I know this is an old post, hope you see it.