Electrical, CCM, ECM experts, HELP
Thread Starter
Burning Brakes
Joined: Feb 2001
Posts: 769
Likes: 0
From: Granbury TX
Electrical, CCM, ECM experts, HELP
I have A nightmare problem. My Corvette is a 1991 coupe, L98, stock. I had several problems including a leaking heater core, bad radio receiver and other malfunctions requring work under the dash. Everything is back together and everything works fine except I have no speedometer, fuel level, cruse control or radio volume auto increase. The car drives and runs excellent and all other gages and the techometer works fine.
Last night I performed diagnostics per section 8D of the service manual for the CCM. I got a CCM code 41 and when I tried to get ECM codes I got "Err". According to the service manual, that means there is no communication between the ECM and CCM.
Problem is there is no clear cut direction on how to proceed. I think ECM/CCM communication may be more than 1 wire but from what I read, it looks like ECM B8 is perhaps the suspect wire.
I disconnected the red "B" connecter at the ECM and observed that the pin 8 and the connecter look fine. That wire goes to the CCM and cruse control These items are probably the most diffacult objects to get to on the entire car.
Questions: Is this the correct wire (and the only wire) that will cause this problem/error?
Is there any easy way to look at/get to the CCM end of this thing?
Any other words of wisdom for this code/problem?
Last night I performed diagnostics per section 8D of the service manual for the CCM. I got a CCM code 41 and when I tried to get ECM codes I got "Err". According to the service manual, that means there is no communication between the ECM and CCM.
Problem is there is no clear cut direction on how to proceed. I think ECM/CCM communication may be more than 1 wire but from what I read, it looks like ECM B8 is perhaps the suspect wire.
I disconnected the red "B" connecter at the ECM and observed that the pin 8 and the connecter look fine. That wire goes to the CCM and cruse control These items are probably the most diffacult objects to get to on the entire car.
Questions: Is this the correct wire (and the only wire) that will cause this problem/error?
Is there any easy way to look at/get to the CCM end of this thing?
Any other words of wisdom for this code/problem?
Le Mans Master
Joined: Aug 2000
Posts: 8,838
Likes: 31
From: Australia
"According to the service manual, that means there is no communication between the ECM and CCM."
i don't think that is your main problem
Lack of communication to CCM will result in no start condition; ECM requires signal from CCM to fire injectors.Car will not start;period.
Even a fault on the CCM self check of some in car systems ( Airbags for example ) will prevent starting
I would be looking to a disconnected/ loose plug as a result of your underdash works.
i don't think that is your main problem
Lack of communication to CCM will result in no start condition; ECM requires signal from CCM to fire injectors.Car will not start;period.
Even a fault on the CCM self check of some in car systems ( Airbags for example ) will prevent starting
I would be looking to a disconnected/ loose plug as a result of your underdash works.
Melting Slicks
Joined: Jul 1999
Posts: 3,214
Likes: 101
From: Decatur, Alabama
it runs on a serial data connection not 12volt on/off . Its part of a complete loop from ccm, to abs pump behind drivers seat, through ac programmer (above gas pedal), to radio controller, climate control,to pcm and to ccm. Any slight wire chaffe or anything not working or plugged up will through the system off. A very hard diagnostic for the back yard. I would check connections at all the above...does your ac / climate control work....will it change vent positions and temp?
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
If those systems worked before you did all this other work, always go back to where you were at least as a starting point before going off into a technical tangent.
Several of those functions are driven by the waveform or pulses from the speedometer input. Fuel gauge would be separate but might be in the same connector as some other connections.
Several of those functions are driven by the waveform or pulses from the speedometer input. Fuel gauge would be separate but might be in the same connector as some other connections.
Thread Starter
Burning Brakes
Joined: Feb 2001
Posts: 769
Likes: 0
From: Granbury TX
After further research, I believe I have identified it as the TAN wires running from ECM pins B5 and B11 to CCM E13 and F12. As pointed out by corvette95, these wires also go to the brake module and the hvac programmer, but what I find more interesting, it also goes to terminal "M" of the ALDL. I would like to perform a continuity check with my Volt/ohm meter between terminal M and pin B5 of the ECM but I am not sure if I will damage anything in any of these other items by back feeding VOM voltage to these wires.
Any help out there
Any help out there
Pro
Joined: Jun 2008
Posts: 610
Likes: 1
An ohmmeter injects a very tiny current, and it will only flow between the two points you have the meter leads connected to. It won't damage anything.
Do disconnect the vehicle battery first, both to protect the meter and against shorting vehicle voltage accidentally with your test probe.
Do disconnect the vehicle battery first, both to protect the meter and against shorting vehicle voltage accidentally with your test probe.
Thread Starter
Burning Brakes
Joined: Feb 2001
Posts: 769
Likes: 0
From: Granbury TX
Thanks M. Schumacher! I have the battery disconnected and it's good to know I won't fry anything. This test would at least tell me if the firewall connector is good. I'll also check it to ground for shorts. I still dread trying to get to the CCM.
Team Owner
Joined: Mar 2001
Posts: 54,124
Likes: 30
From: Cincinnati, Oh USA
Modern VOM's (voltmeter, ohmeter, milliammeter) have a position for measuring solid state diodes (probes have higher than 0.7 volts). The normal ohmeter is designed not to apply a voltage high enough to turn on a solid state junction (less than 0.7 volts). You see in service manuals not to measure resistance on some terminals. The reason being that very old ohmeters had 22 volt batteries in them for measuring high resistances and some solid state circuits can be damaged by 22 volts applied in the ,"off", direction of a silicon diode or transistor junction.
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Melting Slicks
Joined: Jul 1999
Posts: 3,214
Likes: 101
From: Decatur, Alabama
the serial data (tan wires) do NOT work on a 12 volt pulse...its serial data.... cant measure with volt meter... you can test continuity, but it works like a fax machine or telephone line modem. I bet its NOT your ccm... you have something plugged in wrong or not plugged up. also if there is ANY chaffing ANYWHERE on ANY of the serial (tan) wires... it will interfere with communications,
