When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I know this is a long shot... but has anyone ever decoded the bidirectional serial interface between the 84-89 digidash and the ECM? And/or extracted the assembly code on the ECM side to see its calculations and outputs?
This is related to trip/range and MPG calculation and display. The cluster sends information to the ECM, the ECM calculates data based on the cluster provided information, then sends the data back to the ECM for display.
I don't appear to have an ECM binary disassembled enough to read the programming on the ECM side of the equation.
I know this is a long shot... but has anyone ever decoded the bidirectional serial interface between the 84-89 digidash and the ECM? And/or extracted the assembly code on the ECM side to see its calculations and outputs?
This is related to trip/range and MPG calculation and display. The cluster sends information to the ECM, the ECM calculates data based on the cluster provided information, then sends the data back to the ECM for display.
I don't appear to have an ECM binary disassembled enough to read the programming on the ECM side of the equation.
Thank you.
The early C4's used the transmission mounted pulse signal generator to drive the cruise control, dash and send info to the ECM. You could use the pulses per mile to drive / interface other things, the ABS however used its own wheel speed sensors.
From the 90's the Central Command Module handles the speed data from the gearbox mounted pulse signal generator, it despatches the data to ecm, dash, airbag derm module and select ride control. The ccm also logs the odometer the dash only displays what the ccm sends it. I know from my 90s ccm, there are 3 separate outputs one of which I connected to my aftermarket radio which has wireless android / apple car play. So it can show gps map speed when going through tunnels ... There you go some random info no one even wanted to know, but there are uses for 😆
The early C4's used the transmission mounted pulse signal generator to drive the cruise control, dash and send info to the ECM.
Correct, it is this bidirectional serial interface I want to learn more about. The information sent to the ECM from the dash is then used to calculate the trip, range, and instant/average MPG before being sent back to the cluster as calculated data. If I want to learn the format of the info sent from the cluster to the ECM, as well as the return calculated data sent from the ECM back to the cluster. With that information in hand, I want to try a little hobby project to learn some new skills.
I did what I should have done before and broke out my factory service manuals. It turns out, at least for 1987, orange wire A8 from the ECM has a splice then feeds both ALDL port terminal E (serial data) and the Digidash on its port D10. I'm likely dealing with a normal ALDL stream, which may make things easier.
It is not any bi-directional data on the serial interface.
The ECM sends 5 byte packages to the cluster using the 160 baud ALDL interface.
It is a
1 byte running distance.
1 byte fuel usage.
1 byte fuel usage factor (constant)
MPG - calculations are done in the cluster.
The VSS sensor signal goes to the dash and is used for speedometer and cruise control. The signal is also sent from cluster to the ECM.
For some reason the "SERIAL DATA INPUT" and the "SERIAL DATA OUTPUT" are mixed up. Output is on ECM and input on cluster.
If you check this thread you will find information about the 5-byte data stream
Thank you for confirming. And yes…. the FSM definitely has that input/output reversed. Maybe they took the VSS signal fo granted and reused the same text.
The other thread is awesome, and sort of why I’m also looking into this. My use case is an aftermarket ECM, but create a bridging device to convert CAN output from the new ECM to the signal the dash expects. Hoping to keep the factory dash 100% operational without the stock ECM piggybacked.
Yes, I have a MaxxECU with dual CAN outputs and I have the same idea. Create a custom CAN message and use an Arduino to emulate the 160 baud ALDL interface.
I did some simple test using my old ECM, I used an arduino to create reference pulses and VSS pulses. Some potentiometers to simulate TPS and MAP. Then I recorded the 5-byte data packages from the ECM.
I then played the recording to the cluster using another Arduino and a simple interface. I did not see any reaction from the cluster, but engine was not running so no RPM or VSS. I don't know if the cluster needs something more than the ALDL data, or if my data stream was to short or faulty in some other way, or if the electrical interface sa too simple.
There is a guy who built a custom ALDL scanner for the Pontiac Fiero. The Fiero was originally supposed to come with a rally computer that would be located in the passenger side dash area that had all kinds of gauges, a digital compass, and all kinds of other things. There was programming specifically made for that ECM which feeds all of that data to an otherwise non-existing rally computer that never was. It's supposedly the same system and format that the later year cars came with for trip computers and such. The guy that did this Paul Romsky, has a website and a place for you to contact him. If you beg him enough, he'll make a custom system for the Corvette: Paul Romsky | www.romsky.com
84-89 Cluster <> ECM serial interface
The information sent to the ECM from the dash is then used to calculate the trip, range, and instant/average MPG before being sent back to the cluster as calculated data. If I want to learn the format of the info sent from the cluster to the ECM, as well as the return calculated data sent from the ECM back to the cluster. With that information in hand, I want to try a little hobby project to learn some new skills.
