I'm working on a TBI project with the intent of retrofitting a fuel mileage display on the dashboard. I've researched the hardware and read through many of the ALDL data streams, and I think I have an idea of what needs to be done. I have a few questions related to the serial data and was hoping to learn more by presenting my project.

For simplicity, I am using an '8746 ECM. Since it operates at 160 baud, I need to use similar parts for the mileage display. By my research, this leaves the full-size '82-'85 RWD Cadillacs and the smaller '85-'87 FWD Deville as the only options. My intent is to patch the code in the '8746 ECM to emulate the correct ALDL data stream for use with the display.

Here's how the two platforms are configured:

The RWD Cadillacs use the ECM to communicate with the Electronic Climate Control Head (ECC), which in turn communicates with the Fuel Data Panel (FDP). Since 160-baud ALDL is uni-directional, it appears that the ECM has direct inputs from the FDP to handle mode requests for the various display functions. I do not know which device actually calculates the mileage information, especially since the ECC and FDP appear to be dependent upon one another (note the vague "control circuit lines" in the schematic below).





The FWD Cadillacs, on the other hand, use a BCM to interact with the Fuel Data Center (FDC). Again, being uni-directional, there are lots of other data lines to handle interaction between various modules: ECM-to-BCM, BCM-to-ECM, as well as separate lines between the Climate Control Panel (CCP) and FDP. Judging by how these lines are set up, I would guess that the BCM performs all of the processing and just sends the data to the instrument panel for display (something like a 7-segment driver, for instance).







The trouble is, I don't see how the ALDL data stream contains the correct messages for fuel and distance calculation. Regardless of which module performs the calculations, some signal line has to contain either raw data (accumulated fuel, distance traveled, etc.) or formatted data for display only.

I've looked through the various data streams on the Gearhead EFI website, but can't find anything conclusive. The two that apply here are A021 for the FWD models and A047 for the RWD models.

In an earlier post, How to Run Caddy Fuel Mileage Display, RBob indicated that certain ECMs output a truncated data stream when in ALDL "normal mode" that contains the fuel and distance information. This also appears to be confirmed on this website, about halfway down (Buick application). Neither of the above data streams, however, show this.

Can anyone offer some insight? I think I'm close to coming up with a retrofit solution, but this is an important missing piece of the puzzle.

 

That is a significant undertaking. I think all ECM data streams have injector pulse width. With that information and fuel pressure, you know how much fuel you are currently using. If the ECM provides vehicle speed ( I think most, if not all do) you know how fast you are going, and thus can calculate instantaneous miles per gallon. To get average MPG, the display or ECM just keeps a running average of the instantaneous value. How you are going to integrate all that with a different vehicle is going to be tough. Tons of reverse engineering. I would consider grabbing the ALDL data stream with some other piece of hardware and displaying it with a custom display. There are a few projects out there to grab the ALDL stream with microcontrollers. there are bluetooth interfaces too (http://www.1320electronics.com/products.html). Maybe write your own software for your phone or something?

If I stated the obvious, sorry! Good luck!

 

L98_383_Stroker, I appreciate your reply, but that's not really what I'm looking for. I've seen the aftermarket solutions and I'd prefer to use OEM components.

So, nobody has any insight on these data streams?

 

Did you make any more progress with this project?

I'm attempting something similar. I want to get the MPG display working in my early C4 Vette that uses a later TBI truck PCM....

 

Yea, this is interesting. Have you thought about using wireshark to see exactly what is output on the ALDL? Maybe thirdgens have that info already.

 

No, I haven't put much more effort into it. The time will come eventually, but right now I'm focused on rebuilding my engine and trying to put a dent in the project after months of sitting (I'm restoring a Jeep Grand Wagoneer).

The Wireshark product looks interesting, but I haven't actually determined if the ALDL data stream contains the necessary information. I'm sure it depends on the computer and/or PROM mask. When I last looked into this, I was going in the direction of programming a subroutine in the chip that would generate a rolling fuel count and dump the pulses to an unused output (this is how some Ford ECMs work). Then, I'd make a controller board to interpret the pulses, calculate the fuel consumption, and drive a display. It's a lot of work for a novelty, so I'm not sure if it's worth the trouble. Not very DIY-friendly, at least.

 

That's why Wireshark would be useful. You can capture ALL of the output and then look at how many words there are and the value of each word. If there is more/different data than expected, then the necessary stuff may be in there.

That link you posted says that the 1984 year only output 8 words and fuel and distance are two of them, correct? That seems so weird.

Anyway, now that I've brought it up, figuring out how to get wireshark to read the ALDL output seems like fun. I'll let you know if I actually get around to doing it.