Road speed signal and motronic 1.3

[Jean]

After reading various 24v threads it appears that it's important to have the pin14 run to the cluster for the road speed signal. Some e30 cars have pin14 as part of the c101 and some early ones don't.

Reports of the car running much better with this wire connected to car not running well at all with it disconnected are all over the place.

So, I wanted to see if this wire does anything with the motronic 1.3 on m30b35 swap. I've ran w/o this wire for months (simply because the early cars don't even have pin14 (among others) as part of the chassis wiring), my car pulls fine to redline and does not flash or trigger the check engine light.

To clean up the engine bay, I recently converted my early style chassis side c101 to the late model 20pin style (no adapters now) and ran the black/white pin14 wire from the engine side to the " green plug at the back of the cluster. My car does not and never came with cruise control if it makes any difference, and I am using a late model cluster.

Doesn't seem to make any difference that I've noticed.

Anybody with experience on this want to comment?


thanks to 416stroker for the picture:

[e30_kid89]

Road speed signal is only important and needed for the with 24V Vanos equipped motors since the DME uses the road speed signal along with load and RPM to actuate the vanos solenoid. M1.3 is "dumber" and doesn't need/use the road speed signal for that. From what i've seen With m20s/30s the only time I can see motronic uses the road speed signal is to create the "present fuel rate output" which goes to the econometer.

[Brian D]

When I had the wire disconnected at the cluster, my cruise didn't work. No other problems. Hooked the wire up and cruise worked again-nothing else changed. I don't think it does anything except cruise control on an E30.

To clarify, this is on a stock M20, 1991 sedan. Don't know if that's what you're looking for.

[nando]

the road speed signal on later 1.3 cars is for the top speed limiter but it doesn't come from that green cluster wire. that's for the cruise control. the OBC has it's own input for present fuel rate from the cluster, it doesn't get anything from the ECU except for injector pulse lengths.

[Jean]

Good info!

nando - so what happens with the top speed limiter if the wire isn't connected or connected?
my car does not have obc or cruise...but I would like the mpg to work (wire for mpg/fuel rate is connected) but the gauge doesn't seem to work..

Thanks!

[Doughnut]

Good info!

nando - so what happens with the top speed limiter if the wire isn't connected or connected?
my car does not have obc or cruise...but I would like the mpg to work (wire for mpg/fuel rate is connected) but the gauge doesn't seem to work..

Thanks!

I have the later chassis ('89) with m1.3 and I'd like to get my mpg gauge working also, I don't have an obc.

Which wire on the engine side do i connect to on the chassis side and where?

[Jean]

Should be pin 8, white/black according to ETM

[nando]

Good info!

nando - so what happens with the top speed limiter if the wire isn't connected or connected?
my car does not have obc or cruise...but I would like the mpg to work (wire for mpg/fuel rate is connected) but the gauge doesn't seem to work..

Thanks!

the speed signal for the speed limiter/ECU is through a different wire in the harness from the cluster. I tried to disable it but never found which one it was (also tried to unplug the cruise control wire), ended up removing Motronic which fixed that problem. :)

regarding your gauge, basically the ECU sends a conditioned injector pulsewidth signal to the cluster through Motronic pin 32. that is the only communication the ECU has with that gauge (other than RPM which is indirectly routed through the cluster). the gauge then takes the engine RPM and MPH from the cluster, and uses the three signals to calculate MPG. it assumes things like stock injector size, regular gasoline, stock fuel pressure, etc which is why when you change to a motor with different pressure and injectors it doesn't work anymore.

if your gauge isn't working, it's probably something in the cluster - could be a burned/broken trace, dead battery, etc. it's possible pin 32 isn't connected anymore to the cluster somehow, but the MPH signal to the ECU isn't what you're looking for.

[nando]

btw vehicle speed input is pin 29 if you have a speed limiter (maybe only on 5XX ecus?). I didn't know enough at the time to disable it and didn't want to start cutting random wires. It might also be used for idle control though so it's probably not a good idea to remove it anyway.

[Jean]

Well ,the gauge worked fine before the swap so that's why I am thinking it's something else. Before I did the swap I traced what wire did what above the glove box, in the 3 pin connector (I disconnected one wire at a time and made notes just to be 100% sure). One of them was tach (black) and one was for mpg (yellow/white).



I have pin 8 from the engine side go to that same spot for mpg, yet the gauge doesn't work. Very odd. I was starting to think that maybe road speed signal is needed for mpg calculation, but obviously it isn't. My tach and all other gauges are working...

Good to know that road speed signal isn't doing much on m30 cars vs. 24v swaps!

[MattM]

that wire also tells the engine to idle at 1100 when the car is moving, and drop to ~600 when not moving..

[nando]

M30 right? both Motronic 1.3, but the M30 uses a 179?

I haven't done anything with my C101 so I have no idea what pin does what there. I did everything right at the harness connector to the ECU.

also with the M30 I don't think that gauge will be accurate anyway, unless you use one of hoveringguy's (www.seattlecircuit.com) conditioning circuits.

[Jean]

that wire also tells the engine to idle at 1100 when the car is moving, and drop to ~600 when not moving..

That's what I read it does for the 24v guys, but it's not doing that for me.

[MattM]

the guage wont be accurate for an M30, im not sure if hoveringguys thing has enough tunability to use with the M30 aswell? what does an average M30 get for real fuel economy?

[Jean]

M30 right? both Motronic 1.3, but the M30 uses a 179?

I haven't done anything with my C101 so I have no idea what pin does what there. I did everything right at the harness connector to the ECU.

also with the M30 I don't think that gauge will be accurate anyway, unless you use one of hoveringguy's (www.seattlecircuit.com) conditioning circuits.

Correct, m30 with stock 179 dme.

My c101 is on the engine side is 20pin round c101 (m30b35 harness, unmodified), I converted my chassis side to 20 pin late model e30 c101 as well (from the early rectangular kind). Obviously early chassis only has so many wires going to the c101, so extra wires I have to run on their own from the c101 to inside the car (check engine light wire, tach wire, mpg wire).