frankvan

Does CCU get any info from head temp?

Steve Weiner-Rennsport Systems

Nossir,..head temp values are for the DME only.

frankvan

So i've been experimenting with the CCU settings: when the engine is warm and the CCU is at a setting calling for heat the engine revs drop slowly (leading to virtually no engine braking). When the CCU is turned to a setting not calling for heat the revs drop normally. There must be some connection between the CCU heat setting and the DME.

I can be going down a hill in 3rd gear without slowing down, then turn the CCU to a cold setting and within about a second i suddenly get engine braking. And like good scientific method i've repeated the experiment several times with the same result. Living in a subzero environment however, turning down the temp every time i slow down is just not practical

FWIW, the AC compressor is not engaging so it's not an AC/ idle speed related issue.
The intermittent whistling ive been hearing is coming from the wheel wells and i think its the heater flapper valves located there making that noise. so not a vacuum leak.

frankvan

https://rennlist.com/forums/4207948-post3.html

Looking at the DME pins I see that there are some possible sources that might cause the symptons - just not sure what their actual functions are:
Pin 4 - idle speed control (ok, kind of self explanatory)
Pin 5 - tank venting valve at 2000 rpm - what triggers this? Could be a potential vacuum leak source?
pins 40/41 - A/C compressor / switch (bad info from CCU to DME?)
Pin 50 - HVAC regulation (?) - what does this trigger? pins 40/41? Pin 4?

I'll have to get out the old scantool computer and see what codes are on the CCU. (haven't used it in a while, hope i can remember the usb/com port settings and how to hook it up).

F

frankvan

Took out the CCU last night and cleaned it with some contact cleaner (there was some electric corrosion around some of the solder joints but nothing too major, not enough to cause malfunctions - but now it looks new inside) and cleaned the temp sensor fan thing that was full of dust. Still get the same symptoms after re-install.

I'm going to dig into the vacuum lines in the engine compartment - i'm suspecting the tank vent valve, and it's buried nicely behind the intake!

However, doing a bit more digging into schematics, pin K-13 on the CCU controls something labelled "DME Coasting Shutoff" that I think goes to pin 50 on the DME. This sounds exactly like what my problem is. Anyone have any idea? (it's tempting to cut it to see if the car runs normal again, but good sense will prevail)

FeralComprehension

I've heard of similar strategies used as part of the overall emissions control; a fuel cut when backed out of the throttle.

frankvan

yes, I believe Motronic was one of the first FI programs to incorporate fuel shutoff when idle switch is closed and engine is coasting. It seems in my case when i have heat turned on through the CCU it tells the DME to not cut off the fuel.
SO just for an experiment I removed pin 12 from connector T6 in the passenger side footwell (it's a Black/Green wire that goes from K13 on CCU to 50 on DME). Easy enough to do and not permanent. And so far it seems to work.
I'm thinking that when summer comes around (and it's bound to i'm sure) i'll just slide the pin back into the connector so that the engine won't stall when the AC is active.

KaiB

Great work!

frankvan

i'm not sure if it's great work, because i've basically just bypassed whatever the problem is (treating the symptoms, not the underlying disease process).

dfinnegan

Have you checked the idle switch, itself?

Makmov

O2 sensor?

I had a similar problem whenever I got out on the interstate and then got off on a ramp the CEL came on and then it would stop running.

The CEL is a very simple thing my 91 car, you car may be different, but the CEL is only connected to the O2 sensor and the fuel vapor.

I replace the 02 and it fix both problems, the light comming on occationally and the stalling.

mojorizing

On the CCU, the pin K13 output is driven from a pull-up resistor, a BC337 transistor (T10) driven from the CCU micro-processor. The DME is expecting a low signal or a high, I'm not sure if leaving it disconnected (floating) is best.


Where in the footwell is this connector, I'd like to find out more about this signal.

frankvan

idle switch works properly.
Engine is doing the opposite of stalling - the revs won't drop (very similar to vacuum leak). But it only does it when the CCU is set to a heat setting. Separating the CCU signal from the DME seems to have cured the problem.
A BC337 transistor? I got one reference diagram from www.ccu.tore.bergvill.com thats a diagram for 993 but essentially same as ours. Connector T5 (not T6, typo in previous post) is a 14 pin connector tucked under the dash next to the radio, pin 12 (they're even numbered on the connector for ease of abuse) is a black with green stripe wire. mojo, you seem to have significant knowledge of the innards of the CCU, please share!

And the intermittent whistling noise has not disappeared and seems completely unrelated to the rpm drop problem or to CCU heat setting.

mojorizing

Frank,
I have a CCU that's apart right now , and I traced the circuit. I'm not an expert by any means!

This morning I did some looking at K13 output to the DME. It jibes what your seeing as you move your temp. setting ****. When the dial is below ambient, K13 is 0 v, when set above the ambient, calling for heat, it goes high (+12v). K13 has nothing to do whether the A/C compressor is clutched in or out...that signal is coming from someplace else.

Question: Is K13 to keep the engine at a high idle if the driver is wanting heat in the cabin, even as the car is coasting? It appears so.....

There are a number of posts similar to this one trying to identify the reasons for a higher-than-expected revs as the car coasts , like down a slight hill, onto an on-ramp, etc. I'd have to do more reading , but vacuum leaks, injector problems, fuel delivery issues, etc. are always mentioned.

What do you think? Is this how K13 is supposed to affect the engine? I've never seen this problem because I always have the temp. **** set low - never call for heat.

BHMav8r

I had the same problem with mine yesterday.

Fairly chilly with the top down -turned on the heat . . . now the rpms drop slowly when shifting. Sounds exactly like your symptoms. I also had a vague whislte - not sure if it was new or just paranoia.

I'll try it with the heat off and see if it goes away.

Thanks for all your investigation. Please keep us posted.