Originally Posted by
Romad
Went for a morning drive for about 40 minutes. Parked car for about 10 minutes and then had a "no start" situation where I had dash lights but no starter crank. My '05 Xfire has been experiencing intermittent starting problems and was wondering if anyone thinks a too warm ECU/RCM could be part of my problem. Stuck my finger in that mysterious hole in the front bottom of the box and the fan bit me! It does push out air but how much is adequate for cooling the ECU. The air coming out seems a little warm and I do realize the air comes from the cabin. I do have a new cabin filter installed. 1) Should the fan "pull" air from the cabin into the black box via the fan hole in the front of the box? 2) What is the circulation path "in and out" of the black box? With the black box's cover installed seems it might not circulate. Has new cam and crank sensors.
Warming and cooling of the boards in the RCM does cause stress fractures to the solder connections. The ECU appears to have been designed and constructed such that it is immune to this.
Forget the cam and crank sensors, they play no part in starter engagement.
1) The fan draws cabin air from the cavity where the cabin filter is, after the cabin filter filters the air but before it passes thru the heater core or A/C evaporator.
2) The lid on the box is designed to resist air flow, but some still leaks past it, so there is flow.
The theory is two-fold.
A] The air brought in is cooler than what WOULD be in the box had they not force-vented it. The box is in an area that can get to 180F in the summer, sitting at a traffic light. By drawing air in that has passed the cabin filter, it is clean and of cabin temperature. Even with the top down in summer, the air will be no more than maybe 100F, and only then if you can stand to sit there and drive with the top down when it is 100F outside, this is way better than what the underhood temperature would be.
B] By causing there to be resistance to the blower's air flow, the air pressure in the box is SLIGHTLY higher than atmosphere, this keeps unfiltered, dusty underhood air out of the box, keeping it cleaner. Very important, as we all know how much dust can 'insulate' electronic chips and other components. This is why that box's insides looks pretty good in a car with 150,000 miles on it and dust and dirt all over the engine compartment.
The service manual explains the theory behind the blower in the ECU box, there is also mention of it in the Student Reference.
If the no crank appears related to a warm/hot car, I'd suspect the RCM. Otherwise, no cranks can be:
- Interlock issue (clutch pedal switch in manuals, Shift lever assembly in autos)
- Flaky battery voltage caused by flaky battery, corroded/bad battery cables, etc. (lower than about 10.4 at the ECU not the battery terminals or so will inhibit starter engagement in these cars and the SLKs)
- Flaky pulse module (VERY rare)
- Bad starter or starter solenoid (VERY rare and easy to diagnose with only a flat blade screwdriver, I can give you the procedure if you need it.)
- A bad ECU (VERY VERY rare but I know of ONE case since 2009).
Anyone who tells you to change the cam or crank sensors because your starter is not engaging either did not READ your post or should not be giving technical advice. Please read the warning/instructional link in my sigline.