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Re: so its safe to bet the ICU is fried Posted by Justin VanAbrahams [Email]  (#32) [Profile/Gallery]
(more from Justin VanAbrahams) on Sun, 19 Nov 2006 18:14:11
In Reply to: so its safe to bet the ICU is fried, Green9kAERO, Sun, 19 Nov 2006 12:29:06 Members do not see ads below this line. - Help Keep This Site Online - Signup |
I wouldn't bet on anything yet, honestly. You could have a wiring problem, your distributor could be fried, or the tach or the ECU could be having a problem. Since it's virtually one big length of wire which ties the ICU to the ECU (stopping at the tach along the way) any issue in either component or the wiring in the middle could cause this.
The tach works by receiving, essentially, a pulsed signal. The pulse causes the tach to bounce, and as RPM increases the pulses come so fast the tach ends up hovering in one spot. I think it's reasonably unlikely that the ICU would be spontaneously pulsing at such a rate to cause the tachometer to peg at a single RPM, so it's more likely that somehow X volts is going down the tach signal wire and sticking the needle in one place.
Could be a fried ICU, but I've *never* seen one do this. I'd think more likely it's some voltage coming from somewhere and going down the wrong wire. You might start by connecting a multimeter to the tachometer signal wire and seeing how many volts are there, and if that voltage is stable. Knowing what that number is may help you locate the problem.
FWIW, this is the kind of thing I was always worried about when swapping fuseboxes. It's entirely possible that Saab moved the position of the tachometer wire and replaced it with something else on those big 152 connectors at the firewall. You might check to be sure that this didn't happen, and you're accidentally feeding output from the oxygen sensor into the tachometer, or something random like that. :)
I would be sure you totally unplug anything sensitive during this troubleshooting. The ECU and the APC box, for example, could be damaged if there's a short somewhere - no point creating more hassle and a bigger budget for this problem than you have to. :)
posted by 216.57.64...
, Sun, 19 Nov 2006 12:08:50, Sun, 19 Nov 2006 18:14:11 <-- Viewing This Message
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