94 Accord EX
V-Tec, 4 cyl.
160,000


PART I ECU/IGN SYSTEM PROTECTION —

Q: Is removing the ECU fuse the best way to disable and protect the ECU/IGN system and shut down the fuel system for a Honda? ... For any vehicle? ... Because I thought that the fuse is what protects the ECU." If the fuse is removed how is the ECU protected?

I read the archives but still have additional questions:

Looking over various general repair manuals, and various vehicle specific manuals, it seems the goal to protect the Ign/Ecu systems from damage when cranking the engine for the test.

I gather that in the years before ECU's were put into motor vehicles, the usual thing was to simply disconnect the coil secondary high tension lead (often referred to as the distributor center wire), to protect the ignition system.

But after that is when it gets confusing reading the manuals. But the purpose was still the same, to protect the electronic system. The only common ground I see now is holding the throttle down, removing the fuel cap, and a preliminary cranking it to blow out any loose debris before starting the test.

Some manuals mention removing the Low tension lead (primary [negative] coil wire on 80's/90's Volvos). And some of those manuals vary that slightly. One said to remove both primary wires (positive & negative).

The official Toyota manual for the 1985 MR2 says to remove the HT lead at the coil. But Haynes who also published this manual says ...

QUOTE:

"Although disconnecting the coil high tension wire will prevent the coil from sending spark to the distributor, it won't prevent the coil from being destroyed trying to overcome the resistance between the coil terminal and the nearest ground - the better practice in this case is to unplug the electrical connector between the main engine wire harness and the igniter/coil assembly."

END QUOTE

So, I see what the purpose but wonder why all the various approaches to achieve it. Is a compression test all that vehicle specific and not the simple thing we are lead to believe, inviting us to just jump right in and do it? If there has been negligence in the editing of the manuals or they are poorly written I for one would like this mystery cleared up if possible. I can see how so many people may have unknowingly damaged their vehicle due to what they WERE NOT TOLD in the Manuals. In fact, my biggest stumbling problem with most vehicles is not what I am told but what I am NOT told.

Can anyone shed some light on this subject?


PART II - Relieving Fuel Pressure


Additionally, some manuals say to relieve the fuel system pressure by removing the fuel pump fuse and letting the car stall out; the banjo fuel bolt; and some go as far as to mention removing the injector connectors; and the cold start injector. Is all this necessary? If it is, I don't mind doing it. It is just the misinformation or missing information that cause problems for me.


Thanks,
Diyer

 

Maybe the reasons WHY you wanna do this will help... Different ways to do it on different cars, but the goal is the same. You can open the distributor & unplug the ignitor, or you can unplug the wiring harnesses from the ECU, but pulling a fuse is usually the easiest. Maybe on some types of cars, pulling a fuse is NOT sufficient to disable fuel & spark??

Disable the spark. You gotta remove the spark plugs to measure compression. It's easier to pull a fuse vs. making sure all 4 plug wires are safe. Stray sparks can fry the ECU or ABS or whatever, if you're really unlucky with where they happen to be sitting when the spark goes off. Most of the time they'll either NOT arc at all, or they'll just arc harmlessly to a GOOD ground. But if one wire is resting against your arm...

Disable the fuel. You don't want fuel spraying into the cylinders when you measure compression. Besides gas coming out of the other 3 sparkplug holes & getting in your eyes... especially if you haven't disabled the spark. It probably changes the compression pressure a little, too.

The fuse protects the ECU from damage by a short circuit. By removing the fuse you aren't removing that protection. If you removed the fuse & put a jumper wire in it's place, THAT would remove the protection...

 

Oh, thanks for he reply. I guess each vehicle is specific and there is no formula that can be applied to do a compression test.

I removed the ECU fuse for the test. I did a cold engine compression test and got 180-200 psi in each cylinder. However, I forgot to hold the throttle wide open, so does that negate the results?


thank you,
diyer

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by diyer &raquo;</TD></TR><TR><TD CLASS="quote">Oh, thanks for he reply. I guess each vehicle is specific and there is no formula that can be applied to do a compression test.</TD></TR></TABLE>I suppose the bottom line is that you want to disable the spark & the fuel. Some vehicles you have to pull a couple fuses, or maybe unplug something.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by diyer &raquo;</TD></TR><TR><TD CLASS="quote">I removed the ECU fuse for the test. I did a cold engine compression test and got 180-200 psi in each cylinder. However, I forgot to hold the throttle wide open, so does that negate the results?</TD></TR></TABLE>Maybe it just means that you'll need to crank lots of revolutions before the pressure quits climbing. But I'd rather say just check it again...

The cold temperature affects the rpm the starter spins, & also how well the rings seal. I guess it might be OK for comparing the cylinders against each other, but don't compare against other engines. Actually you aren't supposed to compare against other engines anyway.