Hi there, I'm new to the forum, but probably should have joined up a bit sooner, seeing as how many problems my Vo' has had. But this new one really has me stumped. Went to start the old girl, and she flooded. No big deal, i have leaky injectors. I figured this out a while ago, so i unplug the injectors and and crank it. the excess gas burns up, per usual, and i plug the injectors back in. usually at this point the car starts up no problem, but not this time. it seems to start and then struggles and dies. this happens alot too, and im used to it. unplugg the injectors agains, burn up the gas, plug them back in. i try to start it again, but my battery, which is really, really, old and weak, doesn't have it in her. so now the battery is dead. not an issue. i go get some jumper cables and a friend, and hook it up. i go to start it. i turn the key, and for a mili second it sounds like it might have started to crank. but suddenly there is a distinct popping noise that comes from somewhere in front of me. my friend, standing outside, heard it too. and instantally the lights on the dash go out, and the car doesn't start. i try turning the key again, and nothing happens. the dash doesn't light up, the radio doesn't light up, it doesn't crank, nothing. it is as if the battery was removed or something, like there is just no electricity at all available for the car to use. but there is, because the battery is there and being jumped. so i check the fuses, and they are all fine. and i just cannot think of what could have failed so badly that it would do this. I though maybe something to do with the ignition switch, or the neutral safety switch, but i believe that if either of those was bad the lights on the dash would still come on. im gonna check it out a little more, but i figured id ask and see if anyone else had encountered this problem before. any takers?

 

"Has anyone encountered this before?"
Probably. Though I bet most get some non leaky injectors and a working battery.
Such super creative, off the wall "fixes" are apt to create some crazy situations for which it is doubtful you will find an easy answer, and you may cause even more damage. Get some used injectors and a battery.

 

the pop undoubtably was something electrical blowing up under the hood. it should have left some visual evidence of doing so.

Id be under there with a volt meter, checking the battery voltage, checking the grounds, etc.

 

Thanks for the advice you guys. My friend just admitted to me that when he hooked the cables up to his car, he accidentally hooked them up backwards. I guess he didn't notice until he was taking them off, and didn't want to say anything. That makes me think I most likely have a blown computer/ ECU. I believe that could cause the problem I'm having. When I get this fixed I am definitely getting new injectors and a battery.

 

the ECU/ICU's are actually pretty hard to blow, they are diode protected internally. Note that on a LH2.2 740 like this, they are both under the dash, the ECU on the passenger side just forward of the lower right door hinge, and the ICU is up under the driver side dashboard.

I'd be more inclined to suspect things like the voltage regulator, or the ignition power module (near the coil).

why are you so sure you have leaky injectors? there's lots of other possible reasons for things to run too rich, like air leaks between the MAF and throttle body

 

I hadn't thought of the voltage regulator or the the ignition power module, would those going out be able to totally shut down the car? I can't even get the radio or dash lights to come on. I think the injectors leak because sometimes i can get the car to idle with the injectors unplugged. I can't think of any other reason why it would do that apart from the injectors leaking. But I'll for sure check the regulator and module before I go and get a new ECU or ICU. But where you have described the ICU as being is about where I heard that popping noise coming from. Thanks for the info.

 

the ignition power module certainly would crash the ignition and prevent the engine from running, but the lights would mostly still work.

its quite possible some key power (or ground) wiring is fried. on my 92 740, there's a "positive terminal" tucked up behind the driver side dashboard near the firewall, and near the top of the nonexistant clutch pedal. ALL power to the car goes through this, its unfused, its a little black box with a bunch of red wires going into it. 2 of these red wires come directly from the battery +, and 2 of them go to the ignition switch. on my daughter's 87 240, the positive terminal is on the left fender wall near the suspension strut tower.

take a volt meter, set for DC Volts. check your battery (Black wire to -, red wire to +), you should see about 12.6V if the battery is fully charged. now do the same with the black meter wire on the engine block ground rail (thats where the wires are screwed down are near the fuel injector rails). and with the black wire on the alternator body...

if your battery is dead, disconnect the ground lead, and charge it fully with a battery charger before hand, and then after disconnecting the charger wait about an hour before reading the battery charge voltage.

if you can find that positive terminal, pop its cover open, and measure the voltage from a nearby chassis ground (there's one behind the outside kick panel at the driver's feet), and that positive terminal, that should be the same as the direct battery voltage.

I don't have the 740 greenbook wiring diagrams (other than the 1991-1992 740 which are the same as a 940). on the 91/92, fuses 1 through 6, one side of them should be directly connected to the battery power without a power switch, I'd use a thin wire (or spade lug the size of a fuse blade) to probe fuse 1 (if one side isn't powered, try the other side, which ever side is the hot side goes to the red meter wire, and the black meter wire goes to a chassis ground, like the ones near the left or right door lower hinge, behind the kick panels.

if you have power in all these places, then something else is screwy.

 

Cool, thanks for all the info, its looking like I'll definitely have to do a little digging to figure this one out, but the things you've mentioned will for sure be the first spots I check. Much obliged!

 

Was there ever a solution found to this problem? The same thing happened to me the other day.

Thanks.