no check or check enging light.

[e30work]

I got a car that did not start due to the previous owner connecting the battery wrong. since then here is what i have done.

- new battery
- checked wiring harness
- checked relays and fuses
- checked fuel pump, works when jumped on all clicks(off, acc, on)
- checked fuse-link (fusable link), not blown
- interior lights works as well as gauges and locking system
- new dme
- check dme harness and it is getting 12v at pin 54 and 55
- dme harness reads (510 ohms) for the crank position sensor
- crank position sensor reads (510 ohms) i read this is still good
- rpm jumps on crank
- spark plugs do not get soaked in fuel
- car cranks and turns but does not start

i might have a bad alternator but im sure this should not stop the car from starting. when cranking i am also jumping power from another car.

any help appreciated. thanks

[DesktopDave]

That's a tough one to troubleshoot.  A reversed battery could have caused all sorts of havoc...I hooked up a single circuit backwards once & melted a 14 gauge wire in a few seconds!  Worse, about half the car is directly wired to the battery...no fuses at all...but hopefully the damage wasn't too bad.  Most of the Bosch stuff is pretty tough.  Did you take the fuse box apart to verify that the main power bus is still intact?  There are a few diodes that you'll want to check, and some relays are have built-in protection (resistors or diodes) that might have been damaged.

If you don't get a check engine light, either the bulb is burned out or the DME isn't getting power.  You tested the trunk fusible link and the power o the DME though, that seems OK.  Once you get that sorted, try stomp test.   It's not the greatest system, but it's better than nothing.  It'll give good hints, might help to chase down your problem.

Looks like you've been all over that car.  The only thing that looks wrong to me is the crank sensor resistance.  If that sensor doesn't send a perfect signal the fuel pump will never run.

Specifications are 640ohms, +-10% (roughly 580 min/700 max) and the air gap on the trigger wheel shouldn't be any more than 1mm...about as thick as a credit card.  Also, if the crank sensor face is oily & coated with rust, it won't send a good signal.  Then the DME won't ground the fuel pump relay and the pump never turns on.

They're pretty expensive (and usually not returnable), so if you can swap a known good unit it'll save a lot of time & trouble.

You're right that a bad alt won't be part of the trouble during a jump start or with a fresh battery.  It'll kill a good battery quickly though if it's been overheated or shorted out internally.  You might want to have it tested at a local parts store, though I'd be looking for a specialist instead.  Rebuilt Bosch alternators (& starters) are crazy expensive.

[e30work]

Did you take the fuse box apart to verify that the main power bus is still intact?  There are a few diodes that you'll want to check, and some relays are have built-in protection (resistors or diodes) that might have been damaged.

what is the main power bus? is this in the fuse box? the only relays that should effect the dme is the 3 relays on the front of the firewall correct? all other relays in the fuse box are not important right now because they are for other accessories correct?

If you don't get a check engine light, either the bulb is burned out or the DME isn't getting power.

all my other lights work perfect so the chances of the two check and check engine lights burning out are highly unlikely. plus when you do the stomp test even with no light it should make the clicking noise.

Looks like you've been all over that car.  The only thing that looks wrong to me is the crank sensor resistance.  If that sensor doesn't send a perfect signal the fuel pump will never run.

will a bad cps disable the dme to not allow the stomp test? are early obd1 e36 cps the same? can i use it in my e30 m42?

You're right that a bad alt won't be part of the trouble during a jump start or with a fresh battery.  It'll kill a good battery quickly though if it's been overheated or shorted out internally.  You might want to have it tested at a local parts store, though I'd be looking for a specialist instead.  Rebuilt Bosch alternators (& starters) are crazy expensive.

 battery tested ok in both cars. do you think a bad alt can disrupt the power going to the dme?

thanks for you help.

[e30work]

i think i am not getting power to the dme, do you know which pins on the dme harness powers the dme? e36 bently says

pin 54 is input signal (battery voltage from engine control module). this gets correct voltage when in off, acc, and on (all key clicks)

[e30work]

Did you take the fuse box apart to verify that the main power bus is still intact?  There are a few diodes that you'll want to check, and some relays are have built-in protection (resistors or diodes) that might have been damaged.

what is the main power bus? is this in the fuse box? the only relays that should effect the dme is the 3 relays on the front of the firewall correct? all other relays in the fuse box are not important right now because they are for other accessories correct?

If you don't get a check engine light, either the bulb is burned out or the DME isn't getting power.

all my other lights work perfect so the chances of the two check and check engine lights burning out are highly unlikely. plus when you do the stomp test even with no light it should make the clicking noise.

Looks like you've been all over that car.  The only thing that looks wrong to me is the crank sensor resistance.  If that sensor doesn't send a perfect signal the fuel pump will never run.

will a bad cps disable the dme to not allow the stomp test? are early obd1 e36 cps the same? can i use it in my e30 m42?

You're right that a bad alt won't be part of the trouble during a jump start or with a fresh battery.  It'll kill a good battery quickly though if it's been overheated or shorted out internally.  You might want to have it tested at a local parts store, though I'd be looking for a specialist instead.  Rebuilt Bosch alternators (& starters) are crazy expensive.

battery tested ok in both cars. do you think a bad alt or cps can disrupt the power going to the dme? i just need to get the dme and check/check engine lights to activate, after that i think everything will be ok.

thanks for you help.

[318lotis]

did you check the fuse that connects to harness in truck leading to battery?

[318lotis]

sorry not truck, trunk, there is a fuse leading to battery inside trunk.

[e30work]

yes i did, thats the "fusable link".

very good update. this thread is very helpful if you are having similar problem. it will help walk you through everything. http://www.m42club.com/forums/showthread.php?t=8474

so i found out that there are two different types of relays that can fit into the main replay plug. i found the correct white one plugged it in and got my check engine light. still have battery light on all the time so i am assuming bad alt. cps read under 500 now and i do not get fuel. i guess the last thing to do is to clean the cps, test again, and if i have to, replace it.

one thing i read was that if a cps was bad it would read 0 or nothing. i hope its just dirty...

[DesktopDave]

Good to hear you're getting closer.  I'd figure the CPS will have resistance (none of the ones I checked ever had zero resistance FWIW), unless the signal wire got smashed together or something.  It's basically a magnet...so that gap & resistance must be very precise for it to work.  If the DME doesn't get a clean signal, you get no fuel.  It's a very common problem with our cars.

Make sure you get that alternator tested too...they do go bad (especially with reversed terminals) but it might just be a bad wire or a bad ground.  No need to toss good parts out, especially expensive ones like that.  Check the engine grounding strap and the front grounding stud (by the driver's headlight).  There are a few others around the car, but they're usually OK.

[e30work]

are all (e30 and e36) m42 and m44 crankshaft position sensor (cps) the same? thanks

[e30work]

update, so after receiving a bad cps i decided not to waste the whole day and test everything else just to make sure its only the cps.

first was to test the fuel pump as everything else seem fine.  

with half a tank of gas, i went straight to the fuel filter. unplug the fuel line right after the fuel filter (the one going to the engine bay) and let it drain the excess gas. once dry, i plugged in the battery and cranked the car. i got a couple of drop but nothing else. common sense says it should be pouring out. then i said maybe its the dme so i jump the fuel pump from the relay (i can hear it priming) and still nothing. after that i tried this test with the fuel line before the fuel filter and still nothing.

does this conclude a bad fuel pump? jumping the fuel pump relay should bypass the dme and pump gas correct? i am thinking it is a bad pump because the starter and alternator had to be replaced after the PO connected the battery wrong.

thanks

[DesktopDave]

I'd say you have a bad pump as well.  To be totally sure, I'd run some jumper wires from the battery directly to the pump.  There's an article 'round here about swapping an aftermarket upgrade pump into the stock BMW pump frame.

are all (e30 and e36) m42 and m44 crankshaft position sensor (cps) the same? thanks

That's a good question.  They mount in different locations, but I've never verified if they're the same or not.  Pelican lists them as three different numbers, so it's a gamble.  Might just be that the wires are different lengths, or they might be totally incompatible.  Here's a link.

[e30work]

update: car runs and everything seems to be fine. thanks to all that help, especially you dave.

so the results of a reversed battery connection led to

1) bad dme
2) bad starter
3) bad fuel pump
4) bad alternator and wire

luckily all of this was replaced by the original owner who truly believed the car came with a reverse polarity set up from the factory. he had positive to the chassis and even wired the fuel pump wires reversed. from what i can tell he was troubleshooting a no start (which was probably just a bad cps) and upon changing the battery he mistakenly assumed red cable to the chassis was positive.

the main thing to check here when diagnosing a problem like this is to make sure you have the correct relays. there are two 87a type relays that fit in the main relay socket but the switch is different. also 510 ohm on a cps means bad.

thanks again