Main relay power input to dme problem

[Fluffee]

I have an 88 325is that i just swapped with engine out of a 325i convertible and have been having the no spark no fuel problem, I have went through several diagnostics and got parts including CPS ( checked resistance and its good ) and new main relay and just yesterday I picked up a new harness and i'm still having this problem. On one of the diagnostics it says you have to have power to pins 18 27 and 37 for the car to be able to start. I have power on 18, 27 but on pin 37 when i check for voltage the meter only moves a couple centimeters thus not having much voltage at all and definitely not enough to tell the ecu that the main relay has been activated thus allowing spark and fuel to the car. What could make pin 37 (if i remember correctly it is a blue/red wire running directly from the main relay all the way to the dme) not get but a hair of voltage. Is there a ground anywhere that has anything to do with this, maybe i have a bad ground. What are all the causes that could cause barely to no voltage at all at pin 37?

[German Audio Specialties]

Likely a bad relay.

BTW, grab a testlight to check power at the relay, do NOT use a DVM.

[Fluffee]

I don't have access to a test light right now but i do have voltage at pin 30 and 87 ( left and top pins )

[slammin.e28]

Likely a bad relay.

BTW, grab a testlight to check power at the relay, do NOT use a DVM.

May I inquire as to why to not use a DVM? Just curious.

[StereoInstaller1]

May I inquire as to why to not use a DVM? Just curious.

Yes, by all means, I feel it is imperative thing to understand, and also why I tell people NOT to use their eyeballs when testing fuses.

Let me explain this first: dealing with stuff like this is NOT "electronic" work, it is "electrical work", and a meter (wonderful and necessary tool for its job, testing sensors, continuity and stuff) is great for that, but piss-poor for electrical work.

The issue is "ghost voltage". Ghost voltage will often show 12V (or whatever) when in fact, it may be 12 volts, but damn near zero current...thus no power.

Power is the goal here, right?

Power is a ratio of voltage and amperage...no amperage means no power, just like no voltage also means no power. We get a maximum of what, 15 volts? You better have amperage, right? So what are you gonna do to test for amperage, reset your meter and re-test everything you ever test? Sounds like a pain in the ass to me. 10 seconds with a test light will show you quickly and clearly if you have POWER, not just voltage.

I don't have access to a test light right now but i do have voltage at pin 30 and 87 ( left and top pins )

First, the main relay has 2 "87" pins, which is why you can't buy a normal relay and put it in. Not one single relay anywhere else on the car is the same config, but the Main is only slightly different than a normal relay.

Pull yours, if you see "87a" on the center pin, it is wrong.

BMW has always "spec'd" their relays to be different.

As far as "I don't have a testlight", Wal-Mart has a great one for $5. Says "3M" on the package, found in the automotive section.

If you have to, solder some wires on a light bulb.

Do what you gotta do to get a normal incandescent light bulb in your hand to test this stuff, otherwise you are wasting your time.

Luke

[Fluffee]

Thanks, I really appreciate your advice. I will make a stop by Wal-Mart tomorrow and pick up a test light. I will update tomorrow after my diagnostics.

[whodwho]

You might check that the DME is grounding the main relay, you also should check the grounds for the DME, you may have power but it takes a ground to complete the circuit :)

For reference

18 - steady power
19 - DME ground
27 - power from ignition switch
36 - ground to main relay from DME
37 - power in from main relay

[slammin.e28]

Yes, by all means, I feel it is imperative thing to understand, and also why I tell people NOT to use their eyeballs when testing fuses.

Let me explain this first: dealing with stuff like this is NOT "electronic" work, it is "electrical work", and a meter (wonderful and necessary tool for its job, testing sensors, continuity and stuff) is great for that, but piss-poor for electrical work.

The issue is "ghost voltage". Ghost voltage will often show 12V (or whatever) when in fact, it may be 12 volts, but damn near zero current...thus no power.

Power is the goal here, right?

Power is a ratio of voltage and amperage...no amperage means no power, just like no voltage also means no power. We get a maximum of what, 15 volts? You better have amperage, right? So what are you gonna do to test for amperage, reset your meter and re-test everything you ever test? Sounds like a pain in the ass to me. 10 seconds with a test light will show you quickly and clearly if you have POWER, not just voltage.

Awesome thanks Luke! I always though DVMs more or less took the place of test lamps. Thanks for explaining!

[German Audio Specialties]

Aaaaaaaand?

[Fluffee]

CPS and other plug were switched around, i have power to pin 37 now, the only problems thus far are no power going from ecu to coil and after i stop trying to start the car the injectors keep pulsating.

[DaveSmed]

Yes, by all means, I feel it is imperative thing to understand, and also why I tell people NOT to use their eyeballs when testing fuses.

Let me explain this first: dealing with stuff like this is NOT "electronic" work, it is "electrical work", and a meter (wonderful and necessary tool for its job, testing sensors, continuity and stuff) is great for that, but piss-poor for electrical work.

The issue is "ghost voltage". Ghost voltage will often show 12V (or whatever) when in fact, it may be 12 volts, but damn near zero current...thus no power.

Power is the goal here, right?

Power is a ratio of voltage and amperage...no amperage means no power, just like no voltage also means no power. We get a maximum of what, 15 volts? You better have amperage, right? So what are you gonna do to test for amperage, reset your meter and re-test everything you ever test? Sounds like a pain in the ass to me. 10 seconds with a test light will show you quickly and clearly if you have POWER, not just voltage.

Luke is as usual absolutely right.


BUT don't run out and buy a test light just yet. The DVM is better in pretty much every way IF you use it properly.

The key is quit unplugging shit to test for voltage. You will find the really sneaky problems that way. You don't give a shit if it has voltage unplugged cause if its not there when it's plugged in, your crap still won't work.

Also, be very judicious with how you ground. If at all possible, ground your meter to the ground wire of the component.


To use a simple example, think of a US ellipsoid headlight connector. Instead of unplugging it and checking across the terminals for voltage, grab two small paper clips, bend the ends straight, and slide them into the back of the connector alongside the wire. Make sure its plugged into the bulb, turn the headlights on, THEN measure voltage across both paper clips. If you have voltage then, there is pretty much zero doubt that its the bulb at fault. Just by checking that way you verified everything else, including the ground.

[Fluffee]

Its running now, ended up being the ecu..