E21 M42 Wiring Q

[jaredmac11]

I lost my previous thread so Im regurgitating this one...

On the M42 wiring harness there's a battery terminal post and another hot wire (Im told) with a ground. What are the two hot wires powering exactly? Is it a good idea to remove excess wires from the harness that arent used in the C101? I'd almost rather keep the harness whole.

For the e21 fuse box' power line that went to the positive terminal... does that run to the power distribution box?

So I'd go Battery > Distribution Box > Wiring harness Power 1, Wiring Harness Power 2, Starter, E21 fuse box?

[pofo]

My swap was a bit different, but yes the battery will go to the fuse box (4 or 6 gauge wire) and (2 gauge wire) to the stater that then goes to the alternator . Then you can re pin the wiring to the e21 plug to the fuse box and make it plug and play.

It's kinda that simple. Car should turn over after that.

[jaredmac11]

My swap was a bit different, but yes the battery will go to the fuse box (4 or 6 gauge wire) and (2 gauge wire) to the stater that then goes to the alternator . Then you can re pin the wiring to the e21 plug to the fuse box and make it plug and play.

It's kinda that simple. Car should turn over after that.

That's what I was wondering.. if I'm providing power thru the harness I didn't know what could be hacked off.

Are any components that are connected to the harness bypassed by the c101 connector?

I don't really understand what BMW was doing with the E30 harness. . .

[pofo]

I worked with a E36 harness so the wiring was extremely simple. But I think I remember seeing a thread online somewhere showing a diy for the E30 c101 harness, it may have been a 2002 forum but I maybe wrong. I will see if I can find it when I have some free time.

[autox320]

That's what I was wondering.. if I'm providing power thru the harness I didn't know what could be hacked off.

Are any components that are connected to the harness bypassed by the c101 connector?

I don't really understand what BMW was doing with the E30 harness. . .

A lot of the C101 are test points for code readers and voltmeter probe points(sensors, outputs etc). Unless an E30M3 which is OBD exempt ie no codes thrown or help to troubleshoot. A lot of it's plug is empty and dead pins. Have to do it the old fashioned way. I'll be going through the harness on the M3 when get there. Yeah want to delete all unnecessary wiring. Just engine harness to make the motor run so can wire the DOT stuff myself There's a lot going on under a E30 dash. So much I'd really hate to see a 2018 cars wiring.

- - - Updated - - -

someone has to be hosting the ETM's for the M42 in PDF format; it helps a lot to see whats going on

I've a few pages in PDF that are missing from the ETM I think it's for the M42 ignition since it's slightly different from the E36.

[amarino]

someone has to be hosting the ETM's for the M42 in PDF format; it helps a lot to see whats going on

Maybe one of these? http://wedophones.com/BMWManualsLead.htm

[autox320]

Maybe one of these? http://wedophones.com/BMWManualsLead.htm

Yes Dude your still the man.

I'll see if can dig up; post the "missing" page I've got for the 91 318is/i M42 manual.

[jaredmac11]

I decided to go the franken-m42 route. Its an e36 long block with the E30 intake, ECU and harness. The e36 wiring was just too complicated and I needed an expensive chip to delete EWS so I went another way.

I have the wiring figured out, mapping E21 to the E30 C101. But I need to basically remove every unused wire? Or can I just cut and tape the ends? Id hate to open up the harness and literally remove the wires-- seems like an unnecessary project.

[jaredmac11]

Use closed end connectors instead of just cutting and taping. These are like the "wire nuts" that electricians use, only for smaller wires. That's what electricians do if there is a wire which will not be used - put a nut over it so there will be no short.

http://www.delcity.net/store/Closed-...3988.r_IF1003?

Nice, I picked those up and Im cutting off the end for a more clean/tight fit.

[jaredmac11]

I have 2 oddities I need to sort on the E21 harness. I grabbed a harness of a late E21 to keep my harness whole for when I sold my M10. So I dont know if its a difference between early and late or if this particular harness was modified or maybe I just didnt notice these differences on my original harness when I mapped it out... Anyways...

1) On the starter wire for the e21 Engine Connector, there are 2 larger gauge wires coming from the pin. The E30 C101 only has 1 wire of the same gauge. Can I cut and cap 1 of the 2 wires? I have the same story on the switched power to O2 sensor & ECU power-- 2 wires on the E21 connector going to 1 C101 wire.

2) On the fuel pump power wire-- the E21 wire is a larger gauge than the E30 wire. Can I run a smaller gauge wire from the E30 harness to a larger gauge wire on the e21 plug?

edit: One second thought for #1, these each ran to a relay, Ill verify these are sorted as a relay for the E30.

[Tom D]

since i don't keep notes of my builds i can't answer these questions off the cuff. you'll need to study the ETM for all three engines/chassis, see what gets cut out and what doesn't.

for instance, the M42's O2 sensor is wired directly to the ECU and does not get wired to the E21's chassis.

both the E30 and the E21 only have a single switching wire going to the starter solenoid, the second wire is activating something else at the same time as the starter.

no you do not want to wire the fuel pump directly, in my opinion it becomes a fire hazard, be very cautious here. the problem is that both chassis have the fuse, relay and the pump wired differently from each other. on the E21 the fuse is wired before the relay and the E30/M42 the fuse is after the relay. because of this you can end up having a too small wire feeding the pump and without fuse protection. i always re-wire the relay, the fuse box and/or the engine connection.

one more point, the engine connector as well as all the connections have 'pin numbers', it helps you you tell us what pin you're talking about. for example, a quick look shows that the E21 engine connector's #10 pin would be a double large wire, one of which goes to the starter, the other goes to the unloader relay.

[jaredmac11]

...you'll need to study the ETM for all three engines/chassis, see what gets cut out and what doesn't.

...

OK thanks Tom, sounds like I made a bit of a loaded question and made too many assumptions. I think Im gonna slow down and re-read these manuals.

Regarding the fuel pump-- did you just insert a stand-alone fuse after the relay and wire that to the connector? Or did you rewire to the old fuse box? I wonder how other people do it, I havent seen this sort of care in other builds but then again it may just be lost in undocumented efforts.

Also, I dont follow the logic on the O2. Other E21/M42 swap diagrams show pin #6 / green&violet wire tying to the heated switch pin(#13) for the E30, wouldnt I want to maintain that? Or is that a misstep?
edit: seems like a misstep.

This is super helpful and is giving me a lot of food for thought.

[Tom D]

pin 6 on the e21 engine connector is the fuel pump, pin 8 is the O2 circuit.

the lambda circuit for the E21 is powered independently from the run/start bus bar in the fuse box, this is what pin 8 does, supplies the power to run the system.

the lambda circuit for the M42 is powered by the main relay (solenoid) and by the fuel pump relay (operating power) and pump relay is also powered by the main relay. all three relays, main, pump and lambda as well as the O2 sensor and ECU are all part of the engine harness and do not require any wiring to the E21 chassis. the only thing that needs connecting is the main relay and it's power is pulled directly from the battery.

[Tom D]

sorry if this created any confusion, but pin 8 as i explained above is only for the 1.8l e21's. since the 2l cars did not have lambda there is nothing on pin 8 of the engine connector. therefore even though the later engine harness you have will have a wire at pin 8, your chassis being a 78 model will not have pin 8.

- - - Updated - - -

also for the ease of communicating, the 'engine' connector on the 2l models is called a C20h, on the 1.8l cars they called it an engine connector. since my expertise is not in speed typing i will from this point on call the 'engine' connector a C20h.

[Tom D]

regarding the fuel pump circuit.

the e21 uses a 2.5mm (12g) supply wire and a 16amp fuse, the m42 uses a 1.5mm (14g) supply and a 7.5amp fuse. i have to believe the difference is that the e21 pump puts out at 90psi and the e30 only 45psi. therefore the e21 pump has a bigger load and needs a bigger circuit.

since the m42 only requires 45psi to operate then i expect that the e21's pump will never run at full draw and a 1.5mm wire is probably more then adequate. my concern is as i explained in post 11. because the fuses are wired differently if you were to connect C101 terminal 13 (relay gn/vi) directly to C20h terminal 6 (pump gn/wt) then you will be able to run the pump properly, but you have removed all fused protection.

in the past i have used several work arounds. on my 77 there is an unused fuse holder that is not actually wired to the bus bar that is labeled fuel pump. i believe this was used for the non injected euro cars. so for my swap i wired C101 #13 to C20h #6 and then C20h #6 to one side of that fuse. the pump was dissconnected from C20h #6 and wired directly to the other side of the fuse. i did this inside the fuse box for a factory appearance.

for a 81 model swap i did recently with my son we rewired the engine harness by removing the E30's relay from the harness and switched the e21's original relay. doing it this way keeps the E21's pump circuit intact using all of the original wiring and the 16amp fuse.

on another swap a few years ago i simply added a stand alone fuse holder inline between the m42's relay and the e21's pump.

if you choose either the 1st or 3rd options then be sure to downsize the fuse. that way if the pump should overheat the circuit it will blow before the smaller 1.5mm wire melts down.

[jaredmac11]

Damn Tom. Great info. I think Im following along closely, but only because Ive been staring at these engine diagrams for the past umpteen hours. To be honest, the E21 ETM presents info a bit differently than other models and its a wee bit confusing. For instance, pin 8 appears twice on one diagram, but its referring to 2 different functions :/

Let me try to break down a few thoughts I have...

sorry if this created any confusion, but pin 8 as i explained above is only for the 1.8l e21's. since the 2l cars did not have lambda there is nothing on pin 8 of the engine connector. therefore even though the later engine harness you have will have a wire at pin 8, your chassis being a 78 model will not have pin 8.

I see the Lambda in the late models--green wire @ pin 8. However, in the early, there's also a green wire @ pin 8 that goes to the ignition coil. I think I can successfully wire this to the M42 ECU power.

regarding the fuel pump circuit.

...
so for my swap i wired C101 #13 to C20h #6 and then C20h #6 to one side of that fuse. the pump was dissconnected from C20h #6 and wired directly to the other side of the fuse. i did this inside the fuse box for a factory appearance.
...
on another swap a few years ago i simply added a stand alone fuse holder inline between the m42's relay and the e21's pump.

if you choose either the 1st or 3rd options then be sure to downsize the fuse. that way if the pump should overheat the circuit it will blow before the smaller 1.5mm wire melts down.

I was thinking Option 3. I was considering wiring to that fuse in the box (I believe its fuse #16) for the 'factory look' as you said, but at the end of the day, a stand-alone fuse between the relay and pump wouldnt be a big deal.
Im not too knowledgeable on fuses.. what amps would you recommend? 5-7.5A?



I *think* I got it. Unfortunately the early and late arent identical but that being said they are pretty damn close. The only one I really dont get is pin 6.
On the early, it *looks* like there were 3 wires going to pin 6? This only concerns me bc I wanna make sure that the wiring is correct for the chassis. Otherwise, I believe I got it...
early e21 pin 6.JPG

[Tom D]

in the early, there's also a green wire @ pin 8 that goes to the ignition coil. I think I can successfully wire this to the M42 ECU power.

you're right, its been a while.

C20h # 8 to C101 pin # ? ....... refresh my memory

[jaredmac11]

C20h #8/grn to c101 #7/grn/ECU power. I believe. I'm questioning my source and need to confirm.

Should #8 on the C20h be the ECU power?

[autox320]

C20h #8/grn to c101 #7/grn/ECU power. I believe. I'm questioning my source and need to confirm.

Should #8 on the C20h be the ECU power?

IMO main thing with fuel pump powering is make sure wire the relay for safety shutoff. An E30 to the ECU "pump control" to keep safety aspect in a crash or roll over. This kills the pump when the engine isn't running.
On the E21 that's why the relay is strange and has the pin that connects to the distributor pickup. It kills the pump when doesn't see pulses.

Even when carb it's best to have a way to stop the pump when the engine stops. A way to do this is relay is tied to low oil pressure. Ours is on a 20psi switch, so if below that the fuel pump is off. That's right the motor has to turn over with 20psi before the pump is on. But the fuel in the bowl is enough to allow restart.

Other than that make sure wire size is adequate over distance. Voltage drop to the pump diminishes actual output (gpm/gph). Not an issue until trying to squeeze hp out of a car when wondering why the pump tapers off output at high revs. Probably never notice unless turbo or something on a M42.

[jaredmac11]

Thanks for the input, I'll make sure Im introducing a fuse after the relay. Im curious if anyone did re-wire the e30 relay to introduce the larger 2.5mm gauge wire? I was going down this route, but BMW really over-engineered these relay pins. Outside of being unable to even remove the pin, Im not sure I could find a relay pin that fits the relay housing...

Going to go to the hardware store and see if I can find something. Thank god I have a spare harness with relay.. we'll see. Im guessing nobody has had issues with the 1.5mm overheating, otherwise more would take precaution (assuming)

Jared

[Tom D]

.........Im guessing nobody has had issues with the 1.5mm overheating, otherwise more would take precaution (assuming). Jared

i run my m42 swap using the 1.5mm wiring and a 7.5 amp fuse without problems, you'd most likely overheat the wire if the pump was going south or it's connection or ground became iffy.

most folks doing E30 swaps into E21's are using the same wiring schematics that have been posted here for a long time now. the problem is that most of these schematics don't include a fused circuit for the pump and it probably just a matter of time before someone's car burns to the ground.

[Somjuan]

Hey! I'm having these same questions rewiring! Now I feel better. I'll plop this in this thread because they seem like they probably have the same answer.

So for the green wire (pin 8) I actually have the same question with my m50 swap. I'm also wondering if I need the old relays connected to the c10h harness at all, mine is off of a late model so the lambda control is still there, but I could run ECU power off of that, then it would remain switched by relay at the fusebox. Also thinking it would be nice to wire the fuel consumption half of the e28 or e30 gauge into the fusebox for a blank pin so it works properly.

Last thoughts - there are two wires in pin 15 that are black and thin. Which one in the same pin is the tach? Is it better to run the check engine light, which I need for smog, into the fusebox or straight from the ECU and under the dash?

Here's what I have so far, just to put it all out there

Function	1979 E21	1983 E21	1992 E36
Alternator		Blue (pin 5)	Blue (pin 25)
Coolant Temp		Brown/White (Pin 13)	Green/Grey (pin 12)
Oil Pressure		Brown/Green (Pin 1)	Green (Pin 23)
Diagnostic			Green/White (Pin 22)
Ignition		Green (Pin 8?)	Green (Pin21)
Fuel Cons.			White/Black (Pin 24)
Engine Speed		Black (Pin 15)	Black (Pin 20)
Check Engine			Grey (Pin 8)
Fuel Pump		Green/Black (Pin 3)	Green/Violet (Pin 13)
Speed Signal (ABS?)			Black/White (Pin 14)
Starter (Thin)		Black/Yellow (Pin 10)	Black/Yellow (Pin 15)
Starter (Thick)		Black/Yellow (Pin 10)	Black/Yellow (Pin 18)
O2 Sensor heater		Green/Violet (Pin 6)
ECU Power		Green (Pin 8)

[jaredmac11]

Tom, Im going through this post before I execute on the wiring and I think we have bad info.
I dont think EC Pin #6 should go to the Fuel pump. In the diagram that seems to go to the Fuel pump RElay. Pin 3 seems to go to/from the fuel pumps. Am I missing something? I think I should use Pin 3 instead of Pin 6.
Pin 3 EC - Fuel Pump.JPG


John, I did my own wire mapping for the E36 when I thought I was going that route. It was the M42 so the wire colors are different, but the pins seem to match up to the components between our descriptions.

A few notes,
1. Ditch the association between O2 sensor heater and #6 pin. I think thats just bad info. In my diagrams for the E21, pin 6 is to the Fuel pump relay. In the E36 it is the O2 sensor heater but I think we can skip that.
2. In my 78 and 83 diagrams, I have pin 8 connecting to the ECU power, like you have in yours.
3. Im assuming engine speed you mean tach?

Otherwise everything looks good from my end, but Im the one asking for help here so take that for what it is lol

[Tom D]

not bad info, your confusing yourself by using a third harness.

your car is a 2.0l and pin 6 is the fuel pump, that was my directive. if you say pin 3 is the pump on the M10B1.8 engine harness then i believe you, but your car is not wired as a facelifted car.

[jaredmac11]

Ugh youre right. Im overthinking it.