How tell an early harness from a late harness?

#1

My perception is that early and late harnesses aren’t entirely compatible. I’m going to swap the harness out of my early model car. I have one spare harness that I think is early, one I think is late, and one I’m not sure. I thought that I would be able to tell them apart from the shape of the O2 sensor connector, but all 3 connectors are the same shape as the one in the car. They all have the round 4 wire connector.

So how do you conclusively tell an early model harness from a late model harness?

#2

There are two external differences between early and late harnesses.The oxygen sensor connector is different and the early harness has a white three pin flat connector near the DME connector. Either harness can be made to work on an early (up to 8/97) car. You can make an early harness work on a late mode car, but doing so is more work than the other way.

#3

What do the two types of O2 sensor connector look like?

#4

Earily harness=individual injector leads

Late harness=injector leads in a reconnectable/removeable plastic tray

This is one way to tell them apart. Now, are there variations between/within these two obvious harnesses? I don’t know.

RP

#5

[quote=“Patton” post=59019]Earily harness=individual injector leads
[/quote]
I don’t think I’ve ever seen this. Maybe the “early harness” is really early like 84-85?

#6

If you are going through the trouble to swap the harness, why not start with a new known good part? Maximillian has them for $500. It seems like the connectors on these 20-25 year old harnesses are the weakest link and the part that gets the most wear and tear during a motor swap. Or 9. :wink:

#7

[quote=“Ranger” post=59022][quote=“Patton” post=59019]Earily harness=individual injector leads
[/quote]
I don’t think I’ve ever seen this. Maybe the “early harness” is really early like 84-85?[/quote]
Good catch on Robert’s part. I forgot to mention the injector arrangement. The early harness is for car’s built from 9/86-8/87. If the harness on the car has the plastic injector rail it will also have a C191 and will be a late harness.

#8

If I could afford it, I would. I can’t throw $500 at a “maybe” fix.

After I put one of the spare harnesses in, I’m going to see if I can’t hardwire the master relay. I’ve had some good indications that the master relay or the connections to the master relay are one of the possible causes of high rpm miss.

Of course the only way to test a solution is to take the car to a dyno or take it to another event and hope for the best. Sure would be nice to have diagnostic logs. I’d throw $500 at a “sure” fix.

#9

While I was trying to find drawings of engine harnesses so I could see what the connectors looked like, I came up with the idea of looking at fuel injector harnesses.

http://parts.bmwofsouthatlanta.com/showAssembly.aspx?ukey_product=1286641&ukey_assembly=251855

I think that there might be 3 different kinds of fuel injector harnesses and engine harnesses.
-9/87
9/87-5/88
5/88 or 12/88- depending on source of info

Ugh.

#10

Ranger, did you test the TPS? I had this same issue at the June RA event and with Yoda Jim’s help, we traced it to a failed TPS. Of course, disregard this if you’re bored. :laugh:

#11

Yes, after we talked at RA I tested the TPS. I have the TPS pin marked on the harness connector at the DME so I can test the TPS very efficiently. I unfasten the harness from the DME, then sitting in the driver’s seat I run the probes from marked harness pin to ground and floor the throttle.

#12

Is not the injector harness a separate thing? I think I have used both on my car at one point. I have an '88 with build date of 8/87 and it has the square injector harness as that is what I liked best. I remember it being separate and I would say interchangeable. I also used the fuel rail where the fuel goes in the back and out the front. They make one that goes in and out the front. The square injector harness had to be trimmed a little to fit the rear fuel connection. I am pretty sure I used the engine harness that came with the car but not sure. I have an '87 and '90 parts cars so who knows what I used where.

#13

i helped ranger test the tps at mid ohio and we both agreed that it looked good. so that’s probably the culprit.

#14

[quote=“Ranger” post=59026]After I put one of the spare harnesses in, I’m going to see if I can’t hardwire the master relay. I’ve had some good indications that the master relay or the connections to the master relay are one of the possible causes of high rpm miss.[/quote]Why would you introduce another unknown (the new harness) THEN test the master relay fix?:huh:

I really do sympathize with the plight of chasing an intermittent problem but I firmly believe it is false economy to swap used parts for used parts. Especially electrical connectors.

I’m also having a hard time understanding how a wiring issue could cause the miss only at certain RPMs. Is the miss present only when the engine is under load or does it happen revving in neutral?

#15

[quote=“Steve D” post=59058][quote=“Ranger” post=59026]After I put one of the spare harnesses in, I’m going to see if I can’t hardwire the master relay. I’ve had some good indications that the master relay or the connections to the master relay are one of the possible causes of high rpm miss.[/quote]Why would you introduce another unknown (the new harness) THEN test the master relay fix?:huh:

I really do sympathize with the plight of chasing an intermittent problem but I firmly believe it is false economy to swap used parts for used parts. Especially electrical connectors.

I’m also having a hard time understanding how a wiring issue could cause the miss only at certain RPMs. Is the miss present only when the engine is under load or does it happen revving in neutral?[/quote]

Replacing the harness because I’ve replaced everything else and tested everything to death.

Re. wiring issue and certain rpms. I agree that it’s hard to understand. But I saw this happen with the loose alternator (which is fundamentally a wiring issue), I saw this happen when it was fixed by reseating the master relay, and there was one other time of which the precise details escape me. Each time the symptoms were the same…missing under both WOT and the specific rpm range.

Jim Levie fought the same sort of thing for over a year. IIRC correctly it was fixed by swapping out the harness. The combo of those two histories, and the fact that it’s about the only thing I’ve not yet replaced, makes it a logical next item for replacement. Such is flying blind w/o diagnostic logs.

But that doesn’t mean that I’m going to go $500 further in debt because “maybe” it’s the harness. If I had to choose between $500 for the harness and $900 for the Megasquirt’s logs, I’ll get the MS and forever more have diagnostic logs with a simple swap of DMEs.

Re. only under load. I don’t know. I won’t run the engine up that high in neutral. I’m told that it’s hard on engines and I wouldn’t know what to do with the results of the test.

Thankfully I have the fuel pressure gauge on my dash so I can at least rule out some of the possible causes.

#16

[quote=“Steve D” post=59058]I really do sympathize with the plight of chasing an intermittent problem but I firmly believe it is false economy to swap used parts for used parts. Especially electrical connectors.

I’m also having a hard time understanding how a wiring issue could cause the miss only at certain RPMs. Is the miss present only when the engine is under load or does it happen revving in neutral?[/quote]
I have seen this sort of problem caused by:

An engine harness
An injector harness
A CPS
A coil
A TPS
A car body harness
A fusible link
A kill switch
A DME
A bad ground
A main relay
A fuel pump relay
A harmonic balancer

And even combinations of those!

New parts sounds great (except for the cost), but I have seen one case where the engine harness was bad and a brand new harness didn’t fix it because it turned out that the new harness was also bad. Without logs or being able to see live data when an event is occurring you are reduced to throwing parts at the problem while hoping for a favorable outcome.

#17

[quote=“Ranger” post=59059]Re. only under load. I don’t know. I won’t run the engine up that high in neutral. I’m told that it’s hard on engines and I wouldn’t know what to do with the results of the test.[/quote]Really? I ain’t no engineer, but I can’t understand how revving in neutral (i.e. load provided by moving parts in the engine) versus revving in gear (i.e. load provided by moving parts in and connected to the engine) could possibly be any different. I’d be interested in that answer though.

#18

I’ll defend Scott here.
Chased a high rpm/high load problem for several months.
Two, nonrelaed problems: one was a loose vibration damper.
The other was a replacement wiring harness (along with 6 injectors), so I don’t know the real fix with problem two.

RP

#19

Maybe I stated my answer poorly.

I certainly understand the idea of juggling the variables and retesting in order to get a feel for the boundaries of a problem. If you can predicatably manipulate behavior, you are well on your way to understanding the causation. I already know that the behavior only occurs at WOT AND high rpm. If I attempted the test with no load, no matter what the results are, I’d end up sitting in the driver’s seat thinking to myself…“ok, what does this tell me”?

WOT means internal DME maps. High rpm means peak demand on fuel and ignition system…so I kinda understand the confluence of WOT and high rpm. But what I don’t really understand is the unique-isms of high RPM, WOT and no load. Or maybe just high rpm and no load.

I’m still not sure I said that well. Ultimately after any test you have to sit back and ask yourself, “ok, what can I infer from the results of that test”.