Tip: Be wary of trusting old wiring harnesses

#1

Over the past couple weeks I’ve done a couple surgical efforts on the engine’s wiring harness. To my surprise I found that all the wires are black with corrosion under the insulation. That is to say, when you peel away the insulation on a wire, instead of seeing shiney copper, you see blackened wire. A minute with a dremel and a small wire wheel takes off enough corrosion that you can put on a connector and expect a decent connection.

This makes me wonder if I can really trust any old connection in the wiring harness. Tarnish is probably creeping into the contact surfaces between wire and connector in every crimp on connection in the harness.

#2

If properly executed, the crimps should be gas tight. So corrosion between the connector and wire should not be a problem. But yeah a 20 year old harness can be an iffy thing.

A specter from my past has come back to haunt me. I’m getting occasional cut outs (with tach drop) again. So far they aren’t as bas as before, but are just as annoying. While at Barber last weekend I changed all of the easy to change parts (main relay, CPS, DME) and checked all the power and ground connections. I have my IQ3 powered by the the small wire from the battery that feeds the engine management system. The IQ3 logs system voltage, so I know it isn’t a bad kill switch.

Based on past experience my suspicion is that the wiring harness is the next suspect. My theory is that the coax in the harness that routes the CPS signal from the connector to the DME breaks down with time & heat. I may try bypassing that part of the harness with small coax and see if that helps.

#3

When OPM was building my car I pulled the trigger on buying a new engine wiring harness ($508 from Maximillian for p/n 12511724422 for the small bumper coupes I believe). My rationalization was that my breakeven was 7 labor hours to have them chase bad 20 year old connection problems.

Then we turned around and used the relays that came on the donor and chased a high rpm/WOT miss that first weekend…:blink:

#4

[quote=“Steve D” post=57336]When OPM was building my car I pulled the trigger on buying a new engine wiring harness ($508 from Maximillian for p/n 12511724422 for the small bumper coupes I believe). My rationalization was that my breakeven was 7 labor hours to have them chase bad 20 year old connection problems.

Then we turned around and used the relays that came on the donor and chased a high rpm/WOT miss that first weekend…:blink:[/quote]
The high rpm miss was caused by the relays? I’d not considered that. Hmm.

#5

Relays (main & fuel) are plausible. A clue as to where to look, can be from what the tach does. Or if the car has a working econometer it can provide a clue. A drop in the tach indicates a loss of timing data to the DME, a drop in power to the DME, or a problem within the DME itself. If the tach is normal but the econometer drives towards zero there is a fuel system problem.

#6

The high rpm miss was caused by the relays? I’d not considered that. Hmm.[/quote]Don’t put too much weight on that diagnosis. I am almost certain that is how the problem was solved but it was a new car, I was running the Miata that weekend as well, we threw a lot of parts at the problem and I don’t know sh!t about cars. I was so far in the weeds that weekend that my memory of Sept 2008 is weak…

#7

yeah i’ve heard that pot affects memory

#8

My car was cutting out, and I believe both the econ and tach were dropping. The problem was my brand new ignition coil.

#9

A bad coil won’t cause tach and economter drops.