This kind of a long story, so apologies in advance.

Last week I put an aftermarket Tach in my truck (Equus from O'Reilly's). I connected the signal wire by cutting the green/yel wire near the coil connector and hooked it all together with a crimped butt connector. I took it for a test drive of about 15 miles and everything was great.

Saturday came and I was going to drive the truck up to visit my Mom (about 45 miles north) but I only made it about 1/2 mile when I heard it backfire through the exhaust, it surged a bit then died eventually. I got it started again, then opened the hood. When i jiggled my tach wire it died again, so I thought Aha! lousy crimp job. I got it back home ok.

I then bought a new coil and new a coil connector. When I was installing the new connector I found that the red wire had a couple inches bare down near where it was all taped together at the factory. So I cut it back to there and I soldered all the connections, covering them with the hot glue filled heat shrink tubing. I put the new coil in, hooked it up and it still ran like crap.

So I figured the module was failing, either coincidentally or because of the bad wiring. I got a new module from NAPA yesterday, which I was sad to see was made in China. I put it in today and it did not solve the problem either.

So I started looking deeper and found some more bare wire in the harness near the module. I covered it with tape. Still ran poorly. Then I remembered I had tried to disconnect the harness from the distributor, since I wanted to check/replace the O ring. Sure enough those 3 wires were all now bare right by the harness side connector. I wrapped them in tape and taped them all together to hopefully keep it from coming apart again. It now runs normally.

So I've told you all this so you know what I've already done etc. And my question is, is there anyone who sells a new ignition wiring harness? The one on there seems to have a lot of rotten wires. I could try hitting the junk yards but would I find one any better? It might be worth a look but I'd feel better with a new one.

Thanks.

 

Not to my knowledge; liquid electrical tape may help.

 

I don't know of a replacement other than from a salvage, either.

 

It looks like Painless makes one that might work, I'd have to call them to be sure. It's not cheap but might be worth the peace of mind.

If i go junk yard route, are these all the same across engines, or do I have to pull one off a 6 cyl?

 

I need to write this up on my web site as I answered this question only a couple of days ago.

Anyway, electrically the wiring harnesses are the same for all gasoline engines - with one exception. There's a ground in the 8 cylinder harnesses that tells the tach it should show 1 revolution for every 4 pulses. Without the ground it shows 1 revolution for every 3 pulses, meaning for a six cylinder engine.


However, each engine family has the sending units, like oil pressure and coolant temp, as well as the distributor in differing places. For instance, on a 351W the oil pressure sender is by the fuel pump, but on a 351M it is behind the intake manifold on top. So, each engine family has a different wiring harness as the wires need to be different lengths.

Given that, any wiring harness can be made to work, but you will probably have to change the length of the wires to the two sending units as well as add or subtract the ground if you have a tach.

 

Actually, I have written this up on my web site in Engine Swaps here: Engine Swaps - Gary's Garagemahal. It says:

 

We shall make Gary's site be required reading and a test needing to be passed before you're allowed to post here.

 

That would sure cut down on the amount of posts - in several ways.

 

No kidding �� I have read a lot of Gary's site but hadn't run across that paragraph.

On my 300 the OP sender is aft of the distributor and oil filter, and the CT sender is on the opposite side of the block.

I'll probably trawl the junkyards on Friday, and maybe give Painless a call too. At least Phoenix is blessed with a bazillion junkyards!

 

Even I didn't remember it was there, and I wrote it. But, somewhere in the dim recesses of my mind I seemed to recall something about that, so went looking. Fortunately Chris had suggested a search function and there it was.

Anyway, from what I've seen the Painless harness is not very long. Maybe they can tell you what engine it fits correctly and you/we can figure out what other engines it might fit w/o modification.

As for junk yards, be on the lookout for lots of things for these trucks, like air cleaners, instrument clusters with tachs, factory fog lights, factory bull bars, and the icing on the cake - the elusive underhood tool box.

 

Don't forget that you can always make a new one yourself.
You can either use some or all of the special connectors from your old harness, or, get the new connectors.
If you are having any trouble finding them the part numbers will be in the old posts.
Plus some helpful how to's!!

 

JayFinAZ, check your PM box.

 

Thanks for the tipoff. I tried a couple bids on it but his reserve was more than I wanted to pay for one that had been cut up.

I called Painless today and they told me that the 30812 harness was the "factory length" so we'll see. I ordered one with a coupon from autozone which save me a few bucks.

 

Please let us know how it fits. Actually, how about measuring the wires to the two senders from the plug on the fender liner?

 

I'll take some measurements later this week. I did roughly measure out the module to distributor run at about 30 inches. So the OP sender wire would be that plus about 6 inches plus the distance from where it all is wrapped together near the module to the firewall. The coolant sender will be at least 18 inches longer I think. But I'll measure it all when I get it out. I know I need to repair or replace the OP wire, it has the insulation coming off too.

I think a contributor the wire decay was the oil leak, which I now believe to have been from the OP sender being loose. But I also have bare wires were there was no oil, so maybe not. Once I get the wires out of the way I'll get it all cleaned up before I put the new harness in.