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Monday, June 11, 2007

1995 GM 4.3 V-6 Ignition Troubleshooting

Overall this truck has been very reliable, and probably my favorite 4WD work truck ever. It gets a lot of duty hauling heaping loads of firewood across rock creek crossings, etc. and it got hammered around in a 16 inch snowfall last winter. For a fullsize 4WD it’s easy on gas, and I’ve never had it anyplace I couldn’t drive it out of. But ignition problems with this 1995 Chevy K-1500 had been developing for a long time. They were more in the category of ‘annoyances’ than failures. It had suffered for power under loads, and would sometimes miss badly if you sat in a drive-thru too long on a hot day.

Finally the problem degraded into the ‘failures’ category, and it left me sitting alongside the road twice, in 60 days. The first time, a new coil seemed to fix it and the truck ran better than ever for a few weeks- and I thought the earlier problems were solved. Then it started missing again, and finally died and just wouldn’t start at all. Fuel supply problems were eliminated as the cause, and we soon found that it wasn’t firing. Having once replaced THREE alternators in 30 days (thanks, O’Reilly) I suspected the new coil (also from O’Reilly) was a dud. I replaced that, and the truck started and made it about a half-mile before croaking again. It soon became apparent that a new coil was not going to fix it.

Anytime I buy or trade for a used vehicle, the next purchase is a Haynes Repair Manual for that model. These run about $15.00-$18.00 and are the best money you’ll ever spend on a vehicle. Their “Chevrolet & GMC Pickups ’88 thru ‘00”. #24065 saved me considerable money on tow & repair shop bills, in this instance alone- probably 20 times the price of the book.

Once we got it home I started sorting out the ignition circuit.

· The ignition switch was supplying its side of the coil with juice.
· The coil was supplying juice to the distributor module.

The coil itself refused to fire, and I suspected that this was due to a faulty pickup coil, located in the base of the distributor. The distributor module was also suspect, and this one had obviously been in there awhile. Luckily, 1995 was the last year that 4.3’s were produced without a Crank Position Sensor in the timing cover; so that was one less possible cause.

I called a friend who is a REAL mechanic and he suggested that I check all connections and fuses, and if those were to start checking components. I usually buy my parts from AutoZone and while pricing the module, the salesman mentioned that they could test modules and suggested that I bring mine in. I took him up on that, and the module checked fine. When I re-installed it I went all over the wiring harnesses and checked connections- but still no fire. I was getting irritated since the truck had been down for several days, and we needed it running ASAP.

We also needed it ‘reliable’ again. I figured while it was torn down this far, I might as well just replace the distributor pickup coil- since the module had checked OK. Now I’m not a mechanic by any means but I wasn’t towing the truck to a shop for this… so I cracked the Haynes manual and went to work. Replacing the pickup coil requires removing and tearing down the distributor- and the engine needs to be on ‘top dead center’ on the #1 cylinder, before you start.

You begin by removing the distributor cap & making a mark on the plate under the terminal that supplies the #1 cylinder. I got my mark off a tad, but took a photo so I’d have a reference point for the re-install.

When the timing mark on the harmonic balance lines up with the timing notch, and the rotor is pointing at the #1 plug position- you are there. With my wife on the starter, we were able get close by bumping the starter. I turned the engine on into TDC/#1 a socket on the main crank nut.

The next order of business was to mark the Distributor Base in relationship to the block. It is critically important to get the distributor back in, in exactly the position it came out. Since I’m positive that I AM NOT A MECHANIC I decided to make a couple of marks, so I’d have n extra chance to screw it up;)

With that accomplished, I removed the Distributor hold-down bolt & clamp. The Distributor lifted easily from the block and I washed the gear end in gasoline, to remove the grease.

Replacing the Distributor Pickup Coil requires tearing down the distributor shaft- and great care must be taken to insure that it goes back together, exactly the same way it came apart. About the worst thing you can do is to install the distributor gear 180 degrees off from it’s original position, which will often create a ‘no start’ condition- and damage the engine in the event that it does start. To avoid this, you mark one side of the end of the Distributor Shaft in relation to the Helical Gear.

The gears is held to the shaft with a roll pin, and before driving it out you also want to mark the relationship of the Distributor Tube, to any interlocking washers or springs present.

Once the roll pin is driven out, the Distributor Shaft may be lifted free of the Helical Gear, and subsequently the Distributor Body. This exposes the Reluctor Ring Cover and the Retaining Clip that holds it in place. Once these are removed the Distributor Pickup Coil is exposed, and this one was shot! The outer insulation looked like mice had chewed on it, the coil itself was corroded and bare, and the lead wires had actually gotten hot enough to scorch into the Reluctor Ring’s base. It was a safe bet that this was the problem…

The Distributor Shaft was grungy with varnish, but a good dose of Hoppe’s No. 9 cured that, followed by a light coat of Hoppe’s Gun Oil. Hoppe’s No. 9 also removed the scorched copper (result of the short) from the bottom of the Reluctor Ring, and corrosion was stoned from the contact points. Ol’ Hoppe’s did a fine job and the Reluctor looked nearly new after the cleanup. A purty, new eleven-buck Distributor Pickup Coil was set in place, and the Reluctor Ring Cover was fastened down with a new retaining clip. The Distributor Shaft slid easily into the Distributor Body, and due diligence was applied in making POSITIVE that the washers and Helical Gear were in EXACTLY their original relationship. The Helical Gear was then pinned back onto the Shaft, and a new gasket was applied to the base of the Distributor-which was then very carefully lowered into the block. Allowing for a 15 degree offset eased the Helical Gear into place, and the Rotor indexed into it’s original position above the #1 TDC ‘witness mark’ I had made on the plate. The Distributor was then turned until the witness marks on the base and block lined up- and the clamp and hold-down bolt reinstalled. The Distributor Cap, plug wires, and leads to the Distributor Module were reconnected; and all the unrelated air cleaner components.

Life was good! All the major engine ignition components had been checked or replaced, and the dang engine just HAD to run now, right? Right?

WRONG. Engine would crank, but still no fire. I am getting frustrated as hell about now and the truck needed to be running ‘last week’. So I went back to square one, re-checked the wiring, juice to the Ignition Coil, etc. The circuit checked OK. Then it occurred to me that when the Distributor Coil shorted out for the last time, it may have killed off the Distributor Module. So we made one more run for parts, and installing the Module cured it.

The ignition system is essentially rebuilt, and ‘Knock on Wood’- this truck is running fine now. In fact, it’s running better than ever and the mileage has even improved a bit.

The Moral of this story? NEVER trust Auto Parts Store Test Equipment. If by process of elimination you suspect an electrical part is bad- REPLACE it. You’ll save time and money in wasted trips to the Parts Store… where they’ll also tell you that you need to replace your Crank Position Sensor, even if your truck don’t have one.


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