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I am going to re-check the compression on my 1986 5.0L EFI truck. The last time I checked it in 2005, I made no provision to protect the ignition system during the testing and suffered no ill effects for failing to do so. I have absorbed a lot of information since then. I would like to perform the test with the ignition system protected this time. What is the standard procedure for insuring no electronic damage?
Sounds like it's been a while, but I might be inclined instead to measure manifold vacuum with a mechanic's vacuum gauge. This tool doesn't measure compression directly but if the engine pulls a steady 18" to 20" at idle and passes other checks one can be certain cylinder compression is acceptable. It's just a whole lot less hassle to setup and doesn't torture the battery and starter. As far as disabling the ignition though, disconnect the primary so the coil &c isn't energized.
Edit: use a remote starter switch! Nothing is energized except the starter, and it's the only way to do a compression test without a helper, basically.
As far as disabling the ignition though, disconnect the primary so the coil &c isn't energized.
Agreed, this is probably best. Does your truck's coil have the horseshoe connector? If so, that's easy to disconnect.
The other method I was taught is to disconnect and ground the center lead from the distributor. I'm talking about the hi-voltage lead directly from the coil. This is an easier method for vehicles with poor access to the coil, or if the low voltage connection at the coil is hard to disconnect. For example, I have a 1948 ****** CJ-2A and the coil is under the dash, not under the hood, with a long hi-voltage lead through the firewall and to the distributor. The low voltage terminals on the coil are secured with nuts and little lock washers, not slip-on connectors. You've got to be quite the contortionist to reach the coil, and then you're working by Braille.
You might think grounding the coil output would cause harm to the coil, but it's perfectly safe. The way a coil gets damaged is if the output has no way to reach ground. In that case, the voltage inside the coil keeps building and building, until usually the coil's internal insulation fails, creating a new and undesired ground path. Under normal circumstances, with everything connected, the voltage only builds up high enough to jump to ground via the spark plugs, well within the capabilities of the coil. But when disconnected, there's nothing to stop the voltage buildup until something fails. However, with the output grounded, the voltage never builds up in the first place and all is good.
I am going to re-check the compression on my 1986 5.0L EFI truck. The last time I checked it in 2005, I made no provision to protect the ignition system during the testing and suffered no ill effects for failing to do so. I have absorbed a lot of information since then. I would like to perform the test with the ignition system protected this time. What is the standard procedure for insuring no electronic damage?
Just disconnect the coil. It is as simple and easy as that.. Alternatively pull the coil wire off the dist center terminal and stick on the nearest bolt or stud. Either way will do the job.
The compression test results are in. I pulled my old notes and see that the earlier test was in 2001, thirty-two thousand miles ago. The current notes show an extra "wet" reading for a few selected cylinders.
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