Electronics Gone Wild - HELP!

[Basketcase]

I worked in a IH truck dealership parts department for 9 years, plus the 30+ years of messing with cars. one of the oldtimers at the dealership told me "just because a part is new, doesn't mean it's good" that's especially true now with the foreign made stuff.we handled Delco Remy starters and alternators.many a time we would go thru a hole stack to find one good one. and try dealing with the POed trucker that's gotten several bad ones in a row....
might be your tach, might not. but from what you're saying...the only thing not right is the tach. I'd change it or unhook it and go from there.

 

[Ranger]

Oh, don't get me wrong. I understand perfectly well (and preach it myself) that just because it is new is no guarantee that it's not the problem. I just can't bring myself to arbitrarily spend another $100 on a new tach without proving that the present on has failed, though your theory does make sense. The one thing that worries me is that when the ECU failed on me the first time, the tach was acting the same way. If you recall, it was the voltage regulator that was bad and caused the ECU to eventually fail (after replacing the regulator). I just have this nagging feeling that the tach (though I don't fully understand how they operate) is responding to inputs it is receiving. I guess I'm going to have to figure out a way to swap in another tach to test your theory. Don't know of any way to test the ECU.

P.S.
I also have people telling me to use an alternator with a built in regulator and do away with the new one on the firewall and bypass the "fire prone" ammeter system.

 

[Basketcase]

or throw the tach on another car to test it?

 

[Ranger]

That'll be last resort as I really don't want to dismount it and pull all the wiring.

 

[ACME A12]

That'll be last resort as I really don't want to dismount it and pull all the wiring.

You could always cut the wiring at a convenient but hidden location and install male/female insulated connectors. Then tach removal/installation becomes a very quick job - for both this exercise and in the future.

 

[Ranger]

I had a thought and wonder if anyone thinks this is a possibility or not. The voltage regulator, ECU and tach are new. The only thing not replaced is the coil. The coil ohm'd out OK, but considering that the tach gets its signal from the negative terminal of the coil, is it possible for a coil to be bad and cause the tach to jump to the positive side (from 2000 RPM to 6000 RPM), basically over boosting the signal to the tach without causing a misfire? I would expect a misfire to cause the tach to jump in the opposite direction (2000 to 0). Am I making any sense or is this completely illogical?

 

[moparmonk]

Great thread and great responses. Does your tach have a good ground or share a ground with the coil/engine ground? Make sure the engine ground strap on the left head, is tight, clean and bolted to the firewall and not corroded or stripped. Remember, the battery ground hooks to the engine which is mounted on rubber mounts to the car. (rubber don't conduct worth a damn) The only ground the body (IE dash, gauges, lights, regulator, radio etc) share with the battery is through said ground strap. I have seen cars run without them as they can find ground through the mounts and back feeds but often times you end up with weird, temperature and time related scenario's....such as this. Hope you nail it!

 

[Ranger]

I have already checked and cleaned the firewall ground wire.

In doing some online research I have come up with another theory that I will check out later today and tomorrow when I take it for a ride to use a friends lift to change a motor mount. That theory is EMI. I find that someone ran the #8 plug wire over the top of the valve cover along side the fuel line, and that is where the tach signal wire is running. I'm going to reroute it later and see if that solves the problem. If not, then the coil is the next suspect. Will update later today or tomorrow.

 

[69hemibeep]

If you keep the rubber mount wet it will conduct :stupid::basketcase:

 

[moparmonk]

If you keep the rubber mount wet it will conduct :stupid::basketcase:

LOL! That is why so many 318's never had the problem. The valve covers and oil pan side rails leaked so bad, the motor mounts were always covered!

 

[Ranger]

Was not able to reroute it, but I did remove it from the bundle that was wire tied together and strapped to the fuel line. The plug wire now lies below the bundle along the valve cover flange. Should be far enough away. Test ride tomorrow morning. Fingers crossed.

 

[peresvet24]

the first hearing of the voltage 17 volts:huh:, I was sure that there are only 12

 

[Ranger]

Not sure what an unregulated alternator is capable of putting out. All I can say is that the voltage gauge was pegged at 16v, but that is ancient history and has been corrected.

 

[Ranger]

Well, that was no luck. I'm strongly suspecting the regulator, but the ECU could be a possibility as well. Wish I new how to isolate one or the other.

 

[69hemibeep]

Is this all over a tach that might be bad?

 

[Basketcase]

finally.....

 

[Ranger]

No, I believe the tach is operating properly and responding to the signal it is getting.

 

[moparmonk]

Measure the voltage right on the 12 volt line at the tach with a known reliable voltmeter. (not the dash ammeter) What is it? It should not be any higher than 14 with the engine running and slightly reved up. Is your regulator good and tight on the firewall? The 69 and back regulators, even the new style has to shed the extra load to ground through the case and into the bolts of the firewall and through the ground strap wire and back to the negative of the battery.

 

[Ranger]

I'll check both voltage and ground.

 

[69hemibeep]

I have a an old dwell meter with a tach in it I would connect just for a test. It takes 15 seconds to do, any one around you have one?