rpm issue? electronic?

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Dan201

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I was doing a tune up on my darts today and the first went perfect set the idle at 850 rpm and turned it back to 750 using the fuel mixture screw, set timing etc. Then I went to my other car and hooked up the tach it says the idle is only at about 200 rpm which I know from the sounds of it, it is right near the 750 800 rpm mark I tried change the ballast with a spare and the control module to no avail should I go with the coil as well? I am using old parts to swap in so no gurantees on any of them either. I want to set the mixture and timing but without getting the rpm reading right it'll be more by ear and I also worry I might not be getting enough voltage or amperage for a proper spark with the rpm reading that far off on the coil. Any ideas on where to start? Wiring? coil? This car is a DD so realiability is important once worse it is my wifes DD and it's winter and I have kids and she doesn't know a thing about cars so a break down is not an option. Thanks for any ideas
 
Checked the resistance on the distributor and it is good also checked it on the coil and it was off by 0.1 so just in case I swapped it with one right in the middle of spec but still no difference at high idle I am reading about 650 rpm and at low about 200 rpm which is way off. any ideas? If I wasn't worried about the drivability issues this might imply I wouldn't worry about it but low amperage or voltage depending on what a tach reads most likely means there is a problem somewhere.
 
It's just a hand one for checking rpms and dwell I have 2 different ones and both are saying the same thing one car reads fine and one reads 200 rpm vs on the other car 750rpm engine sound to be very close by ear I am going to start checking with a meter all the way back to source I think one goes to the ballast and one to the ecu if I recall correctly
 
How exactly is the tach connected? Coil negative terminal, inductive clamp around a plug wire, pull a plug out and put a spring on it?
 
Two terminals one to the positive of the coil and one to the negative that is the way it says to hook it up in the book if you have points then one goes to the positive one to ground but I have electronic ignition as it is a 74 and the meter works fine on the one car just not the other.
 
I would connect the + lead of the tach to the negative of the coil and the other to a solid ground. Don't know why the instructions would tell you otherwise since the electronic ignition is no different than points other than it uses an electronic switch to open and close the coil path to ground insteand of mechanical points. I have never seen any tach that hooked up any other way on an inductive igntion (points or electronic) + of tach on negative of coil and other lead on solid ground.

I would guess that you are right on the edge of getting a big enough pulse to tigger the tach with it attached across the coil and that would account for one car working OK and the other not.

Also, do you have the coil wired correctly? The wire from the ballast resistor goes to the + side of the coil.
 
I have gone through the wiring since some ends have been changed and ensured that both cars are wired exactly the same I ran some test with a meter yesterday and the car that doesn't work properly does have a lower amperage and voltage I did a complete tuneup by ear and have it running better now (car with problems has been in a major accident since it last ran well large enough to brake the front frame, wreck fenders inner and outer etc. engine did well with it just hitting the firewall and the blades hitting the rad) I will try hooking up the tach differently and see what readings I get. Thanks for the input.
 
This one may be powered through the positive terminal of the coil. If this is the case, try powering it through the positive terminal of the battery instead. The ballast resistor may be dropping the voltage too far.
 
I tried using any combination of hookup with the tach still reading 400 rpm even tried going straight from the ballast and battery still 400 rpm will continue experimenting thanks for the advice. Right now I have electrical on my hardtop and steering on my sedan going to be a busy week. Car also has a miss have tried changing out everything and have done compression tests.
 
Dan201 said:
I tried using any combination of hookup with the tach still reading 400 rpm even tried going straight from the ballast and battery still 400 rpm will continue experimenting thanks for the advice. Right now I have electrical on my hardtop and steering on my sedan going to be a busy week. Car also has a miss have tried changing out everything and have done compression tests.
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Have you tried swapping (complete swap of) the alternator, voltage regulator and battery from the good running car to the poor running car? See if symptoms follow.

Also, read this: http://www.fixya.com/cars/t2612175-following_rear_end_accident_damage

With that type of hit in your accident the cells in the battery may be damaged or perhaps there was a short created somewhere in the wiring.

While you are doing these tests make absolutely sure you have a good fire extinguisher nearby -- and , hate to panic your wife, but she should be carrying one around in the car. You also might want to consider doing the tests outside of your garage in case of fire.
 
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