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Aftermarket Tachometer


Tanken

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Does anybody have information on wiring these up? i have everything connected but the rpms just jump around and is definitely not accurate. is there something i'm missing? my signal wire is on the negative terminal of the coil closest to the fender.

 

'85 720 z24

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It should work on both. Pull the lead for that coil off the distributor cap, stick an old spark plug in the end and lay or hold against a grounded surface. Have someone turn the engine over with the key. Got spark?

 

If by chance there is no spark, change the first fuse on the left end of the fuse box.  Still no spark?

 

Turn ignition on and touch the negative terminal of the coil with a jumper wire to ground. You should get a spark every time the wire is removed. Got spark?

 

YES there is spark. The distributor module is bad.

NO there is no spark. Check that the + coil terminal has 12v. If it does the coil is bad.

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1 hour ago, datzenmike said:

It should work on both. Pull the lead for that coil off the distributor cap, stick an old spark plug in the end and lay or hold against a grounded surface. Have someone turn the engine over with the key. Got spark?

 

If by chance there is no spark, change the first fuse on the left end of the fuse box.  Still no spark?

 

Turn ignition on and touch the negative terminal of the coil with a jumper wire to ground. You should get a spark every time the wire is removed. Got spark?

 

YES there is spark. The distributor module is bad.

NO there is no spark. Check that the + coil terminal has 12v. If it does the coil is bad.

will do. i'm powering my tach with a fuse tap from the ignition coil fuse. could that affect the way the tach operates? should i be running the power wire to somewhere else?

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5 hours ago, bottomwatcher said:

More like where is your ground running too? If not direct to the battery try a jumper wire directly to the negative battery terminal. For me 90% of 12v issues can be connected to ground issues.

mine is under the dash. i also have my stereo grounded around there. i will definitely try grounding to the battery.

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13 hours ago, Tanken said:

will do. i'm powering my tach with a fuse tap from the ignition coil fuse. could that affect the way the tach operates? should i be running the power wire to somewhere else?

 

Definitely confirm that the second coil is firing. This is important as the correct timing is based on both firing and if one is not the engine will act like the timing is retarded.

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Some tachs require a "tach adapter", but those are mostly on modern engines with EFI. Another issue may be tach voltage. Some tachs require a "jump up resistor" to give the tach more power. I have to do this mod when wiring in my Speedhut tachs in the LS swaps.

 

https://www.holley.com/products/gauges_and_gauge_accessories/tach_adapters/

https://speedhut.com/content/instructions/2_and_2_58_tach.pdf

 

image.png.e83cedfb019d50d1549b4d299022738b.png

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20 minutes ago, datzenmike said:

 

Definitely confirm that the second coil is firing. This is important as the correct timing is based on both firing and if one is not the engine will act like the timing is retarded.

just tested both coils. strong spark 

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Just now, datzenmike said:

Dealer's choice as both coils fire exactly the same.

oh gotcha. i thought i read somewhere one of them cuts out at a certain range. it seems to be working i just went for a short drive. although im not sure if its accurate. it seems like the rpm are very low. i was opening up the throttle but i never went above 3000 because it almost seemed like it was sounding too high. i've never put a tach on these trucks so do they just have a low red-line? 

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53 minutes ago, Tanken said:

oh gotcha. i thought i read somewhere one of them cuts out at a certain range. it seems to be working i just went for a short drive. although im not sure if its accurate. it seems like the rpm are very low. i was opening up the throttle but i never went above 3000 because it almost seemed like it was sounding too high. i've never put a tach on these trucks so do they just have a low red-line? 

 

Im not 100% sure on that set up,

but the double plug set up on the CA20e the exhaust side cuts off after 3k rpm.

 

Either way run if off the coil running your normal plugs.

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Didn't know about the CA engines and there is a lot of misinformation about the dual plugs on the 720. However both are NAPS engines and the 720, at least, fire the plugs together at all times. There is a function that shuts off the exhaust side under very heavy load to reduce engine noise. Low intake vacuum switch. The distributor automatically brings the timing more advanced to compensate for the longer burn time.

 

Could be that the CA is the same and misinformation and lore has got around till it's now believed as true. The whole point of the dual plug is to shorten the burn time allowing the timing to be more retarded and less time to produce oxides of nitrogen. Nothing to be gained turning them off at an arbitrary 3k.

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