Author Topic: Installing Tach on 79 Eldorado with HEI system.....How??  (Read 96 times)

Offline nanaimogene

  • Posts: 48
  • Name: Gene McCormick
Installing Tach on 79 Eldorado with HEI system.....How??
« on: February 08, 2018, 12:29:03 AM »
I have installed Tachs in cars with the points and coil system like older Caddy's but never on one like 79 Cadillac with HEI. I have googled but information
is not good with warnings about blowing your module.  The HEI has markings bat - tach. Since my car is fuel injected a suggestion was to tap into one
of the injector wires. So the point is where safely do I hook up the Tachs green wire ??  Do I need a tach filter with the HEI system? Thanks Gene
Oh Im not trying to be cool I want to see what RPM the shifts are doing on the tranny.
I should also mention the 15" (rare) mag wheels I installed last year with largest tires that would fit gave it a better ride and control. Had to add
spacers to rear springs.

Offline Dave Shepherd

  • Posts: 1984
Re: Installing Tach on 79 Eldorado with HEI system.....How??
« Reply #1 on: February 08, 2018, 08:18:20 AM »
Why not fhe the tach port on the hei?

Offline TJ Hopland

  • Posts: 6950
Re: Installing Tach on 79 Eldorado with HEI system.....How??
« Reply #2 on: February 08, 2018, 08:24:54 AM »
I would just use the tach terminal.  The signal there is pretty much the same as it would be with points so its a universal signal.    That terminal is literally the minus wire to the coil.   In my aftermarket EFI system that is where I am driving the coil since I don't have the HEI module installed.    I'm really not sure where the 'be careful not to fry the module' came from.  Its already designed to be firing the coil and you look at how quickly a coil destroyed something as rugged and heavy duty as points its kinda amazing that they are/were fairly reliable.   I don't see how adding a tiny little gauge even if it was a marginal design would increase your chances of having a module failure.  If you were doing something that would risk feeding the secondary (spark plug side) of the system into the module then you may have a problem but there shouldn't be much risk of that installing a tach.

I don't know what the signal is for the EFI on those, its coming from the 'pods' on the distributor stalk.  I do know that there are 2 signals per dizzy rotation which would equal 1 per crank if you were able to read both signals but its 2 different sensors so reading a single would only be every other crank.    A V8 tach is going to be looking for 4 pulses per crank rotation.  If you have one that can be set for 4cyl that would be 2 per crank so you are still wrong,  maybe if you have one meant for a motorcycle?   Even if that worked for you that signal may not be a ground so that could get you .   IF it was a ground you may be changing the load/impedance of the circuit so the EFI is going to get confused or damage so I think that is just overall a bad idea.   

Re reading your post I see the suggestion was tie to an injector.  In this case like more modern and possibly even current stuff the computer does modulate the ground signal so that does have some possibilities but still has all the concerns and issues of the above.    In this car injectors are batch fired so that is the 2 signals from the special EFI pickups in the dizzy so we are seeing only 1 signal every other crank turn and the tach is likely expecting at least 2 for a 4 cylinder engine.   If its electronic maybe it can be set for 1?  But I would still have the concerns of it loading or changing the impedance of the circuit since its not designed to piggyback on that circuit like it is an ignition circuit.   If these were sequential injection I would think you would still have a problem because hooking to a single injector would only give you a pulse every other crank turn.   Analog tachs were never designed to do such a thing that I know of.  Digital?  Maybe, its just software so you could in theory make the same gauge work on anything from a single 2 stroke to say a big radial aircraft engine. 

Would you need a tach filter?    I hope not, if its designed for an old car and needed one its a pretty poorly designed tach.   If its an old school electro mechanical gauge it doesn't care.   IF its electronic which is very possible these days it should have the 'filter' built in.   In this case the tach signal is coming from the coil.  The coil being the bit that is generating the 1000's of volts tends to spew some of that voltage in all directions which is generally referred to as noise.  Analog stuff didn't tend to care much unless it was say an AM radio in which case something as simple as a 'condenser' would do the trick.  Electronics like say an aftermarket EFI system?  They don't like that so much and can easily be confused and or damaged by that noise mixed into the signal.   There are/were also things called tach filters that were more converters than filters.  They were often used when there were things like the classic MSD6 'ignition box' in use that was a capactive discharge unit which is kinda like having a coil feed the coil so you are getting at least a double boost and at least double the noise potential.   The 6 series may not be a good example because I think it always had the converter built in hooked to a special old school tach emulator terminal on it.  I think there were some devices like that that didn't have those so you needed the converter.   In some cases they could also do mulit spark per cycle so the converters/filters would weed out the extra signals so the tach stayed reasonably accurate.  There were also ones that would convert the signals from coil packs to an old school tach if the tach wasn't smart enough to be calibrated for the 'wasted spark' modes these earlier 'distributorless' systems ran in.   
« Last Edit: February 08, 2018, 08:27:20 AM by TJ Hopland »
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
75 Eldo rusty but trusty
80 Eldo Diesel
And other assorted stuff I keep buying for some reason

Offline bcroe

  • Posts: 2127
    • CLC Member
      CLC Member #14630
Installing Tach on 79 Eldorado with HEI system.....How??
« Reply #3 on: February 08, 2018, 08:43:27 AM »
The 79 EFI uses a standard HEI coil, so the usual tach terminal is
next to the power connection.  Any tach designed to connect to an
HEI this way should do it.  Not sure what you have, some cars have
a tach option that will work.  Some of these were not stable till I
added a 10 ufd cap to their 12V power input.  There is an ESS tied
into the ignition, but will not affect this function. 

In my opinion a car can't have too many gauges, for EFI I add a
Wide Band OX sensor to the list.  It will tell if the EFI is maintaining
the mixture, or a problem is developing, highly intuitive.  Currently
I am recommending
APSX V2 Anti-Glare D2 Digital Wideband O2 AFR Gauge & Sensor Kit

This unit has a moving electronic pointer around the outside, which
is better for changes too fast for mechanical parts or digital meters.
Match this with an electronic fuel pressure gauge (mechanical will fail). 

I spent a huge amount of effort getting my 79 Eldo to handle properly,
including touring T bars and springs, heavy ADDCO sway bars with poly
links, every possible brace above and below the engine, rebuilt rag
joint, and very important, KYB shocks.  The KG4531s in front really did
it, and KG5573 bolt into the rear, but with no air adjustable height.  So
the springs must be strengthened with them.  Bruce Roe

 

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