Tach Noise

J

jon payne

New member
May 28, 2008
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Hi Guys!

Need you help please. Last fall I installed the FiTech EFI. Only put on around 300 miles on the system since then. When I first took the GMC out
this past April it was the first time I had the handheld controller set up where I could monitor some parameters like AFR, RPM, Coolant temp, etc
while driving. One thing I noticed right away was the RPM would spike to 3000 to 5000 randomly and the AFR was really rich, around 10.8. The RPM
spike would last 1 second but no obvious affect on the engine operation. This also happens during idle including the very rich AFR. I performed a
data log and noticed that when the RPM spikes the fuel pressure spikes to 100psi and the injector duty cycle spikes as well. This only lasts 1 second
and I think this is the reason the AFR is really rich. The ECU might think there is a mis-fire and is trying to adjust.

Anyway, I spoke with FiTech and they said there is noise getting into the tach wire so I isolated the tach wire and put braided shielding the length
of the wire but no help. During further trouble shooting, while the engine was running attempted to move around the tach wire and when a touched it I
got a huge shock so for sure there is noise in the tach signal. I then put an alligator clip on the tach wire, about 5 inches from the distributor,
touched the other end to ground and the engine quite. Moving the clip another five inches or so away from the distributor, touched ground and had no
effect. For the heck of it I swapped out the coil with a spare, made sure the installation was correct but no change.

Not positive if what I am seeing is normal or not with the tach wire but if it is not could it be the RF capacitor is bad? I know that is for RF
noise but not sure if it helps to keep noise out of the tach wire??

Hoping for some ideas here as I am really lost on this one. Got a big trip to Sleeping Bear Dunes next week and got to get this sorted out ASAP. Any
help would be greatly appreciated as always!

Jon
--
Jon Payne
76 Palm Beach
Westfield,IN
 
> Hi Guys!
>
> Need you help please. Last fall I installed the FiTech EFI. Only put on around 300 miles on the system since then. When I first took the GMC out
> this past April it was the first time I had the handheld controller set up where I could monitor some parameters like AFR, RPM, Coolant temp, etc
> while driving. One thing I noticed right away was the RPM would spike to 3000 to 5000 randomly and the AFR was really rich, around 10.8. The RPM
> spike would last 1 second but no obvious affect on the engine operation. This also happens during idle including the very rich AFR. I performed a
> data log and noticed that when the RPM spikes the fuel pressure spikes to 100psi and the injector duty cycle spikes as well. This only lasts 1
> second and I think this is the reason the AFR is really rich. The ECU might think there is a mis-fire and is trying to adjust.
>
> Anyway, I spoke with FiTech and they said there is noise getting into the tach wire so I isolated the tach wire and put braided shielding the
> length of the wire but no help. During further trouble shooting, while the engine was running attempted to move around the tach wire and when a
> touched it I got a huge shock so for sure there is noise in the tach signal. I then put an alligator clip on the tach wire, about 5 inches from the
> distributor, touched the other end to ground and the engine quite. Moving the clip another five inches or so away from the distributor, touched
> ground and had no effect. For the heck of it I swapped out the coil with a spare, made sure the installation was correct but no change.
>
> Not positive if what I am seeing is normal or not with the tach wire but if it is not could it be the RF capacitor is bad? I know that is for RF
> noise but not sure if it helps to keep noise out of the tach wire??
>
> Hoping for some ideas here as I am really lost on this one. Got a big trip to Sleeping Bear Dunes next week and got to get this sorted out ASAP.
> Any help would be greatly appreciated as always!
>
> Jon

Jon,
Redoing my dash board, I choose a Tach and Speedo from Speedhut. After the install, I noticed similar issues as you with that new tach. It would
suddenly peg the needle over 6K for about a second then cycle to "0" and back to normal engine speed. I contacted Speedhut tech support and they
asked questions that you might want to answer, like...."What sort of plugs and wires are you using? Solid core wires can cause quite a bit a feedback
through a dizzy." I have carbon core wires. His response..."Alright. Well there must be noise coming off the ignition system somewhere. I can send you
a tach filter wire inline with the signal wire of the tach to reduce the noise going into the tach." I got that "filter" and installed it inline with
the ignition wire to the distributor. The anomaly went away. Sounds like you need some kind of filter.

FWIW, I love the Tach and Speedo from Speedhut, and found their tech support to be outstanding. Maybe FiTech has a noise suppression unit of some
kind. Speedhut said that " it is somewhat common for one to be needed."

HTH

--
Larry
78 Royale w/500 Caddy
Menomonie, WI.
 
Let me state that I know absolutely nothing about FiTech FI.

I have chased noise problems all over vehicles running ham radio equipment. Almost every noise problem I have chased is improper grounding somewhere.
So here are a few suggestions concerning the unique grounding system on a GMC.

So here are a few simple ideas:

1. Body ground and engine ground are not the same thing in a GMC. Because the Fitech is mounted on the engine, ALL other electrical components for
the FI system need to get their ground back at the engine and NEVER from something connected to the body.

2. Never have two paths to ground from a given component. If you do this you establish an tuned antenna at some unknown frequency that can cause
havoc for propagation of electrical noise throughout the vehicle.

3. I assume that the FI tech is mounted to the intake manifold with a fiber gasket. Add a short ground jumper between the base of the Fitech and the
intake manifold. Attach the jumper to any bolt on the intake manifold.

3. I assume that the Fitech unit is getting power from someplace that is powered by the engine driven alternator and engine battery. If their is
more that one place that the Fitech system gets power, make sure all places have their +12 volt power traced back to the engine system and NOT the
house system.

4. If you add a shielded cable anywhere like that lead to the tach, it must be grounded on ONE END only. If you attach both ends, then the shield not
becomes a current carrier for -12 volts and provides an alternate path for -12 volt current. It also establishes a ground loop (two paths) which
makes an excellent antenna at several unknown frequencies.

5. Add an additional ground wire from the base of the distributor to any bolt on the engine. There are a couple of pre-threaded holes, I believe they
are 3/8", on the back of the engine near the distributor that could be used. Actually any other bolt on the engine will also work.

I should note that I had one guy stop by here with another brand FI system that had a control unit mounted under the driver's seat. He was having
major problems with the engine quitting every 100 miles or so. He had replaced just about everything in the ignition system. Adding a ground wire
from the control unit direct to the engine solved his problems. Again I knew nothing about his system, and at the moment do not even remember the
brand.

I'm sorry that I'm not more help but I have never seen an FiTech system.

Good Luck

Ken
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Jon,
Can you clarify a few points?
When the RPMS spike, is it actually spiking in the engine or just on the handheld?
Your 10.8 AFR, is that all the time or only during these 1 second instances?

--
Justin Brady
http://www.thegmcrv.com/
1976 Palm Beach 455
 
>
> "attempted to move around the tach wire and when a touched it I
> got a huge shock so for sure there is noise in the tach signal. I then put an alligator clip on the tach wire, about 5 inches from the distributor,
> touched the other end to ground and the engine quite. Moving the clip another five inches or so away from the distributor, touched ground and had no
> effect."

Jon
Did you place the alligator clip on the insulated jacket, or directly on the metal wire core?

If moving the clip another 5" along the wire gives different results, I would investigate a potential faulty tach wire, especially since you also got a shock when moving it.
 
Thanks everyone for your suggestions! Got some grounds to check, thanks Ken for that. Also I'll look into a tach filter.

To answer some questions that were asked:
- I placed the alligator clip on the braided shielding then touched ground.
- The shielding I placed over the wire is not grounded at all, just encases the wire. I first was thinking it was outside interference but I do
understand that it should be properly grounded on one end.
- Inspected the tach wiring and could find no defects.
- RPM spike is just an indication on the handheld.

Thanks again!

--
Jon Payne
76 Palm Beach
Westfield,IN
 
Ken B gave many good points, but SHOCK from 12V system tells me to look else where. What ignition wires are you running? If not good magcore type I
would start there
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
> Ken B gave many good points, but SHOCK from 12V system tells me to look else where. What ignition wires are you running? If not good magcore
> type I would start there

Agreed, that's why I asked what coil he's running.
Shock usually comes from voltage much higher than 12
--
Justin Brady
http://www.thegmcrv.com/
1976 Palm Beach 455
 
I called Dick Paterson to discuss the issue too. He says no voltage should be on that wire accept maybe 12 volts or so. Certainly should not receive
a shock when touched. He went on to say that for some reason the voltage from the secondary coil is bleeding over to the primary and suggested to
replace the coil so fortunately I have a brand new coil never been installed and will try that first. Another possibility, ask Ken eluded to earlier,
is grounding. He said to make sure I have no grounding issues and also on his later distributors they have an external ground wire that is connected
to the stud on the bottom of the distributor (where the fastener that holds the module in place) and that should be connected to a solid ground on the
engine like the intake for example. I think my distrib had that wire but I didn't connect it for reasons I do not recall.

So after work I'll replace the coil, add the ground to the distrib, check all the other grounds as Ken suggested and I'll report back.

Thanks again guys for all your input...very helpful. I'll keep you posted.

JP
--
Jon Payne
76 Palm Beach
Westfield,IN
 
If you get a shock to ground at regularly spaced intervals on an well-insulated wire and no shock in between those intervals, there is a radio wave
present on the wire. This could be coming from the ignition system amplifier either leaking a signal backwards or possibly wired or grounded
incorrectly. What type of ignition system are you using?

If you have a stock ignition (points and coil, no amp or CD setup), check/replace the condenser and check the coil tower for cracks and arc trails,
and make sure there isn't just 1 strand of wire from that negative (distributor) wire having a wild hair day.
--
Terry Kelpien

ASE Master Technician

73 Glacier 260

Smithfield, Va.
 
Guys its fixed!! It was the ignition coil. Evidently it was the secondary winding bleeding over to the primary coil inducing a voltage into the tach
wire. No more RPM fluctuations and the AFR is spot on!

Thanks,

Jon
--
Jon Payne
76 Palm Beach
Westfield,IN
 
Oh well, so much for my grounding guess.

Do not let Colonel Ken know I was wrong.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Tsk, tsk.

Ken H.

> Oh well, so much for my grounding guess.
>
> Do not let Colonel Ken know I was wrong.
> --
 
Factory wiring does not have any connection between the aluminum dash panel that has the gauges and lighting and the body or frame. There is one
ground wire between that aluminum plate and the aluminum plate up front that has the isolator and boost relay on it. Then there is a second wire on
the isolator aluminum plate that runs to the negative terminal of the engine battery. Finally the big ground cable from the battery goes to the
engine block which is the master ground and completes the circuit.

So the circuit goes: dash panel -> isolator panel -> Engine battery -> engine block.

There are no ground loops this way but it is a rather round about to get it done. That is the GM did it.

That wire connection at the dash panel aluminum plate occasionally loosens up or corrodes causing strange and sometimes intermittent gauge readings or
dash light problems.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
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