Bypassing tach filter prevents engine from starting?
Thread Starter
Racer
Joined: Sep 2009
Posts: 292
Likes: 58
From: Freeport, Illinois
Bypassing tach filter prevents engine from starting?
The tach isn’t working in my ’79 and I tried the method Willcox suggests to test the filter – bypass the filter by plugging the tach wire directly into the distributor and see if the problems go away. I did that and the car wouldn’t start, it just cranked. Plugged the filter back in the way it was, and it fired right up.
I’m not sure why the tach wiring would prevent the engine from firing. This doesn’t sound right. Can anybody tell me what’s going on?
I’m not sure why the tach wiring would prevent the engine from firing. This doesn’t sound right. Can anybody tell me what’s going on?
Race Director
Joined: Sep 1999
Posts: 11,361
Likes: 383
From: Plano TX
The tach wire between filter and tach is touching a ground. When you bypassed the filter, you grounded the coil = no start.
You either have bad insulation on the wire or the tach itself has failed in a way where the signal wire gets grounded.
You either have bad insulation on the wire or the tach itself has failed in a way where the signal wire gets grounded.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
That 'filter' removes unwanted spikes and spurious pulses from the tach signal line, so that only the ignition pulses are detected. If that is not in the circuit, the tach usually reads high, is jumpy, or is erratic.
If the filter is in the circuit and it is defective, you are liable to get anything with the tach reading, depending on how that filter circuit has failed.
If the filter is in the circuit and it is defective, you are liable to get anything with the tach reading, depending on how that filter circuit has failed.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
The tach isn’t working in my ’79 and I tried the method Willcox suggests to test the filter – bypass the filter by plugging the tach wire directly into the distributor and see if the problems go away. I did that and the car wouldn’t start, it just cranked. Plugged the filter back in the way it was, and it fired right up.
I’m not sure why the tach wiring would prevent the engine from firing. This doesn’t sound right. Can anybody tell me what’s going on?
I’m not sure why the tach wiring would prevent the engine from firing. This doesn’t sound right. Can anybody tell me what’s going on?
Tech Contributor
Joined: Jun 2004
Posts: 20,910
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
The tach isn’t working in my ’79 and I tried the method Willcox suggests to test the filter – bypass the filter by plugging the tach wire directly into the distributor and see if the problems go away. I did that and the car wouldn’t start, it just cranked. Plugged the filter back in the way it was, and it fired right up.
I’m not sure why the tach wiring would prevent the engine from firing. This doesn’t sound right. Can anybody tell me what’s going on?
I’m not sure why the tach wiring would prevent the engine from firing. This doesn’t sound right. Can anybody tell me what’s going on?
Analogy: The Tach circuit board in the dash is acting just like a pair of points that never open. Result: No spark.
Thread Starter
Racer
Joined: Sep 2009
Posts: 292
Likes: 58
From: Freeport, Illinois
I test drove it with the filter bypassed and the tach has the same erratic behavior. When I just turn the ignition on without starting the engine, the tach needle pegs past 7,000 and just sits there. So that indicates the problem was not the filter, correct?
I guess the next step is removing the tach and trouble shooting it per your info. I'll let you know what I find.
BTW, I already have a new filter from you on the way. Is it best not to install it until I figure out the problem with the tach, or will the filter not be damaged by a faulty tach?
Thanks,
Ken
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Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Since you are getting the same behavior its safe to say the issue is not the filter and more than likely the board.
I'd try to figure the issue out first and then either keep the original filter when fixed, and keep the new one as a spare back up.
Willcox
I'd try to figure the issue out first and then either keep the original filter when fixed, and keep the new one as a spare back up.
Willcox
Cruising
Joined: Jul 2016
Posts: 10
Likes: 0
76 c3 tach filter
When i took the tach wire from fire wall out of plug (No Filter) and put it into The Dist cap,The tach works got a new tach filter wired up then put into filter nothing ,Put wire back into cap works but when you shut car off tach does not go to zero but if you turn key on no start goes to zero confused Do i need Filter thanks
8th Gear
Joined: Aug 2019
Posts: 8
Likes: 3
From: Sicklerville, New Jersey
Hello, reading this thread after posting the following (potentially in relation to the tachometer filter):
"Hello, looking for some guidance with an inexplicable engine malfunction; engine will crank but will not turn over. After cranking for some time, with no signs of firing, there is no noticeable scent of fuel, as may be expected after pumping the gas pedal repeatedly with no firing. Leading up to this malfunction, the following series of events / maintenance occurred in this order:
Car was driven without a tachometer filter and there were not noticeable issues (other than an inoperable tachometer, which was expected).
Air cleaner was removed, to better access top of the engine block for cleaning.
Vacuum pick-up line for head lights was temporarily disconnected.
New tachometer filter was bought and installed.
Various wires located near the firewall on the drivers side of the distributor were disconnected for ease of access to cover in braided wire wrap, and plugged back in the same location.
Attempted to start the car, and engine only cranked, but would not turn over. Smoke was observed at the newly installed tachometer filter. Removed the tachometer filter and car still would not start.
My initial thought with the smoking tachometer filter is that there may not have been a suitable ground, or it was installed backwards. This is an issue that needs to be fixed, however i am at a loss for ideas, as to why the engine will not turn over. Is it possible that the tachometer filter issues may have damaged something in the distributor? Any guidance / suggestions / lessons learned would be appreciated. Prior to this malfunction there have been no issues with starting or driving our 1980; car has been approximately 75% restored, most of the engine compartment is new and has been operating properly for over two years."
What seems to be different in my case, is that the car fired with no issues while the tachometer filter was removed.
Let me know if anyone has a thought as to where to go from here.
Thanks!
"Hello, looking for some guidance with an inexplicable engine malfunction; engine will crank but will not turn over. After cranking for some time, with no signs of firing, there is no noticeable scent of fuel, as may be expected after pumping the gas pedal repeatedly with no firing. Leading up to this malfunction, the following series of events / maintenance occurred in this order:
Car was driven without a tachometer filter and there were not noticeable issues (other than an inoperable tachometer, which was expected).
Air cleaner was removed, to better access top of the engine block for cleaning.
Vacuum pick-up line for head lights was temporarily disconnected.
New tachometer filter was bought and installed.
Various wires located near the firewall on the drivers side of the distributor were disconnected for ease of access to cover in braided wire wrap, and plugged back in the same location.
Attempted to start the car, and engine only cranked, but would not turn over. Smoke was observed at the newly installed tachometer filter. Removed the tachometer filter and car still would not start.
My initial thought with the smoking tachometer filter is that there may not have been a suitable ground, or it was installed backwards. This is an issue that needs to be fixed, however i am at a loss for ideas, as to why the engine will not turn over. Is it possible that the tachometer filter issues may have damaged something in the distributor? Any guidance / suggestions / lessons learned would be appreciated. Prior to this malfunction there have been no issues with starting or driving our 1980; car has been approximately 75% restored, most of the engine compartment is new and has been operating properly for over two years."
What seems to be different in my case, is that the car fired with no issues while the tachometer filter was removed.
Let me know if anyone has a thought as to where to go from here.
Thanks!