73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Interior & Equipment => Instrumentation => Topic started by: BlackTomC on September 02, 2015, 03:10:21 pm
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Anyone have any ideas why when I press my horn, my tach needle drops a bit, then when I let up, the tach needle goes back to it's original position?
PS - Tach doesn't work right, reads really low, unless I hose down the engine bay, then it magically works for a few hours (already replaced the distrubitor harness for it, no difference).
Thanks peeps.
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still sounds like a poor ground. i know on my 91 i have a ground that runs from the aldl box (obd1) dracs box, pcm and speedo. its runs through the fire wall connection and turns into an engine ground. i dont know where it was connected but i know without it the speedo doesnt work. its just indicated as a black wire.
for those just tuning in here is his original post
http://forum.73-87chevytrucks.com/smforum/index.php?topic=30685.0
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Alright then, this weekend I'm ripping out all the wiring for the tach and redoing all of it. Stay tuned.
All the wiring
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how do you have the ground and power ran?
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As it sits now, I've got 3 wires running out of the back of the tach
Brown is running straight down and out the firewall to the distributor.
Red/Pink is running down to the fuse box for power
Black is running to this 6 prong deal for ground (common ground is what I've heard it referred to as)
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That's off the top of my head, I'll revisit it when I get home just to make sure.
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do you have another tach that you could hook up and see what its reading? the only other thing i can think of is running a new ground to the engine or directly to the battery and see if it helps your problem
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Unfortunately I don't, but as of now, the plan is to rip the tach out and manually run 3 new wires to it (1 ground, 1 power from battery, 1 straight from the distributor).
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Check the ground strap from the left rear head to the firewall and ground from batt to rad support.
Sent from my iPhone using Tapatalk
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Alright folks so I replaced the pigtail connecting the tach and distributor, ran a power wire straight from the tach to the + terminal on the battery/ and a wire from the tach ground to the battery negative. So far it's still reading low.
I started poking around the fuse block a noticed the pink power wire from the tach was plugged into the spot in the fuse block for the horn (probably explaining why the tach bounces down anytime I press the horn). I ended up moving it down 1 fuse position in the fuse block, so the tach still gets power but now the horn doesn't work at all. Anyone have experience with how this is is all supposed to work? I'm confused as heck at this point.
Also the pink power wire from the tach has a plug that plugs into the fuse block, HOWEVER it also has a wire coming off of it to some weird looking female connector..... also, can't seem to locate any wiring for the horn. Pls help.
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Conventional ignition or HEI? Does the brown tach wire connect to a filter on its way to the distributor? Crimp a new terminal end on the signal wire. Is the wire connected to the negative coil terminal?
Check the fuses and make sure there is no oxidation or corrosion between the metal caps of the fuses and the fuse box clips - a common problem.
Horn wiring connects through a 3-prong relay that hangs out of the harness under the dash. The horn button grounds the relay to energize the horn.
If you want help with the "weird looking female connector," post a pic.
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From what I know, it's got an HEI distributor. The brown tach wire connecting to the tach has a small little spacer on it, not sure if it's a filter or just rubber to protect it passing through the firewall.
Already crimped a new terminal on the end of the signal wire (pigtail going into distributor) (now has green and red wire coming off the distributor), didn't seem to help the issue at all. I did notice however when I rev the engine, the tach reads wrong until I let off the gas, then the RPM's shoot up, then back down.
From what I can tell, there is only one wire running from the distributor, to the tach, a brown wire.
Will check horn fuse in daylight tomorrow, would you happen to have a diagram pointing out which fuse belongs to the horn?
Picture coming soon.
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Can't upload high quality images here, 256kb slaughers image quality, uploaded to Dropbox, click the link and all the images should show up.
https://www.dropbox.com/sh/9v1qq8r8lwqlfq5/AAD5b8jt7LLaP3GzG1Uw0d3ea?dl=0 (https://www.dropbox.com/sh/9v1qq8r8lwqlfq5/AAD5b8jt7LLaP3GzG1Uw0d3ea?dl=0)
Not sure why you think it slaughters the quality, but this is the same pictures resized to 800x600... Looks fine to me. - Admin
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As of now, the tach works (somewhat, still doesn't read accurately, reads low).
Horn doesn't work at all now, it did when I had the pink wire plugged into the socket above where it's at now, the socket that says HRN.
I think I understand this now, before I plugged the tach into the right spot, the horn was grounding out through the tach, causing the needle to dip... now onto where togo from here.
-This is the other 2.. -admin
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At the top of your 2nd image, above the gold flasher, is a little black box with red, green and black wires attached. That black box is the horn relay. Upon inspecting the wiring diagram, there is no fuse for the horn, so don't waste your time looking for one. The red wire from the relay connects directly to battery through a fusible link at the firewall junction block over the back of the engine. The green wire connects to the horn(s). The relay energizes when the black wire grounds through the horn button in the steering wheel. Does the relay "click" when you press the horn button? Momentarily jump the red to the green and the horn(s) should blare, bypassing the relay entirely.
The 'weird looking blue connector' on the short pigtail is just a convenient location to plug in another accessory if the fuse box sockets are occupied by other connectors.
The pink power wire for the tach is located in the proper fuse box cavity in your image. Interesting enough, the socket directly above the pink wire (where the pink wire was previously located) is electrically identical - that is both power tap sockets are protected by the same fuse located immediately beneath, the 15-amp "B/U LPS DIR SIG" fuse. That pair of 'stacked' cavities is marked "IGN FUSED" not "HRN." The horn has no electrical connection with the fuse box. Hence, this...
...before I plugged the tach into the right spot, the horn was grounding out through the tach, causing the needle to dip....
...is unlikely. However, voltage loss due to other causes is possible.
Since you have already thoroughly checked the vehicle grounds (correct?), check the 12-gauge red wires and associated fusible links connecting to the starter solenoid 3/8" battery lug and the firewall junction block. Give a gentle tug on the fuse links - if they stretch, they're toast and need replacing. Any less than perfect connections in the main feeds entering the cab can cause a loss of voltage to the fuse box and distributor. The greater the current demand of an appliance, such as the horn, the greater the voltage drop across an imperfect connection. Don't make any assumptions when checking for poor connections and voltage loss. The best method for determining voltage loss employs a volt meter. Do you have one?
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Just read the message, I'll respond with what I know now, then do the troubleshooting as soon as work is over.
The relay does click when I press the horn button, unfortunately there's no sound (unless I have that pink wire plugged into the spot above where it's currently located).
Yes I've checked the grounds (even replaced a few), I'll double check again and start tugging on the main wires from the starter tonight. I appreciate the thorough response.
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Tugged on all the wires connecting to the starter, absolutely no slack. Anything else you'd recommend I check?
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@Admin - My bad man, next time I will put more effort into reducing them to the correct size. The images were initially over 2Mb's in size, each time I reduced the resolution of the image, I noticed the image quality drastically reduced and assumed if I hammered it down to 256Kb, it would reduce it far too much to depict any valuable details that could leading to resolving my little fiasco. Thank you for the heads up.
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Are all of the connections clean and tight? Any discoloration or evidence of heat stress at any of the wire connections? Did the horn sound when you removed the horn relay and jumped red to green? Do you have a voltmeter?
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Alright, so to very that the connections are clean, I will need some time in the daylight (when I'm off work) to closely inspect, I will remove the starter and thoroughly check all connections.
Just jumped the red and green wire behind the horn relay, horn are loud and clear. I don't have one, but can get ahold of one tomorrow morning.
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Alright, so to very that the connections are clean, I will need some time in the daylight (when I'm off work) to closely inspect, I will remove the starter and thoroughly check all connections.
Just jumped the red and green wire behind the horn relay, horn are loud and clear. I don't have one, but can get ahold of one tomorrow morning.
You shouldn't need to remove the starter. Just use a bright flashlight and peer at the connections, try to wiggle them. If the terminal ends don't appear to be factory installed, look at the wire strands to see if there is any sign of discoloration from heating due to a poor crimp. You get the idea? Just perform a close visual inspection for anything obvious. Then you can use the meter to check for any voltage loss. You may need the voltmeter for a few days.
Since the relay clicks when you press the horn button, and the horns sound when you bypass the relay, replace the horn relay.
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Will do. Picked up a new relay on the way to work this morning for $11, installing tonight after work, will update with results. As for the starter, I probably should mention this, sometimes when I go to start the Burb, I turn the key, and the starter just goes "CLACK", then usually the second of third time I turn the key, the burb cranks over and fires right up.
Tonight I will verify the connections are solid and inspect for any weirdness going on down there with the terminals.
Another piece of history on this burb, before I bought it, the engine was removed due to a burned valve, it was replaced, along with the cam, all the gaskets and distributor.. I'm not sure if the tach worked before that.
Will update accordingly.
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If the battery is in good shape and all the electrical connections are clean and tight, it sounds like it could use a new starter, too.... And, maybe a couple of shiny new 2-gauge copper battery cables. :) To diagnose the B+ feeds into the cab you'll need to use a voltmeter.
Regarding the tach, I suspect a component is bad on the circuit board attached to the back of the tach head.
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Battery is brand new. I'm sort of learning as I go here, any cons to using a replacement gear reduction starter?
ACDelco - Starter Motor
Part # 337-1016
I'll go ahead and grab new cables as well, it's all old under there, I'd really like to get everything into better shape.
Regarding the tach, what I don't understand is that sometimes, when I soak the engine bay (during cleaning), the tach comes back to life for a few hours, then goes right back to reading low again.
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I didn't check the P/N you posted, but a compact gear reduction starter is usually a good choice. On the battery cables, make sure they are all-copper 2-gauge or heavier cables, not copper-clad aluminum. The tach thing is still up in the air, pending further diagnosis.
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So I replaced the relay, horn works perfectly now, absolutely no delay, and no interference with the tach. Also picked up a multimeter.
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Alright, so quick status update. I found the resistor for the my blower motor, YES! Also, the horn works great, but now when I press the horn button, the tach drops down even more, drops down almost 500 RPM's. Tomorrow I am replacing the starter, giving her some new battery cables and cleaning all the connectors.
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Progress?
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Hey bd,
Thanks for sticking with me, I really appreciate it. The starter comes in today and will be installed Friday morning. This 4/10 work schedule has me waking up when it's dark, and leaving work when it's dark.
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im still with you also keep at it. but bd's knowledge is much more vast and complex than mine. so im still lurking around learning off the two of you
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Hey bd,
Thanks for sticking with me, I really appreciate it. The starter comes in today and will be installed Friday morning. This 4/10 work schedule has me waking up when it's dark, and leaving work when it's dark.
Been there and done that - don't miss it one bit. I've had a few backcountry epics hat started and ended that way. I learned to carry a flashlight! 8)
im still with you also keep at it. but bd's knowledge is much more vast and complex than mine. so im still lurking around learning off the two of you
Boy! Have I got you buffaloed. ;D
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I appreciate your help as well Irish, I've already learned quite a bit from both of you here on the forums
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We all learn from each other. It's part of the fun of our little community.
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So before Friday rolls around, can anyone speculate as to why the tachometer drops substantially more after replacing the horn relay?
* Tach needle drops anytime the horn is pressed *
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Four scenarios come to mind: (1) a floating common ground, (2) excessive circuit resistance resulting in a voltage loss/shift under load, (3) the voltage spike generated from the current surge when energizing the horns is interfering with/mistriggering the tach, or (4) a faulty component on the tach head circuit board. Each scenario has its own correction.
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Made a video this time to show the problem.
https://www.youtube.com/watch?v=p6rwVK3qN4E
Tach used todo this everytime the horn was pressed, now it does this every time the fan speed selector is changed on the HVAC controls.
Any ideas?
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Needle jump was most pronounced when switching high blower. Does the symptom persist if you unplug the horn(s) and the blower motor?
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This is from memory, I'm almost positive it did. Tonight when I get home I will disconnect the horn relay and unplug the blower motor, test it out again.
A few months ago, I hosed the engine bay down real good to clean everything, right after I did that, I drove the burb, and interestingly enough the tach worked just fine, for about an hour till it started reading slowly, doing this whole needle jump thing.
If it does/doesn't, what are you thinking is the issue?
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The tach has electrical noise filters built into its circuitry. The filters depend on electrolytic capacitors. Electrolytic capacitors contain liquid electrolyte.
If when the electrolyte dries the capacitors become ineffective, defeating the associated filters. Without functional noise filters, spurious voltage surges can mis-trigger the tach, causing its needle to jump. The tach's circuit board capacitors are typically surface mount devices that are not practical (easy) to replace. The question about whether the symptom occurs even with the horn(s) and blower motor unplugged is related to whether the voltage spikes affecting the tach are generated by the loads (horn and blower) or the relays controlling the loads.
Based on your response I recommend installing a 680 Ω, 1/2 watt resistor across the control coils of the horn relay and the high blower relay. In addition, install a 35 - 50 µF, 50+ volt electrolytic capacitor across the tach power leads (cathode to black wire, anode to pink wire). Adding these components (the two resistors and capacitor) is an attempt to suppress the voltage spikes generated by the relay coils and any other spurious electrical noise that may spike the power bus.
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Would these noise filters cause the tach to respond slowly, and not read correctly ALL the time? (Unless I hose down the engine bay?)
I read somewhere that these tach's have a resistor that goes bad and that's what causes the issue.
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Four scenarios come to mind: (1) a floating common ground, (2) excessive circuit resistance resulting in a voltage loss/shift under load, (3) the voltage spike generated from the current surge when energizing the horns is interfering with/mistriggering the tach, or (4) a faulty component on the tach head circuit board. Each scenario has its own correction.
Did you ever follow up on any of these suggestions from one year ago?
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I did, my apologies for not addressing this sooner
1) grounded straight to battery
2) ran new wiring straight from the distributor to the tach
3) Horn issue was resolved, now the tach drops when the AC switch is switched to the fast blower speed
4) Not sure how to test this
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Made this vid to show the issue
https://www.youtube.com/watch?v=p6rwVK3qN4E
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Okay. Since you established a dedicated ground from the tach head directly to battery, you effectively eliminated a floating ground. Next, monitor the voltage on the B+ power lead into the tach head. The best tool for this is an oscilloscope, because of the brief duration of the effect. Alternatively, a good voltmeter may suffice, although a sensitive analog meter (i.e., VTVM) might outperform a digital meter under the circumstances. Watch for a momentary drop in B+ voltage that's coincident with de-energizing the high blower. A momentary dip in voltage suggests a reverse voltage spike generated off of the high blower relay coil. Under this scenario, adding a 680 Ω, 1/2-watt resistor across the relay coil leads may suppress the voltage spikes sufficiently to relieve symptoms. Otherwise, a high-value electrolytic capacitor inserted between the tach head B+ and ground leads may be indicated.
Before attempting to create the supplemental noise filter, if you wish, visually inspect the tach head circuit board for obviously damaged components. If you're unsure of what to look for, post or link a pic of the tach's PC board that clearly shows all of the discrete components in as high a resolution as you can provide.
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There are 2 issues, with the voltage spike being the lesser issue.
The issue I am trying to fix is the incorrect tach reading.
Would the voltage spike also cause the low RPM reading the tach is producing? Also, the board looks good.
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I think tach calibration issues are likely related to faulty board components or perhaps shorted gauge windings. So, none of the board components are discolored from heat, cracked or show subtle staining on the PC board directly below the components?