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cjwilson Member
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Hello and thanks in advance for input. I have a 73 JH I am trying to bring back. My tach is giving troubles. Before I dive in, I thought I'd glean some wisdom here. The tach appears to work while starting the vehicle, but then immediately drops to zero while driving. Any thoughts or advice where to start looking? |
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DonBurns Member
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I'm sure other members will be able to help you work it out with the original tach, but I'll mention one option- I was having issues with both the tach and speedo. I tried to rebuild the tach myself and failed miserably. Ended up buying both new from SpeedHut. GPS speedo. They are custom built and you can specify faces to pretty closely match the original appearance.Cheaper than sending out to be professionally rebuilt if that turns out to be necessary. I am holding on to originals in case I ever want to restore. Last edited on 03-13-2020 02:39 pm by DonBurns |
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cjwilson Member
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Thanks Don. I feel like it might be related to the key being in the start position. That seems to be the only time the tach is energized. |
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discogodfather Member 
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What are you running in terms of ignition coil? Still have the ballast resistor? |
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cjwilson Member
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Yes. Original points and resistor. I had pulled them out once and put in a Pertronix. But then fried it months later. So the points and resistor went back in. It's been converted back for years. |
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redracer Member
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It is possible the green wire entering your tach from the #2(middle fuse--check it) may not be getting any power. The car will still run with the white wire to the tach sending current out to the coil either through a ballast resistor(W/S: white with slate line) or nickel chromium resistive wire(white with pink stripe)--not sure which wire system you have). I would advise pulling out the tach out through the dash; if you're "not lithe & limber" maybe a grand kid to go upside down under the dash to loosen the two knurled finger nuts should work. keep us posted, bruce |
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cjwilson Member
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OK. I'll check power from the middle fuse first while standing next to the car. If that's functional, I'll find someone lithe to assist. |
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cjwilson Member
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I went and checked again. First, I have 12v to both sides of every fuse. And the tach is not really registering the engine rpm's at all. It is bouncing a bit, but mostly staying at zero. Independent of during starting or idling or stabbing at the gas pedal while it's running. I did get it to bounce to 4000 rpm once, but it mostly stays at zero. |
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cjwilson Member
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I dusted off my schematic. 4 wires to tach. Ground. Fused power off Fuse #1. Hot in from Ignition Switch. And finally hot out to the Coil. It looks like if the car is running ok, then the last 2 are working, signal into and out of the coil. So I'm thinking either hot from Fuse #1 or Ground is my culprit. And of course to check these, I'll have to find someone lithe. |
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redracer Member
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yes, the tach must be grounded(this is actually one of the main problems with the fuel & temp gauge: the 10v voltage stabilizer has a bad ground contact between the housing/case and the sheet metal screw attaching to the metal backing plate).So pull the tach(and maybe the gauge panel--much more difficult) and do some preventative maintenance |
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cjwilson Member
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Gotcha. So, I'm probably looking first for the voltage stabilizer ground as the culprit? Is it nearby? What does it look like? |
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cjwilson Member
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Or is it internal within the tach? |
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redracer Member
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The "voltage stabilizer: controls ONLY the temp & fuel gauges(NOT the tach). If the ground for that is not working, the needle for those two gauges will read 22 1/2 degrees higher(I.e. CCW rotation-your temp gauge will look "HOT"). ALL the gauges MUST be grounded for not only the night lights to work but any internal parts that need grounding(such as the TACH) |
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cjwilson Member
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Ok. So I just need to trace the ground from the tach. On my schematic, that is a black wire labeled B-126-C. It heads off to a wire nut then on to terminal 4 of a 5 way plug. |
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cjwilson Member
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Redracer, I have both good ground and good power to the tach. And fuel and temp gauges are both working and registering at key on. So I'm wondering if I need to rebuild my tach. |
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Tom Bradley Member
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cjwilson wrote:So I'm wondering if I need to rebuild my tach. Possible, but unlikely. When I did a Spiyda conversion on my tach, the tach internals looked very robust. What you described earlier sounds to me like a poor or intermittent electrical connection. The main culprit I have found over the years is the slide-on connectors that are used all over the place on these cars. Before going to messing with the tach internals I would check to make sure that the connections to the back of the tach are clean and tight. The connectors should be difficult to pull off. If not, try tightening them with a pair of pliers so it takes a bit of muscle to push them back on. If the connectors on the wire ends are tight enough, they should push through any oxidation on the spade connectors on the tach. To be safe, though, it is better to clean off the spade connectors with fine sand paper or steel wool. If possible, also check the wires going into the connectors. Normally these connectors do not move much, so the wires should be OK, but if they have been moved much for some reason, the wires could be loose or frayed in which case the connector needs to be replaced. |
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redracer Member
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Assume you have pulled the green wire from the back of the tach and push it in & out a few times to slightly clean the connection? If all that has been done(including the ground connection for the tach) then you'll need to pull it. Either get it reworked/whatever or get a good used one(I have about a dozen N.O.S. ones in case you want a "new" one; somewhat pricey but they're "new"). |
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cjwilson Member
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Thanks Tom. I pulled the dash and the tach and have reinstalled it a couple of times. All of the connections are tight. The only issue I see is the wires in the male bullet connector are starting to fray. I'm guessing that 50% of the wires are still connected. It's at least a good enough connection for the car to run. I'm guessing that's not my issue. |
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cjwilson Member
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And the tach internals looked good. Nothing frayed. Solder joints still solid. Nothing to my untrained eye looked amiss. |
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cjwilson Member
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I do have 12V DC into the tach. Someone told me that I need to check the signal wire into the tach for AC voltage. It could have up to 90V AC on it. That will confirm if I have signal to drive the tach. And if I do have AC voltage present, the tach is the problem. Can any of the much more experienced people here confirm this? |
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Tom Bradley Member
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cjwilson wrote:The only issue I see is the wires in the male bullet connector are starting to fray. I'm guessing that 50% of the wires are still connected. It's at least a good enough connection for the car to run. I'm guessing that's not my issue. If there is fraying, it often is worse than it appears. To be on the safe side, I would replace that connector. Better to waste time doing that than go to the expense of replacing, repairing or upgrading the tach to find out you have the same problem. The only other relatively easy fix I can think of is that the internal calibration trim pot connection may be bad. This happens sometimes on these types of potentiometers. If you have an ohmmeter, you can measure the resistance between points 1 and 2 on the pic shown below. It should be approximately 20 ohms. If the resistance is very high, then try rotating the orange knob back and forth to try to clean off the connection. Also check to make sure the small nut on the back is tight. If neither of these work, my guess is that you will have to replace or modify the tach. Attachment: tach internal.jpg (Downloaded 60 times) |
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Tom Bradley Member
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cjwilson wrote:I do have 12V DC into the tach. Someone told me that I need to check the signal wire into the tach for AC voltage. It could have up to 90V AC on it. That will confirm if I have signal to drive the tach. And if I do have AC voltage present, the tach is the problem. No way. If this is the original, unmodified tach, then it works by measuring the AC current, not voltage. It is only if the tach has been converted to modern voltage-pulse system (which requires some rewiring) that you will be able to measure a voltage signal. |
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cjwilson Member
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I am pretty sure it's original. So I should measure AC Current into the tach? My meter goes up to 10A. Is that enough? Do you know what amount of current I should be looking for? |
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Tom Bradley Member
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cjwilson wrote:I am pretty sure it's original. So I should measure AC Current into the tach? My meter goes up to 10A. Is that enough? Do you know what amount of current I should be looking for? The amount of current will vary depending on the RPM, but will certainly be less than 10A. But measuring this is a waste of time. The current has to be flowing for the engine to run. So if the engine is running OK, then there is no problem in that part of the circuit. The problem would instead be either the +12V or ground connections or internal to the tach. |
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cjwilson Member
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And I've checked both the 12v and ground. They both are good. Are there any good tutorials on the tach? I'm pretty good with a soldering iron and enjoy tackling things as opposed to shipping them off. Worst that can happen is I destroy it and have to buy another. |
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Tom Bradley Member
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I don't know of anything that explains the internal circuitry much. You might try googling "Smiths RVI tachometer" and see what you can find. On this particular model you might also check the 39 ohm carbon composition resistor on the board. These are notorious for going bad when they age. Also carefully examine the small coil wires for breakage or bad solder joints. Otherwise, I don't see anything that should go bad readily. Since you are good at soldering, you might consider ordering an RVI to RVC coverter board from Spiyda. This is what I used and am happy with the results. https://www.spiyda.com/smiths-rvi-rvc-conversion-board.html |
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Jim Ketcham Member
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If I recall correctly, the tantalum capacitor in the rvi tachometers is a common failure point as they had about a 20 year life expectancy. An equivalent electrolytic should work (2.7uf?). I converted to RVC using the Spyda kit to work with my Pertronics Ignition system. It was easy to install and works well. |
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cjwilson Member
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I put a new 2.5 mf 20v cap in about an hour ago and it still doesn't read anything. The old one was reading zero mf. Double checked power and ground. I checked the 39 ohm resistor and it's good. Don't see any broken solder joints anywhere. I haven't checked ohms through the pot. I'll do that next and report back. Thanks to folks that are pitching in. |
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cjwilson Member
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I checked ohms through the pot and it's working well through the entire range. I also desoldered the 39 ohm resistor and it's actually out of spec. It's reading about 50 ohms. I'll run into town and get one to see if that helps. |
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cjwilson Member
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My electronics store is closed due to the virus... I've ordered the resistor on line and will post back once it's arrived and soldered in. |
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cjwilson Member
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So I've put in a new 39 ohm resister and a new cap. Pot reads 12-83 ohms. Still no reading. Car is running great, and when unplugged at the tach, car dies instantly, so I know the through loop is good. Not sure where else to go in attempting to self diagnose and repair. Anyone else that can share wisdom?? |
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cjwilson Member
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Since my tach has never functioned since I bought the car, I decided it was probably stupid of me to trust resoldering replacement parts into the same holes they were desoldered from. Now that I am looking at the tach with cynicism, I see that the red wire feeding into one of the coils is not connecting to anything via the board. It is soldered to the board, but nothing else is soldered to the same contact. And there is an empty hole at the other end of the solder point. Does anyone have a few good pics of what is to get soldered where on their tach? |
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cjwilson Member
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It's working now. Along with several fried parts, the previous owner also soldered in a transistor to the wrong points on the board. But it's all sorted and seems to be accurate. Thanks to those who pitched in! Attachment: Tach 1.jpg (Downloaded 26 times) |
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