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i opened the DELCO box today of my NOS 4000 rpm tach and discovered that switch on the back is not actually a switch at all. I believe I have it figured out and will post photos and my theory tomorrow.
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factory tach install will it work?
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Winston Wolf
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You’re right, it’s not a switch. I soldered jumpers to the 3 pins that it can connect and was then able to test all options without taking the gauge apart. It It changes some resistors on the board, but doesn’t do squat from what Ive seen.
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OK here are a few close-up photos of my new Delco 4000 rpm tach. The little "switch" on the back is actually a plastic plug designed to be inserted or left out. Included in the Delco box with my new tach is a small brown folder with 4 of these plugs. Not sure why there are 4, as only 2 can be used at any given tilme. Maybe the fine folks at Delco figured us idiots would probably lose a plug or two over time. Whatever. Anyway, the way I see it, there can only be 4 combinations of plug configurations; 1)- no plugs, 2)- a plug in the top hole, 3)- a plug in the bottom hole, or 4)- both plugs installed. So it seems to me that the plugs, in varying configurations of installment, may change the signal coming from the signal source to the tach. I am confident that somewhere in the world of GM bulletins there exists a paper detailing these configurations, but I guess we will just have to experiment to find out the true meaning of the little plugs. Maybe some SS member who knows more about these types of issues can chime in.
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Winston Wolf
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I don’t think both plugs is an option. The spring on the one plug is a contact jumper. Like I said, I tried it all 3 ways and it doesn’t make it work properly. I found a box on eBay that the guy thinks should make it work with the alternator. In the big trucks they used a flywheel sensor with more pulses.
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Thanks Winston. Please let us all know how you get this thing to work.
Winston Wolf
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Well, I got the tach to work with a magic box I got on eBay. It allowed me to change the ratios and test things out.
At 2000 rpm engine speed, the tach read 1500 with a 1:4 ratio. With a 1:6 ratio it read 2250. So, I calculated out the correct ratio should be 1:5.33
i can get a custom box made with the specific ratio, so I’ll get one made up next to test out. Should solve the issue.
At 2000 rpm engine speed, the tach read 1500 with a 1:4 ratio. With a 1:6 ratio it read 2250. So, I calculated out the correct ratio should be 1:5.33
i can get a custom box made with the specific ratio, so I’ll get one made up next to test out. Should solve the issue.
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Thanks for your determined efforts. Please keep us posted.
Winston Wolf
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The tach is apparently designed to read at 134 pulses per revolution, as that is the tooth count on the Cat flywheels.
134 pulses / 5.33 = 25.14 alternator pulses per revolution on the 6.2 is approximately what I'm getting with the stock CUCV pulley ratio. I didn't measure the pulleys yet, but it must be just over 4:1 ratio on the alternator to crank pully if it's 6 pulses per rev.
134 pulses / 5.33 = 25.14 alternator pulses per revolution on the 6.2 is approximately what I'm getting with the stock CUCV pulley ratio. I didn't measure the pulleys yet, but it must be just over 4:1 ratio on the alternator to crank pully if it's 6 pulses per rev.
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What do you think about abandoning the thought of using the alternator as the signal source and instead use an aftermarket crankshaft position sensor? These are readily available and attach to the timing index plate next to the harmonic balancer.
Winston Wolf
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Really no need to go away from the alternator now that I have figured out the correct ratio and found an adapter that will do the conversion (it’s only 60$). If you used a crank sensor, you’d need to output the 134 pulses per rev anyways with some kind of pulse converter. You could sense off the 6.2 flywheel, but I think the tooth count is off by 4 or 5, so you’d still have an inaccuracy of a few percent.
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Hey Winston - did you figure out how to get the tach to work?
Winston Wolf
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Yes, but it's not finished yet. It works as it should with my first box, but it read off by a couple hundred rpms. I had a custom ratio box made, but there was a mistake in the firmware with the blanking time, so it would go to half speed after 1700 rpm. https://youtu.be/7JGEcP-wDnA. You can see the VDO reference tach in the video down by the test plug in.
They fixed the firmware, but it hasn't arrived back yet. I'll put up all the details when it finished up, probably this weekend or next week.
i checked the custom box/tach reading with my laser tach, and the ratio I calculated is right on the money.
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Winston Wolf
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Awesome!
Can you put up a guide? I have the tach laying around myself and I know others want to do this mod as well!
Can you put up a guide? I have the tach laying around myself and I know others want to do this mod as well!
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Ditto Winston. I have 2 of those tachs. A step by step guide will be most appreciated. Also did you have to cut out the back of the cluster housing?
48cj2a
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Please tell us exactly what and how to order.
Winston Wolf
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Here are some photos detailing where I needed to cut the stock dash, and how the unit is wired up. You will need to purchase a tachometer adapter from Widget Man.
They can be found on E Bay under CUSTOM-PROGRAMMED-TACH-ADAPTER
The unit will need a ratio of 1 : 5.33, with a 12 Volt output. 1.5 millisecond blanking time.
When you wire in the tach, do not tap into the ignition 12+ wire in dash wiring harness, when the glow plugs cycle, it kills power to the dash, so the tach will read zero when the glow plugs are lit or cycling.
Here is where it hits and where I had to cut out, you do not need to cut the circuit board. I used a dremmel with a cutting disc:
Here is the wiring. I took the diagrams off the site here, but I'm not sure if they are 100% correct. You only need to worry about the fuel sender input, ground, and hot. The tach is wired separately. I grounded to the grounding block/terminals under the dash. I did also buy a small brake light indicator and put that up between my voltage gauges. Just tap into the brake light wires if you want to add that. Fuel sender wire is terminal 18 on the board, and the pink wire in the harness. I added a second tank, so that's why I showed it switching in the video. Don't worry if your tach doesn't have the wiring plug, it will work with just normal spade terminals. The tach is clearly marked, so you won't mess it up.
The tach has a jumper, and I broke mine. So, I soldered wires to allow me to test the different positions. Use the jumper position shown, or do what I did below and just connect the 2 wires. If you don't have the jumper, or yours is in the wrong position, be sure you take off the plastic backing plate or you will break the jumper trying to get it out. If you are installing the jumper, make sure you have the metal tab under it, or it won't complete the circuit. In the picture below, you would want the red and white wires to be connected. I was doing some testing, that's why I made it so I could change the circuit without taking the dash back out.
They can be found on E Bay under CUSTOM-PROGRAMMED-TACH-ADAPTER
The unit will need a ratio of 1 : 5.33, with a 12 Volt output. 1.5 millisecond blanking time.
When you wire in the tach, do not tap into the ignition 12+ wire in dash wiring harness, when the glow plugs cycle, it kills power to the dash, so the tach will read zero when the glow plugs are lit or cycling.
Here is where it hits and where I had to cut out, you do not need to cut the circuit board. I used a dremmel with a cutting disc:
Here is the wiring. I took the diagrams off the site here, but I'm not sure if they are 100% correct. You only need to worry about the fuel sender input, ground, and hot. The tach is wired separately. I grounded to the grounding block/terminals under the dash. I did also buy a small brake light indicator and put that up between my voltage gauges. Just tap into the brake light wires if you want to add that. Fuel sender wire is terminal 18 on the board, and the pink wire in the harness. I added a second tank, so that's why I showed it switching in the video. Don't worry if your tach doesn't have the wiring plug, it will work with just normal spade terminals. The tach is clearly marked, so you won't mess it up.
The tach has a jumper, and I broke mine. So, I soldered wires to allow me to test the different positions. Use the jumper position shown, or do what I did below and just connect the 2 wires. If you don't have the jumper, or yours is in the wrong position, be sure you take off the plastic backing plate or you will break the jumper trying to get it out. If you are installing the jumper, make sure you have the metal tab under it, or it won't complete the circuit. In the picture below, you would want the red and white wires to be connected. I was doing some testing, that's why I made it so I could change the circuit without taking the dash back out.
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Winston Wolf
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I should also add that I checked everything with my laser light tach, reading off the blancer, and it is as close as you will get with a factory tach.
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Thank you very much! Going to order the adapter.
I really feel motivated to get the instrument panel ready now. My girlfriend will be so happy having that big box with instrument panel parts gone from the dining table!
I really feel motivated to get the instrument panel ready now. My girlfriend will be so happy having that big box with instrument panel parts gone from the dining table!
GKGoodCheapParts
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Hello all! I'm Gene K, the eBay vendor for the "magic boxes" Winston Wolf used to get his tach rig working. Since Winston's custom firmware is already written and validated, we can offer a $5 discount to anyone who needs the exact same item. Just contact us through the forum or eBay and mention Winston. For Rutjes, we do ship to the Netherlands and can ship almost anyplace by special arrangement. FYI, here are links to our tach adaptor products that might be of interest to forum users: Full Custom Adaptor (any arbitrary ratio and timing) https://www.ebay.com/itm/142512806637 Universal Adaptor (31 standard ratios) https://www.ebay.com/itm/141931549970 Fummins Adaptor (31 Ford-Cummins specific ratios) https://www.ebay.com/itm/142758902981 Square-to-Sine Signal Converter for 2-wire sensor inputs https://www.ebay.com/itm/142982817997 Voltage Booster for back-EMF tachos https://www.ebay.com/itm/142171581596. Questions are welcome and our tech support is second to none. Happy Holidays everyone!
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