AMC86Kid Posted December 18, 2016 Share Posted December 18, 2016 ok so i put the 3.4 in my 86 MJ but here is my problem with the RPM this is how it looks when the vehicle is in park and turned off this is how it looks when its in park and in idle and here's what it looks like when its in park and you press the gas any ideas on how to fix this Link to comment Share on other sites More sharing options...
Eagle Posted December 19, 2016 Share Posted December 19, 2016 It depends on what year vehicle the tachometer came out of. From the design of the dial, I'm guess it came from an 84-86 (or early 87) Cherokee or Comanche. Those didn't have a potentiometer on the back for calibrating. That said, six cylinders is six cylinders, and the tach doesn't know if the engine is a 2.8L, 3.4L, or 4.0L. Those readings look WAY off. I wonder if the tachometer is any good. I think it's theoretically possible to pull the needle off and reinstall it pointing in a different direction, but I've never been brave enough to try that. But I think that's what people do when they install those decals to make their instruments have a white background. Link to comment Share on other sites More sharing options...
AMC86Kid Posted December 19, 2016 Author Share Posted December 19, 2016 so are you saying that i should pull the dash out and look at how its connected behind it ? Link to comment Share on other sites More sharing options...
krustyballer16 Posted December 19, 2016 Share Posted December 19, 2016 Buy a tiny tach Link to comment Share on other sites More sharing options...
AMC86Kid Posted December 19, 2016 Author Share Posted December 19, 2016 can you post a link of that ? Link to comment Share on other sites More sharing options...
krustyballer16 Posted December 19, 2016 Share Posted December 19, 2016 can you post a link of that ?Just google tiny tach. They make them for gas and diesel Link to comment Share on other sites More sharing options...
Eagle Posted December 19, 2016 Share Posted December 19, 2016 so are you saying that i should pull the dash out and look at how its connected behind it ? Not how it's connected -- whether or not there is a potentiometer on the back of the tach. There have been photos posted here in the past showing the back of the old and new style tach, that show what the potentiometer looks like. But yours is off by so much at idle that calibration isn't the problem. Let's say idle speed should be 1000 RPM (I know, it isn't, but let's make the match easy). It's a 6 cylinder, and a 4-stroke, so the for 1000 revolutions tach "sees" 3000 impulses per minute. But a 4 cylinder only fires 2000 times in 1000 revolutions, so if a 4-cylinder tach is installed in a 6-cylinder vehicle, it will display 3000 impulses as 1500 RPM, not 1000. In other words, a 4-cylinder tach will read 50% higher than it should, across the scale. Yours is starting out at 3500 to 3600, which would mean if it's a 4-cylinder tach you're still idling at 2000 RPM. I think the needle is out of position on your tach. Do you have an idle tachometer? Can you check to see if a difference of 500 RPM on your tach is really 500 RPM? Link to comment Share on other sites More sharing options...
AMC86Kid Posted December 19, 2016 Author Share Posted December 19, 2016 i think its way off cause when i press the gas i barely press it and it jumps up like 3500 rpm I'm afraid of pressing the gas all the way down the rpm gauge will probably do a full 360 Link to comment Share on other sites More sharing options...
Eagle Posted December 20, 2016 Share Posted December 20, 2016 Do you have an idle tachometer? Your second photo shows it reading 3600 RPM at idle -- it should be about 750 RPM. You need to compare what this tach is reading against a tachometer that you know is reasonably accurate. Link to comment Share on other sites More sharing options...
AMC86Kid Posted December 20, 2016 Author Share Posted December 20, 2016 how do i figure out if i have a idle tachometer ? i know my mph gauge is working correctly and I'm going to take the dash apart to repaint it so shouldn't i just replace it wile i have it apart? Link to comment Share on other sites More sharing options...
Eagle Posted December 20, 2016 Share Posted December 20, 2016 how do i figure out if i have a idle tachometer ? i know my mph gauge is working correctly and I'm going to take the dash apart to repaint it so shouldn't i just replace it wile i have it apart? An idle tachometer is a hand-held diagnostic instrument. You would know if you have one because it would be in a drawer or on a shelf in your garage or workshop. Link to comment Share on other sites More sharing options...
AMC86Kid Posted December 20, 2016 Author Share Posted December 20, 2016 oh ok i see what you mean but even if i did have one and i hooked it up i would definitely be getting different number then my rpm gauge Link to comment Share on other sites More sharing options...
Dzimm Posted December 20, 2016 Share Posted December 20, 2016 That's the point... If you use the idle tach you can get a reading of what the rpms should be at idle and then adjust your tach to match, whether that be by using the potentiometer if it's equipped or by moving the needle to a new location. Judging by how quickly you say it jumps with the slightest amount of gas, it may need replaced all together but you will want to start by trying to adjust it. Don't just replace MJ specific parts "just because." When you do that, they usually end up in the garbage when a simple adjustment is all it needed. Link to comment Share on other sites More sharing options...
AMC86Kid Posted December 20, 2016 Author Share Posted December 20, 2016 ok i will check it out when i take it apart and see what kind i have and if i can adjust it Link to comment Share on other sites More sharing options...
Eagle Posted December 20, 2016 Share Posted December 20, 2016 One thing you can do with an idle tach (or any aftermarket tach you can hook up) is to see if a change in throttle results in the same change on both tachs. Say your dashboard tach reads 3500 RPM at idle, and an external tach reads 800 RPM. Move the throttle enough to bump the reading on the aftermarket tach to ... let's say ... 1500 RPM. That's a change of 700 RPM. Then you look at the dash and see if the tach in your cluster also moved by 700 RPM -- if it was 3500, it should now be reading 4200 RPM. Then run the idle tach up another 500 or 1000 RPM, and see if the tach in the cluster also changes by 500 or 1000 RPM. If the change is consistent, then the tach in the cluster is reading properly but the needle isn't positioned correctly. I ***THINK*** (based on reading posts about installing decals to make the gauges into white-faced instruments) that you can carefully pull the needle off and reposition it. Link to comment Share on other sites More sharing options...
AMC86Kid Posted December 20, 2016 Author Share Posted December 20, 2016 i will try to fix it if not then i will just throw in a new one now i have to figure out how to get the stalk to work correctly Link to comment Share on other sites More sharing options...
johnj92131 Posted December 21, 2016 Share Posted December 21, 2016 i will try to fix it if not then i will just throw in a new one now i have to figure out how to get the stalk to work correctly Before you decide to just throw in a new tach - you need to know that your tach starts at the 8:00 position and goes to the 4:00 position. Most other year tachs you will find will not match the tack you have. AND all the other gauges in you cluster match the tach colors and type fonts. Not so with most other clusters you will find. The tach sweep is also different from later tachs. Link to comment Share on other sites More sharing options...
Eagle Posted December 21, 2016 Share Posted December 21, 2016 And, lastly, on the first generation clusters the instruments did not go to zero when you shut off the ignition. They stayed wherever they were pointing when the power was shut off. I'm not 100% certain that applied for the 1986 model year, but I know that was the case with the '84 XJ Wagoneer I once had. Link to comment Share on other sites More sharing options...
Eagle Posted December 21, 2016 Share Posted December 21, 2016 http://www.wranglerforum.com/f210/replacement-gauge-cluster-needles-221183.html Link to comment Share on other sites More sharing options...
AMC86Kid Posted December 21, 2016 Author Share Posted December 21, 2016 If I can't get it to work then I can always just throw in a whole new dash gauge display area right Link to comment Share on other sites More sharing options...
Blue88Comanche Posted December 21, 2016 Share Posted December 21, 2016 here is a pic of the Pot they where refferring too. Its the white thing with 103T on it Untitled by Blue88Comanche, on Flickr Untitled by Blue88Comanche, on Flickr Link to comment Share on other sites More sharing options...
AMC86Kid Posted December 21, 2016 Author Share Posted December 21, 2016 does the potentiometer have 3 leads on it or 2 Link to comment Share on other sites More sharing options...
Eagle Posted December 22, 2016 Share Posted December 22, 2016 The potentiometer is the white plastic thing that in the first photo says "103T" on it and has a + shaped opening, for a Phillips screwdriver. It doesn't have any leads -- it's soldered to the circuit board. If your tachometer has one, it's calibrated for 4-cyliner or 6-cylinder by turning that + shaped thingie. My friend in Greece went through that. What he found was that the adjustment range was basically all the way to one limit of travel for 4-cylinder, and all the way to the opposite limit for 6-cylinder. Depending on which way you're going (4 to 6, or 6 to 4), the difference is either a factor of 1.5: 1, or .67:1.Your tach is off a LOT more than that. Assuming your swapped-in engine idles at something like a normal idle speed, that's around 800 RPM. So a 6-cylinder tach should read 800 RPM. A 4-cylinder tach would read 1200 RPM. Yours reads 3500 RPM ... "calibration" is not your problem, there's something else going on. Link to comment Share on other sites More sharing options...
AMC86Kid Posted December 22, 2016 Author Share Posted December 22, 2016 So what does it have to be connected to as far as wiring goes maybe mine has a poor connection or no ground Link to comment Share on other sites More sharing options...
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