Factory tach from hell
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Junior User
Joined: Dec 2012
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I am so upset. I have no idea what to do about this problem anymore. I replaced the tach this evening with the NOS tach with the microchip and everything was going fine until I started up the truck and the tach turned on, yes, which was great, and idle is perfect, but the revs do not translate at all, I put my foot to the floor at idle and it only goes to 3000 rpms. The tach is also very slow to translate RPM changes. I checked the v8 ground, It is there, I checked the power going to the tach, it is there, I checked the regular ground, it is there, and I checked the coil input and it is there. the coil voltage is 6-10v. so bummed...
Cargo Master
Joined: Mar 2008
Posts: 2,569
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From: Washington
I am so upset. I have no idea what to do about this problem anymore. I replaced the tach this evening with the NOS tach with the microchip and everything was going fine until I started up the truck and the tach turned on, yes, which was great, and idle is perfect, but the revs do not translate at all, I put my foot to the floor at idle and it only goes to 3000 rpms. The tach is also very slow to translate RPM changes. I checked the v8 ground, It is there, I checked the power going to the tach, it is there, I checked the regular ground, it is there, and I checked the coil input and it is there. the coil voltage is 6-10v. so bummed...I use a cigarette lighter voltmeter like this one. Its about $2.50 on ebay.
http://www.ebay.com/itm/111411914237?_trksid=p2060778.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT
When the tach acted weird the voltmeter also did, rapid fluctuations. voltage jumping around.
You could just disconnect the ALT and see if it work correctly.
good luck
Jim
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Thank you for the advice but I had just got the alternator fixed along with the charging system by a car electrician a couple weeks ago. I mentioned it in the thread. So It couldn't be that problem.
Thank you for the help though. Ill take all the help I can get.
Thank you for the help though. Ill take all the help I can get.
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Junior User
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Should I go and spend the money on yet another tach from ebay?
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At the very minimum, at least I know the wiring in my truck is on point I guess.
OH BTW Gary, I made that ICVR from your walkthrough and I have to set mine to 6.2V for my gauges to read properly, I filled up my tank all the way and it still read 3/4 tank at 5.4V, so I went home and took a multi meter and looked at the gauge and found out it needs 6.2V. The temp gauge and oil pressure gauges read correct now too. IDN if that would maybe explain the tach? Coil voltage being 1V lower that normal I HAVE NO IDEA, just brainstorming.
The old tach would read right sometimes when it wanted to work, so it couldnt be that, could it???
I was thinking about it and the tach reads 1000 rpms off of what it should be (from the readings I was getting from the old tach when it wanted to work), 2000 rpms at 45mph, when it should be around 3000.
Is there a switch on those newer tachometers with the microchip that you are supposed to flip to tell the tach that it is installed on a v8? I know the aftermarket tachs are like that, but I thought I would ask before I tear the dash apart again for the billionth time. haha
OH BTW Gary, I made that ICVR from your walkthrough and I have to set mine to 6.2V for my gauges to read properly, I filled up my tank all the way and it still read 3/4 tank at 5.4V, so I went home and took a multi meter and looked at the gauge and found out it needs 6.2V. The temp gauge and oil pressure gauges read correct now too. IDN if that would maybe explain the tach? Coil voltage being 1V lower that normal I HAVE NO IDEA, just brainstorming.
The old tach would read right sometimes when it wanted to work, so it couldnt be that, could it???
I was thinking about it and the tach reads 1000 rpms off of what it should be (from the readings I was getting from the old tach when it wanted to work), 2000 rpms at 45mph, when it should be around 3000.
Is there a switch on those newer tachometers with the microchip that you are supposed to flip to tell the tach that it is installed on a v8? I know the aftermarket tachs are like that, but I thought I would ask before I tear the dash apart again for the billionth time. haha
FTE Legend
Joined: Jul 2010
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From: Northeast, OK
I don't think there's a switch on them, just the V8 wire. But, let's explore this a bit as your comment of 2000 RPM at 45 when it should be 3000" tells me the new tach may be correct. This site has an RPM calc and, not knowing your rear end ratio or tire size I plugged in mine of 3.5 and 29". That gives 1874 as the RPM, which is pretty close to 2000. And, as a check, I then told it to calc 65 MPH and got 2706. Yesterday when driving Rusty I saw 3000 on the tach at 65 MPH, so that's pretty close. Maybe your tach isn't bad?
As for the ICVR, I don't know what to tell you. In my testing of several instrument clusters with fairly precise resistors simulating the senders I found that 5.4v gives full scale with a 12 ohm resistor, which is what the book says the gauges are to read with that resistor. But, if 6.2 volts works for you then so be it.
As for the ICVR, I don't know what to tell you. In my testing of several instrument clusters with fairly precise resistors simulating the senders I found that 5.4v gives full scale with a 12 ohm resistor, which is what the book says the gauges are to read with that resistor. But, if 6.2 volts works for you then so be it.
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WELL.. I am confused. I used this RPM calculator
Engine RPM / Engine Speed Calculator
in addition to the other one that you sent gary and today I noticed on the freeway I was going 55 and the RPMS were at 2000, almost on the line, and that is exactly what this calculator says at 1955 rpms.
My axle gear ratio is 2.75, and my tire diameter is 26 inches.
GUESS IT ISNT BROKEN??
I am just flabergasted on how low the trucks RPMS are when the truck sounds like it is screaming. what is the rev limit of the 302 from 1980? I put my foot to the floor in neutral and only get it up to 3000-3500ish, dont hold it very long cause i dont want to blow up the motor. but yeah, are these 1980 v8's really just low reving engines? I had a 1985 300 straight 6 and that engine would sound like it was screaming at 2500, is the 302 the same way?
Engine RPM / Engine Speed Calculator
in addition to the other one that you sent gary and today I noticed on the freeway I was going 55 and the RPMS were at 2000, almost on the line, and that is exactly what this calculator says at 1955 rpms.
My axle gear ratio is 2.75, and my tire diameter is 26 inches.
GUESS IT ISNT BROKEN??
I am just flabergasted on how low the trucks RPMS are when the truck sounds like it is screaming. what is the rev limit of the 302 from 1980? I put my foot to the floor in neutral and only get it up to 3000-3500ish, dont hold it very long cause i dont want to blow up the motor. but yeah, are these 1980 v8's really just low reving engines? I had a 1985 300 straight 6 and that engine would sound like it was screaming at 2500, is the 302 the same way?
FTE Legend
Joined: Jul 2010
Posts: 32,875
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From: Northeast, OK
Well, it sounds like the tach is ok. But, you may either have no fan clutch or a frozen one. The 302 has always been a high-winding motor as its short stroke doesn't give much torque until the R's are up. But it shouldn't sound like it is winding - unless the fan is running at engine RPM. That would happen if you don't have a fan clutch or if yours is bad.
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That's probably it. With your gears and with stock tires, you should be around 2K @45mph in top gear. My 80 302 with a fixed 5 blade fan IS fairly loud once you get above 2500-3K rpm. .Until the intake and exhaust roar drown it out 
I have the Granny 4 speed, 3:50 gears and 31" tires.
I have the Granny 4 speed, 3:50 gears and 31" tires.