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Discussion Starter · #1 ·
My 1996 Dodge 2500 CTD 4x4 5 speed has alittle problem. I just bought the truck yesterday and the tach has been acting up. The tach will bounce up and down and the gen light will come on, the check engine light and some other lights like the wait to start and the small light beside it. The volt gauge in the dash seems to still read ok though I was just wondering a possible solution
 

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Tach signal comes from PCM and PCM gets it's signal from engine speed sensor (ESS). You could have a failing ESS, PCM or wiring issue. Intermittent problems can be a nightmare to diagnose and repair. I guess you could also have a bad ignition switch, loose fuse somewhere or something else along these lines.
 

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Discussion Starter · #4 ·
ok somemore on the issue it comes on and goes off and the gen light and check engine light is on along with the WTS light and the one next to it. It only does this idleing and as soon as I start down the road it reads just fine, but as soon as im back to idle it goes to zero I can rev it and it jumps up alittle. The volt meter seems to be lower at idle but at road speeds its at 14. Im totally lost on this. on occasion the engine oil pressure is jumping around also. Could this be charging related. Im going to clean my battery connections and load test seperatly. The batteries have some corrosion so ill start at the simple thing first. How much is a PCM and tach sensor??
 

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charging is controlled by the PCM, The PCM goes 'dead' when it does not see a minimum 500rpm (IIRC) tach signal from the ESS.

A new PCM is super expensive, dont start throwing parts at it!

The ESS is up by the harmonic balancer, check its adjustment, should be appx the thickness of a business card away from the balancer.
 

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You might try making sure the gap between ESS and harmonic balancer is set correct. It should be .049" to .051". Everything you describe as wrong seems to point at a problem with the ESS. You don't even want to know what PCM costs. Check gap first and then go from there.
 

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Click> 1996 DODGE FACTORY SERVICE CD-Dodge Cummins Diesel Truck Accessories – Geno’s Garage

Above link is one of the most inexpensive place to get a factory service manual. The PCM is located on the firewall behind the stock air cleaner box. Completely remove the air box (takes a minute or two) and look back towards firewall and you will see the PCM under a little plasic cover. The factory service manual has a complete wiring diagram and is priceless in some repairs.
 

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Discussion Starter · #11 ·
I checked the ESS last night I ohmed it and check the one in my other truck and they both read the same but its intermittent when the truck acts up so it could be vibration. Also if I give it just a touch of throttle the lights will flash on and off and ive noticed my oil pressure gauge acting up sometimes. Im gonna clean everything good wed night and maybe I can get it to help just looking for more insight on the cituation. Also when the truck is rolling just alittle the tach works fine even if im just rolling 1 mph in neutral. Also my ABS light is on and stays on...
 

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Discussion Starter · #13 ·
well to update I replaced both batteries as one was weak. I cleaned and checked every ground in the engine area. I cleaned the terminals and replaced anything that was too corroded. My problem with the tach fluttering is getting worse. I can unplug the ESS and the tach goes from 1000-3000 RPM but when I plug it back in it reads 0 if I rev it up it might get to 1500 and going down the road it bounces everywhere and the lights go on and off.
 

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Discussion Starter · #14 ·
So does anyone have any insight on the issue. Should the tach still flutter with the ESS unplugged I didnt know if someone was willing to unplug theres and see how there truck reacts to help me out alittle.
 

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I would think with ESS unplugged the tach would not work. The ESS sends a signal to PCM to let it know what engine RPM is and the PCM in turn gives signal to tach and tell it how far to go up. With ESS unplugged the AC, CC, alternator charging and other gauges in instrument cluster should not work. Reason for them not working is because the PCM doesn't know the engine is running so it won't send a signal to them for correct operation. Sounds like an issue with wiring or PCM itself. The wiring just takes a long time going over to properly check. The PCM may take a trip to dealer to properly diagnose or if you have a buddy nearby that will let you borrow his for about 15 minutes. Having a truck nearby also lets you check and verify on yours immediately.
 

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Discussion Starter · #16 ·
Yeah all my buddies have 24v's mostly. Well the only thing that messes up now is the tach it just goes crazy if I throttle in to it hard the tach will come up and alternator will charge but as soon as it comes down to idle it drops to zero and bounces to 1000 rpm. Im gonna trace wires possible install a new ess if it doesnt work ill have a spare then.gonna do some wire tracing. Do the wires from the ESS goes directly to the PCM and are they the same color at the PCM connector???. If someone reads this and doesent mind unplug your ESS connector and tell me how you tach reads.....
 

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i would be willing to be this has nothing to do with the ESS, unplug your PCM for 20 or so minutes, it should restart its self when plugged back in IIRC. that and you need to inspect plugs and wiring, either the PCM is just havin a bad day, or somthing is feeding the PCM bad signal and throwin it off. I HIGHLY doubt this, but pull the black cover around your dash and unscrew your gauge cluster and check whats behind it and make sure its all clean. and finally cycle the key 3 times( on off on off on) and the pcm is gonna show you some trouble codes by way of the check engine light. you have to count the number of times it flashes and translate that through a code book. lemme try and find it and ill post a link.
 

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Discussion Starter · #19 ·
the only code I get is..Direct Battery input to PCM disconnected within last 50 ignition key-on cycles. Normal if battery has been disconnected, otherwise check battery power and ground connections. does anyone know what wire the PCM gets its power from so I can check this and also the ground
 
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