I have had problems with my tach acting like it has a loose wire. It works fine for a while then starts bouncing around then starts working again. I don’t think the problem is in the tach itself, when problem occurs , cruise control is also erratic. Occasionally the tach quites completely for few seconds and this causes the check engine light to come on and checking the trouble codes gives a lower than expect engine speed error.

At about the same time this problem started a glitch with the A/C appeared. The problem with the AC is that there is not steady 12 volts to the clutch. Oddly, it works reasonably well around town but kicks in and out on the highway, it’s not the over pressure switch, have checked that already.

I have check and cleaned connections with no change. I am beginning to suspect PCM may be the problem but am hoping to find a cheaper solution.

96 2 wheel, automatic, 250,000 kms.

Any suggestions appreciated.

 

we had the same problem, just not interminent, twas aways like that, snince we bought it, tranny didnt work good either!

we ordered a PCM, no success! after a few weeks we found out it was the crank sensor!
Check conections, and or change crank sensor before anything else! it also has to be very close to the pulley (wheel) to work properly!

its on the front of the engine, on top of the lowest pulley!

hope it helps!

 

I second the crank sensor idea. The cruise control, tach, and a/c will all fail without that. What happens with the a/c is that while it's not directly dependent on the crank sensor signal, the pcm will think the truck is not running if it loses that signal. If the truck is not running, it does not command the compressor clutch to lock up.

 

Since my original post I purchased a used PCM and gave it a try a couple of weeks ago, no change and since I assume there is little chance of the second PCM having the same intermittent problem I returned it and assume the problem is not there.

Last week I took it into dealer, spent $120.00 and they couldn't find problem although they did see the problem occur. Mechanic suggested problem is probably in crank position sensor, so I purchased a new one, $105.00 and installed it today. Went for a test drive, appeared OK at first but about 5 miles down the road same problem.

So I am back to checking connectors and grounds and hope I find something I missed. Was really hoping the dealership would have more insight into troubleshooting and diagnosing problem but they don'r seem to be able to suggest a solution and I'm getting tired of paying to find nothing.

Any ideas?

Dave D

 

Hey I am having the same problem with my 98 ext cab 12v 5 speed. All of my gauges flicker on and off and check engine lights, wait to start light go on and off as I drive down the road. Left me begging for a jump start at Wall-Mart this afternoon. First time it has drained my batteries. I removed the alternator Had it test off the truck they said it was bad. So I replaced it. Same symptoms exist going to look at the CPS tomorrow just curious if you ever solved yours?

Thanks Chad

 

Rockwagon, I have been having the same issue. Since my issue is all of my gauges and not just my tach I don't think it's CPS related. I think it's a grounding problem. My truck died the other night, but fortunately it was running so I was still able to get home. I'm frustrated as well.

 

OK, funny as it sounds, i had a problem with my, tach inop, power windows inop, ac/heater blower inop, power mirrows inop.

I looked and looked every day for over a week. Finally i took it to the dodge dealer they troubleshot it for about a hour and half. came in and said they finally found the problem.. Cost me $105.00

BLOWN FUSE LINK. Yep $105.00 blown fuse. Machanic said the way the link was seperated allowed it to make contact and vibration would allow it to touch and then open, touch and then open.

It was the "eng run" fuse located under the hood in the power dist box.

Sounds crazy but it works now. (who would think all of that would be dedicated to that "named" fuse???? eng run)

Hope it helps you.

 

lgp9999
Keep me posted on what you find as I will post what I try and find. Going to mess with it for a couple of hours today.

jtandgwen Thanks for the heads up I will start looking there and other links as well

Wish me luck

 

I made the original post and guess I was negligent in not posting the solution once I solved the problem. Turned out my problem was somewhat unique so don't if it will be helpful but here is the rest of my story.


After spending way to much at the dealership I decided I had to troubleshoot the problem myself so I borrowed a portable oscilloscope and started monitoring circuits while I was driving. After only a few minutes of test driving I found that the 5V power supply to the crank position sensor was intermittent so for a quick fix I cut the wire and ran a new wire from the sensor to the 5v supply on the PCM. This fixed my problems so I drove with this temp fix until I had a few days where I could rip apart the wiring from the sensor back to the PCM to find the actual source of the problem.

There is short piece of rubber fuel line on a fuel return line located behind the head of the engine and just getting far enough under the firewall that it is hard to work on. Prior to experiencing my issues in this post I had a problem that after sitting overnight the engine was hard to start. The problem was that this short piece of rubber hose had a crack and was allowing air into my fuel lines and fuel in the injector pump was draining back to the tank and hence the hard starting after sitting for 12 hours or so. Turns out this is a fairly common problem and I have needed to replace this hose twice since I have owned the truck.

I had this repaired at the dealership the first time and when the mechanic cut the old hose away with a knife he also unknowingly (I hope) cut away the insulation on several of the wires running beside this hose. One being the 5V power supply wire to the crank position sensor. Eventually the bared wires started to make contact, shorting the 5V supply causing the tach to drop out and the other issues I explained. I repaired all the wires and haven't had a problem since. I still own the truck but now only use it in the summer and fall for yardwork, hunting and misc stuff, still runs great and I still love the old 12 valve engine.

As I said this problem was unique and may not help you out but thought I should follow up.

Dave D in cold Saskatchewan

 

OK I believe I solved my Problem. On the positive post on the driver side battery there are two fusible links that were crimped in to the same eyelet connector. This went too the battery terminal bolt. The eyelet was about corroded in two. I put two new eyelet connectors on the fusible links and the problem seems to be solved. Thanks for the help

 

Well I definitely have some direction that I did not before. I will take a look when I can get 5 minutes without rain.

 

It's not all that unique.
Yours is the first I've heard of cut wires but have run into rubbed though wires in the same area a couple times.

 

Well, mine does the intermittant tach thing as well, but ONLY on cold mornings. All summer it works great. On a cold morning the tach will bounce to zero erratically, works a little better when warmed up, but if it's really cold it can happen any time. Also, when the tach is doing it's weird thing, the lights will dimm a little every time it bounces to zero. You think it could be those wires behind the fuel return line? PS Bill, thanks for the advice you gave me a few years ago suggesting the fuel shutoff relay was why sometimes wouldn't start. Ordered a cheap relay from the source you suggested, changed it out and no more starting probs.