i've had a little gremlin in my tach ever since i bought my truck a year and a half ago. didnt notice it when i was test-driving the truck, of course, but not sure it would have stopped me from buying it even if i did.

so, about 25% of the time the tach works just fine. about 25% of the time it reads some rpm value that is clearly not accurate (somewhere between 1/3 - 1/2 of the actual value) and about 50% of the time it is just pegged at zero.

considering there are a few lights out behing my gauges anyway, i finally pulled the gauge cluster apart at lunchtime the other day. unfortunately, though, the fix for the tach was not obvious. i took it out, inspected all the connectors and circuit board componets, cleaned and closed up the contacts that it slides into in the dash and put it back in. it worked fine for about 5 minutes at idle, then bounced to zero and stayed there like usual.

the symptoms strike me as a bad ground or bad connection condition, but the question is obviously: where? in the tach circuitry itself, or in the dash/vehicle wiring?

i pulled it out again and tested all the sockets in the dash with my multimeter with the engine at idle, though i wasn't really sure what readings i should be getting. checking across the 'SIG' and 'GND' terminals in the dash, i got reading of ~7V DC, though this was independent of engine speed. ...idle = 7V, rev it up pretty good, still 7V. so, i switched the multimeter to its tach/dwell mode, thinking maybe it was a digital signal coming in. no reading in that mode. AC Voltage mode gave some nonsense reading as well.

i've since read through the service manual and the pinpoint electrical tests section and may have a few more things to try, but has anyone out there come across a similar problem? - and found a fix.

for anyone interested, the back of the gauge and mating socket in the dash are below.

also - any chance anyone out there has a parts trucking sitting around with a diesel tach guage they don't need? the easiest check is obviously to replace my tach with a new/known good one and see if the problem goes away. not sure what these things go for, though.





also - any chance anyone out there has a parts trucking sitting around with a diesel tach guage they don't need? the easiest check is obviously to replace my tach with a new/known good one and see if the problem goes away. not sure what these things go for, though.

 

LMC has the gauge

Truck Parts and Truck Accessories

 

I did see that, but for $200 i'd like to make sure the gauge is the source of the problem, first...

thanks though.

 

Check the grounds behind the drivers kick panel.

 

sorry for the dump question, but what is the kick panel? is that inside the dash or is just the lower portion of the firewall? or something else entirely?

 

Little panel next to the parking brake. Some major grounds down there including the one for tach.

 

thanks... i'll have to check there.

i did spend my lunch break checking things out a little more. first, i plugged my multimeter (set on dc volts) into the ground and power terminals in the dash (the two side-by-side lower ones in the image above. several miles of driving could not get the voltage to drop below ~14V, so i think i can eliminate suspected issues with the power and ground connections from the dash back to their respective sources.

i also played around with the trim pot (variable resistor) on the circuitboard of the back of the gauge. i set it all the way one direction, then put the gauge back in - still the same drop-out behavior. set the pot all the way the otehr way, put the gauge in - still the same behavior. so, it doesnt appear that that little guy had gone bad.

i did notice something on the back of the tach while i was playing with it though - there seems to be evidence that it might have gotten hot at one point in time. you can sort of see it in the image, above - there's a band of darker brown going from the top left to bottom right on the surface of the circuit board in the image. it seems like a coating on the circuit board might have melted...

if anyone out there in FTE-land happens to have their dash apart at this point in time, i sure would appreciate any pictures or descriptions of the back of your tach gauge. from this finding, i'm thinking i may be due for a new gauge, though...

i'll definitely check all the wiring going to the guage before spending $200 on a new one, though...

 

thanks... i'll have to check there.

i did spend my lunch break checking things out a little more. first, i plugged my multimeter (set on dc volts) into the ground and power terminals in the dash (the two side-by-side lower ones in the image above. several miles of driving could not get the voltage to drop below ~14V, so i think i can eliminate suspected issues with the power and ground connections from the dash back to their respective sources.

i also played around with the trim pot (variable resistor) on the circuitboard of the back of the gauge. i set it all the way one direction, then put the gauge back in - still the same drop-out behavior. set the pot all the way the otehr way, put the gauge in - still the same behavior. so, it doesnt appear that that little guy had gone bad.

i did notice something on the back of the tach while i was playing with it though - there seems to be evidence that it might have gotten hot at one point in time. you can sort of see it in the image, above - there's a band of darker brown going from the top left to bottom right on the surface of the circuit board in the image. it seems like a coating on the circuit board might have melted...

if anyone out there in FTE-land happens to have their dash apart at this point in time, i sure would appreciate any pictures or descriptions of the back of your tach gauge. from this finding, i'm thinking i may be due for a new gauge, though...

i'll definitely check all the wiring going to the guage before spending $200 on a new one, though...

 

I think I still have a gauge cluster in the shop. I can get you a tach for a lot less than $200 and I'm sure it will be better quality than the POS that you would get from LMC too. I will look tonight and see what I can come up with but I think I have the whole gauge cluster. I will let you know what I come up with.

 

awesome - thanks Nate. shoot me a PM on what you'd like to get for it. i had a sneaky suspicion you might have one.

 

Sorry man, I got 'nuthin! I must have sold the gauge cluster that I had.

 

oh well! thanks for checking Nate. anyone else out there might have one? i guess i'll keep looking for parts trucks on craigslist around here and keep driving around with my dash ripped apart. ...i never see powerstrokes in the junkyards here in PA.

i got to thinking though - i've been wanting to buy a scan tool for a while, so instead of a $200 replacement tach, i could just buy a scan tool, and mount it somewhere semi-permanaently to serve as the tach... we'll see, i guess.

gotta take care of a more serious problem that 'popped up' yesterday, though. ...gotta start the thread for it, first, though, since i'm stuck at work.

 

Finally sorted this one out for anyone who's interested.

turns out the two ground wires from the tach-side gauge connector were slightly bad, but that the tach was also bad.

with help from you guys (above), and the wiring diagrams (thanks TJ), i found that it was a bad connector about 2/3 of the way down the aluminum dash support just above/forward-of the e-brake assembly. somewhere between there and the dash, the ground wires in that tach-side connector merge into one big black ground wire - probably with a bunch of other grounds as well as that sucker is a big wire - maybe 12 or 10 gauge stranded.. so, from the manual 'pinpoint diagnostics' the resistance to ground at the gauge cluster connector grounds should be 1 ohm or less. mine was 2.7 ohms. not terrible, but not '1 or less' so tracking it back to the connector metioned above and cleaning and repairing the connection, got it down to 0.8 ohms. still not zero, but within spec now.

so, with that fixed, i put the gauge cluster back together and back in the truck - but the tach was still dropping out on me. maybe the bad ground had killed the tach over the past 16 years? who knows, but at this point i was pretty certain a new tach was needed.

a lucky trip to the junkyard on Friday and i had a whole new (to me) diesel gauge cluster. i just plugged the whole thing into my dash first and the tach worked perfectly for the ride home. however, the speedometer didn't work at all (!!?!?) and the odometer read the mileage from the truck it came out of. i'd have thought the PCM told the odometer what to read, but i guess not.

anyway, after frankensteining the junkyard tach into my gauge cluster, everything seems to be working properly - of course another bulb in the cluster had blown from all the handling and swapping, so its been in and out once more since, but should be in for good now. time will tell, i guess...

 

Odometer memory is in the PSOM.