Tach only works when cranking

[Kalashnikov]

89 F350 7.3 IDI E4OD-I'll update my sig later

Tach randomly stopped working and of course sent my E4OD into open loop, OD light on, locked converter in D, etc. Tach sensor is good/new, harness from sensor to fender tested good, swapped relay 3 times, all fuses good. The strange thing is the tach works when cranking but after that it stays at zero. Ideas?

 

[rhkcommander]

Loose connection n dash or firewall probably. Mine works a third of the time tops an not accurate. A hard bump or acceleration will make or stop it from working. If you hit the dash hard enough you might be able to get it on

I noticed it's a few issues in my case, including bad ground - lights on/off make it move about 50 rpms

 

[icanfixall]

Might be the ignition switch too. These switches have severl places where power is directed to differant electrical needs. When you attempt a start all the power goes to the starter and what the engine needs to start. Probably the tach is wired in both the start and run locations on the ignition switch. Maybe its not making contact in the run position on the switch.. Then when you release the key it rolls back to ignition on and power flows to all electrical needs like radio etc. Have you been experiancing any odd ignition issues.

 

[Kalashnikov]

Loose connection n dash or firewall probably. Mine works a third of the time tops an not accurate. A hard bump or acceleration will make or stop it from working. If you hit the dash hard enough you might be able to get it on

I noticed it's a few issues in my case, including bad ground - lights on/off make it move about 50 rpms

That was the first thing I tried!

Might be the ignition switch too. These switches have severl places where power is directed to differant electrical needs. When you attempt a start all the power goes to the starter and what the engine needs to start. Probably the tach is wired in both the start and run locations on the ignition switch. Maybe its not making contact in the run position on the switch.. Then when you release the key it rolls back to ignition on and power flows to all electrical needs like radio etc. Have you been experiancing any odd ignition issues.

Ignition has been fine except it'll stick once in a while sometime just enough that the water in fuel light is on and sometimes so the starter stays engaged. All I have to do it tach it a little and it'll release and be where it is suppose to. Volt gauge doesn't work properly and will move if I hit the dash. it always read low, anywhere from just below half to only like a 1/4. Temp doesn't work. It'll move up to the line at least if I hit the dash. Oil pressure works fine.

where are the ground for the cluster/how can I test the actual gauge in the cluster to rule it out?

 

[rhkcommander]

Hitting mine is how I fixed the sticky volt meter and how I fixed my friends blower motor that wouldnt come on. *Whach whack -vrooom* problem solved .

if I was home I'd post a picture of the back of the tach gauge in the cluster. There are 4 connections, labeled and everything on the traces nearby IIRC. I wanna say 8 S B and another...

edit, found it:
"S" goes directly to the sensor.
"B" goes to a fused power source(they show it going to the 10 amp warning lamp circuit fuse).
"G" goes to ground and the other wire of the sensor is grounded also. Instead of grounding it at the sensor location they show the other sensor wire grounded near the tach plug.

but i also heard for similar gasser tachs:
1 or C =Hook the "Tach" (neg side of coil) wire here.
B=This goes to 12v switched power
G=Ground if your truck has an Inline 6 or V8.
And also
8=Ground only if your truck has a V8.

Test the tach sensor first, its easier. At idle the voltage should be ~ .85V AC from what I've read. Theres a thread on testing it ala:
http://www.oilburners.net/forums/showthread.php?t=54186

Let me see if i can find any of the threads I participated in on google for the tach gauge outputs