What is the model # and engine. What is the truck doing..no start.???? you cab add 12 volt feed direct to coil from a relay. If that gets it going and it runs,

Please help me I can't figure out how to post a question but I have a GC 25K forklift propane it has very low voltage to the coil 6 V a mechanic of mine told me I needed a new ECU do you have any recommendations

a few other things to note about this system.
this system will give the idle speed only if there is any problem with the accelerator pot, which includes if the setup with the computer is set close to the published specs, there can be a real lead foot that bends down the accelerator pedal and creates a reading that the ECU thinks is the accelerator pot out of range, and then shuts down to idle only, and if someone messes with the pedal stop bolt on the floorboard under the accelerator pedal trying to get the machine to go faster, that can set the accelerator pedal out of range error too.
As Jplayer noted, the new ECU does not require the vac hose, and you can tell which is the new version, when you look into where the round cover that you remove to hook up the computer, if you see the resistors, transistors and diodes, then it is the newer version with the electronic MAP function, and if it is all beige/white plastic "potting", then it is the older version that NEEDS the vac hose to be right and on the correct port of the throttle body.

it is important to keep the correct size vacum hose on that MAP sensor line, the MAP pressure has to be within a certain range for it to work correctly. Also make sure it is conneted to the correct port on the throttle body, MAP Sensor line should be on the bottom port and the top port is for the Vacum Advance on the Distributor.
In the service grams on this system they mention many times about making sure the vacum lines are not damaged, cracked or connected incorrectly.
Another issue they mention about this same symptom is an incorrect dwell setting, incorrect dwell setting will force the system into open loop and not accelerate, also the dash MIL light should be on in this condition.

And to properly setup this S15G LPG system you should use the laptop program, it would be very difficult to get all the settings within specs without it.

As for trying to replace that ECU?
good luck with that, they did away with that vacum operated MAP sensor and went to a total electronically controlled ECU MAP function.

That would make sense, because before I left the 1st time I secured the vacuum firmly on the ECU. The second time I replaced the hose to a smaller fitting hose

The ECU vacuum line is the map sensor which with manifold vacuum tells the ECU to switch from cranking mode to running mode. If there is no vacuum the ECU thinks you are still cranking and does not allow throttle. This system had a few bugs and this is one of them.

When I arrived on site, I placed the ECU vacuum hose from the throttle body, directly to the intake manifold. I then returned to the shop and ordered the pot sensor assembly.

Returning on site days later w/ the part; I asked if the problem returned. The operator(s), (who happen to be as intelligent as very young camels) replied " no".

I still replaced the pot, and check the voltage sweep. It was basically the same as the original. I did notice, that on start up, the engine slightly rev higher momentarily then dropped to normal.

The problem has not reoccared, so the answer is a coin toss. Too many variables to know for certian.

Did the pot fix this problem?

ok
put a pot in it

:o)

I tied the vacuum line directly to the intake manfold. I think the issue is related to the pot. When the pot is set at below.3/.25v, as read at the ECU wire, then the problem occurs most of the time. The pot will not give a full sweep of.25 - 4.7v.

If the pot is set at.5 -.7v idle, the problem is less frequent.

Thank you for input Jplayer

yep it does give me some more thoughts :o)
that has the vacum line connecting to the ecm directly so this is actually pre tier1 or early tier 1, the later versions of the tier1 updated came without the vacum line and calculated dwell via the ecm instead of reading the vacum from the intake/carb/mixer

check the mixer diaphram, if its dirty clean it, pay close attention to where the button in the center seats in the center, if the seat is dirty it will not close properly and cause a leak inside the mixer. It will not want to idle very well if this is found.
clean the mixer body and also down inside where the butterfly in the throttle body is. You can use brakecleaner or if your not comfortable using that just use some carb cleaner.
also check the vacum line going from the carb base over to the air cleaner pipe at the engine, does it have a check valve inline? not the pcv valve but there is another line that should have a check valve inline the hose that goes over to the air intake hose just on top of the engine. I think the older versions had a check valve in the fitting where the hose connects at the air intake hose on top of the engine where the bracket holds the hose to the valve cover.

it sounds to me like your not getting vacum on this line to the ecm, ya wanna make sure the hose is intact and no cracks. make sure the pcv valve isnt clogged or stuck, if there is that vacum check valve in the hose to the air intake pipe make sure its not damaged or dirty or blown out.

if you find all the above good then there could be something causeing low vacum like bad valves or lifters, worn out engine.
I'm just throwing out all this stuff to give you things to check that MAY cause this problem.
If anyone else can think of anything please chime in ;o)

Good Luck

Thank you Jplayer. In one instance, when the truck was in slow mode, I took the vacuum hose that was attached to the ECU and manually *** a vacuum in it while sealing off the vacuum port.

This allowed the truck to accelerate, any thoughts?

if the ecu detects any voltage abnormalities in the accel circuit it will put the engine into 'slow mode', this will also happen if there is a problem with the throttle body sensor voltages.

there is probably nothing wrong with the ecu itself.

try replacing the accelerator pot first
if that does not change it then the throttle body is next
if the internal mechanism in the throttle body gets dirty and keeps the sensors from making thier full sweep, or in some cases i have seen the wiring connections to the board inside them break, this can cause it to error out and put the engine into 'slow mode'. And as far as i know on those particular TB's they are not accessible, they are riveted shut and are not meant to be accessed and repaired or cleaned.