Freightliner Century 1999 I can't idle up......

If it was equipped with opti idle, I’d find that neutral safety circuit and jump it - if the problem arose directly after the transmission swap - I think that’s the culprit.

I had the exact same problem with my 95 Pete - but my problem was that someone had hacked into a ground circuit off the park brake sensor circuit - my truck also had the opti idle from factory - but hood sensor had been removed already.

Let us know the results when you find out - help others that search later.

 

That 2 wire connector, if I had to guess. Check it’s ground with a multimeter, (just in case it’s positive voltage) and jump , then test.

Frustrating problem.

 

Your asking about fast idle & also state you have optimized idle. I noticed the one of the safety interlocks is transmission range needs to be in high for optimized idle. Here's the trouble shooting for that if that might be helpful.

 

My 13 speed has two switches on the top, one on either side of the shifter. One is backup reverse light switch, the other is neutral safety swich.

Does your reverse light turn on when transmission is in reverse? If not try switching the connectors.

You can access the switched from inside the cab. Remove the boot at the bottom of the gear shifter.

Test by shorting/jumping the switches.

Perhaps the switches were not connected when the transmission was swapped.