NE frmhnd

Up to 90 there is a regulator on the firewall above the exhaust. 91-up the PCM controls charging. You can convert it to internal regulation, I think Jim Lane has a sticky on it, and you can convert it to the old style regulator as well, also in the sticky.

Mark Nixon

Actually, all 1991s have the voltage regulator on the firewall, as well.
The only 1st gen era DIESEL trucks to have a CEL are 1992 and 1993.

There are reportedly some very late run 1991s without the regulator that have the later ECM and the crank sensor, but I have yet to actually see one, much less seeing one that was actually a 1991 built truck, with the 1991 emissions label.

Mark.

NE frmhnd

Oh yeah, 1990's have a fish excrement electrical system don't they?

Mark Nixon

Not sure what you mean by this?
The electrical on the 1990 diesel trucks is much simpler, but the 1991 non-intercooled trucks is very similar.

Mark.

NE frmhnd

It's a 1-year-only deal.

Mark Nixon

In reality, the engine harness is a 1-1/2 year deal, the '90 and early '91 harness interchanges.
Also, you CAN use an intercooled 1991 engine harness on a '90, as well, because the pinouts are the same.
Either way the dash harness recognizes both.

But we're sorta' straying off-topic, here.

Mark.