I have a 1995 Toyota pickup, and am the original owner. I have used the jumper method to read trouble codes on the dash many times in the past. Now I can't get the check engine light to flash. I have at least one problem going on, the check engine light is lit during normal use. When I have the jumper in place, the check engine light just stays lit, does not flash. I need to get this vehicle inspected. Any ideas what the issue is?

 

My guess, off the top of my head, would be the ECU. Just a guess, though.
Pat☺

 

I'm hoping not, but suspect the same.

 

So, when you put the key in the ignition and turn to "ON" the MIL light comes on? Then shuts off a second or two after starting?
The ecu can check many things, but it can't check itself. So Pat's logic is correct. But I'm always hesitant to condemn an ECU.
I'd open up the ECU and inspect the circuit board for any obvious issues. Depending on what you see/smell (or don't) might help with the next decision.

How's the wiring loom around the battery and fuse block under the hood and under the dash? Fairly intact, or hacked with wires twisted together and covered with electrical tape?
Some pictures of the engine bay and right kick panel may help our thought process.

 

You are correct, the MIL light burns whenever the key is on.

A little more history, several years ago, there was damage to the wiring harness by rats. The motor would randomly shut down, completely loosing ignition. A short on top of the gas tank was repaired by my local Toyota shop after much time and diagnosis . They did a poor job of putting the wiring back, under the fuse block, its a mess. The most recent issue started with the MIL light flickering or randomly coming on for short periods. This came with a noticible loss of power, and when flickering, could jar the truck at lower RPM's So I'm anxious to diagnose it. This was going on for several months, and is when I discovered I couldn't read codes on the dash anymore.I suspect another short. It would make sense if the ECU was damaged. The light came on full time a couple of weeks ago.

It came home from the Toyota house with the vacuum hoses misrouted to the EGR valve and I never took the time to fix that. It has thrown a code for EGR fail in the past and it's possible thats why it's lit full time now. I need to reset it and see what happens. Maybe that will fix the issue with reading the codes on the dash too. (I can hope, right?)

 

Thanks for the additional insight.
I'm not a fan of crimp wire connectors, and think those wretched quick-splices should be banned. I’d sit down and start going through all the wiring and properly repair each connection. I solder and shrink tube my repairs, but I might start using open barrel crimp splices covered with shrink tubing. Proper wiring repair is time consuming, but done right you’ll never have to worry about it. Eastern Beaver can get you excellent Yazaki brass connectors and crimp splices of all the crucial shapes and sizes. Ships from Japan, but he doesn’t dawdle
Vacuum tube routing looks daunting, but it’s not that bad. Post pictures of the components in question and we can help get it routed correctly.

 

I was a radar tech, and E-Shop Supervisor, my entire career. Not counting a couple of years as a rifle/pistol marksmanship instructor in the Corps. I am whole-heartedly, 100%, behind the solder/heat shrinks method of wire repair. I got some good soldering schooling, however, and I got good at soldering. To me, it's the only way to go.

You need to spend a little to get established with a decent/good soldering kit, but it pays for itself very quickly. Do a couple wire repairs, and the kit has paid for the cost of assembling it. Always use the meltwall, or FIT-300, type heatshrink for automotive applications, BTW. It really pays off in the protection it provides from the harsh environment that is automotive.
Also, once you get some experience in soldering, learn to recognize, and repair, a cold or bad solder join, etc, repairing wires by soldering them is quick and easy. Very reliable, too. Technique is the secret. I can't tell you how many cold, globbed on, solder repairs/component replacements in ECUs, or even just wires, by people that can't solder to save their lives, but sure THINK they can. Very frustrating to me to see garbage like that.

Ok, ok shutting up now. Good luck, and if you need advice, let me know.
Pat☺

 

I had done some very careful and clean repairs to damage that I could find. Heat shrink tubing and all. The Toyota house tore it all apart and returned it to me without fixing it back. Those guys were butchers. I was told the short was against the frame over the gas tank, but "he managed to fix it without dropping the tank." I am having fuel flow issues now, either the fuel pump or filter, or both. So I've got a great excuse to drop the tank and look at the wires.

 

Worth dropping the tank and inspecting, though that was never a common area for rodents. But hey, they can access just about anything under your vehicle if they want.

Rodent damage is a pain to track down. The most common areas they nested were behind either headlights or around air cleaner box.
The EWD manual shows the Check Connector, along with the wiring running to the ECU. Besides cleaning up the wiring around the fuse block you may want to get a schematic and start checking the wiring at the Check Connector. I think you just have an open circuit somewhere.