I need to pull off the connectors on the ECU to make sure they aren't corroded and making poor contact before I'm completely sure the ECU itself is the culprit. Here's what's going on....

90 NA vert

The TPS check connector (passenger side near pressure sensor--3 pin) has 12vdc on one pin (main relay connection) and about 4vdc on the others (goes to pin 2O and pin 2P on the ECU). These two pins also connect to the "solenoid relief" and "solenoid switch" coils. The TPS cannot be set properly because I have voltage on all three pins.

I have the following ECU check engine codes coming up....
17 feedback system, possible O2 sensor
18 TPS fixed full open
31 solenoid relief valve
32 Solenoid switch valve

The car will also stall at idle with the AC on (hot day only). I tested and cleaned the BAC and it does work. I just bought a 10K resistor today and will measure the voltage tonight.

What do you think....bad ECU?

Does anyone know where to have them rebuilt or should I just buy a used one (6 mo warranty) from Mazda Recycling or Mazmart?

Do the people that install the performance chips, like MDS, rebuild the computers to meet factory specs or do they just solder in a new chip?

 

If you've ever had moisture on your floors, there's a very good chance your ECU connectors are corroded.

When I pulled mine off ('90 vert also), they were green/blue like the Statue of Liberty.

To clean them up, soak the wiring harness connector in Tarnex for 30 minutes, then dry it and spray it with WD40.

For the ECU pins, you can take a toothbrush and clean them with baking soda and water. Then dry them and rinse them with WD40.

If your lazy, you can just keep removing and reconnecting the harness until the pins rub their way thru the corrsion to a good connection.

Make sure you disconnect the battery and discharge the system, by pressing and holding the brake pedal for 10 seconds before messing with anything.

 

Thanks! I would have forgotten to discharge the system as you noted. And my carpet did get wet a few times...

oops!

My 4VDC measurement was wrong! Bad DVM ground. It's actually 12.5 vdc on all three pins (DVM reference now battery neg) with the ignition in the on position. Disconnecting the TPS connector makes no difference. Looking at the TPS 6 pin connector (engine side) I measure the following voltages with the ign ON (TPS is disconnected):

Top Row (left) beginning with
green / red wire (pin 2F on ECU?)= 0.04
center pin = 5.0
far pin Black / green (pin 2G on ECU?)= 1.83

Bottom row beginnning under green / red wire,
0.02
0.02 center pin
5.0 far pin

I also tested the BAC with a 10K ohn resistor across the pins (engine side) and meaured 6.5 vac which fits with other measurements I found on this site. I've had this off recently and it functions OK with 12 vdc applied.

 

I opened the ECU and here's what I've found....

Pin 2O (solenoid switch): the first component on the trace is burned. It's blue and looks like a resistor, but I don't think it is. Maybe it's a protective device of some kind. Hard to read the lettering.

Pin 2P (solenoid relief): the first component in the trace is burned. Same as above.

There are diodes next in series and they check out OK with the DVM.

What are these and does anyone know the value and where to buy them. Any idea of the cause?

I found a place that rebuilds computers for $250...1 day turn around. 800-254-3556. Hate to spend the money if all it takes is a $2 component.

 

You could just buy a used ECU from another '90. I've seen a few recently for around $150. I found out 2 weeks ago that a '90 'vert will run on a '90 GTU ECU even though the numbers don't match.

 

Here's what I've decided. I wouldn't send it to the previous phone number I posted. They wanted cash up front and I had a bad feeling. This guy gives a year warranty and checks all functions. His company repairs 25 ECU's / day. 1 Day turn.
http://www.autoecu.com/
I'll post a note when I get it back.