Simsalas

Hello

I am searching a stock PROM for an 1987 Pontiac Trans Am GTA, 350 TPI automatic. Where can I find this? I heard that GM don't produce new ones...

Fit the 88 GTA PROM also for the 87? Or are they different?

Thank you for an answer.

Simon

bigal55

iTrader: (1)

Yeah, you can use an 88 prom, same ECU. 88 prom may actually have upgrade so you might be better off. What makes you think your prom is bad?

Simsalas

My voltage reference is only 4.1 volts instead of 5 volts. I changed ECM, relais, temp sender usw.

At least, I think my PROM is bad.

RBob

iTrader: (1)

The reference voltage is hardware based. You can remove the PROM (MEMCAL) and it won't change.

RBob.

Simsalas

Ok, thank you.

RBob

iTrader: (1)

I saw your post on the tech board about this issue. If you checked the reference voltage at the CTS with the CTS plugged in, it will be lower then the +5 volts you expect.

You can also check the reference voltage at the TPS connector. Key-on, engine-off, if an inline connector measure between the two outer pins. It should be in the 5.0 - 5.1 volt range. Can do this with it unplugged from the sensor.

As for the malfunction code, it is common for the CTS sensor and connector to need replacing at the same time. They always seem to corrode, both the sensor pins and the connector terminals.

RBob.

Simsalas

Ok, I will check this.

But how can I measure the CTS, when the connector is plugged in? I only check the voltage at the connector and it's 4.1 volts...

RBob

iTrader: (1)

Back probe the connector. Note that the CTS circuit is based on ECM voltage, not the reference voltage. Also, it has several pull up resistors to provide the 5 volts or so to the sensor. The sensor being a variable resistor that pulls that voltage down as the engine gets hotter.

RBob.

Simsalas

Also, are the 4.1 volts on the cts connector ok? Sorry but I'm confused.

First, on the TPS there should be ca. 5 Volts beetween the two cables on the side?

But on the CTS, is not the 5 Volt reference neccassary?

Thank you for your response.

RBob

iTrader: (1)

No, something is wrong. Without the CTS plugged in there should be +5 volts on the connector yellow wire. The other wire, black, is ground.

Yes, the two outer terminals/wires (A & C).

RBob.

Simsalas

Today I tested the TPS. 5 Volts and it's ok. I disconnect also the MAT and the voltage at the CTS connector was still 4.1 volts.

my mechanic don't know where the problem is... grounds are ok, fuses are ok, and ecm is ok.

Simsalas

Update:

Another mechanics tested yesterday the reference voltage. All connectors have 5 Volts!

We also adjusted the TPS at 0.54 Volts at idle and replaced the CTS connector.

I will see if my problem whit the fans is gone...

Thank your for your help!