So, this car was in my shop almost a year ago with same issues & they are back & continuing after more work...

Primary complaint: engine shuts off (electrical not fuel pressure) when AC system is on.

No ECM codes, no CCM codes.

Over the course of system diagnosis & other minor system failures we have:

Replaced ECM again w/ rebuilt (same chip of course)
rebuilt stereo head/CDM & speaker amps (Dr Don's service)
rebuilt AC head & control box (again Dr Don's service)
replaced ignition switch (electrical switch stuck in run one time scared customer, lock cyl & rack system are working fine)
Checked all wiring between CCM/ECM/AC controller (no opens or shorts)
Checked wiring between ECM & AC compressor
Replaced cooling fan relays (one was intermittent)

Now the car is fairly stable, however, have AC on & turn on headlights, car will likely stumble a little, relays behind IP may stutter (AC clutch relay & fuel pump relay #2 are in that area) & car will often shut down. Again its an abrupt shut off, not a failure to idle under the extra load.

I'm wondering now if alternator is not putting out enough amps?

Voltage is 14 volts but it is a reman that has been in there since before client got car. I have another 105 amp rebuilt on the way...

Any OTHER ideas on what could be causing this issue?? I'm really at my wits end & not sure what else could be causing this...

TIA

Ps. after it shuts off, it will restart, don't even have to cycle the key, just from on to start.

 

Check your alt and clean you IAC.

 

Voltage in an improper place "appears" as very high resistance. It sounds to me like every time an additional electrical mechanism is activated, the ground path for any electrical component that is already operating is being interrupted.

I would literally start at the negative battery cable and work backwards all the way to the ecm grounds. Manually, meaning by visual inspection, disassembly and reassembly.

Is that the problem ?? I don't know, but that where I would start.

Alternatively, if you have a Fluke 87 mulitmeter, use the min/max function and measure voltage drop on the ecm grounds, and other important grounds, as you actuate each and every electrical function in the car. I'm betting you measure some whacked out shizz. Keep in mind that this wouldn't necessarily mean the ecm ground is bad. An example would be the A/C relay. Lets say the relay coil ground path is messed up. Well what's going to happen is the current will find any ground path available. Even if it momentarily (read-micro seconds) partially interrupts the ground path for something else.

You do need the Fluke 87 ... the 88 or another version may or may not catch it.

You have an interesting problem. I'm betting you don't need any new parts to fix it.

 

This is a big time important clue. The "stutter" your describing makes me think of exactly what I posted earlier.....A momentarily interrupted ground circut.

I hope you have a wiring schematic and a component locator, that will make this quick and easy.

 

Ok, I'll go over the grounds (again I think)... Pretty sure I had reviewed most of the primary ones before but I will check all the grounds for the AC system & ECM for sure.

I have '90 FSM as I actually own a '90 myself.

A side note, client has a battery disconnect, gold plated thing w/ **** that you loosen to break the connection. Thinking I might take that out of the equation as well.

Thanks again for the input.... I'll get back on it tomorrow.

oh... IAC/TPS etc are all set & working correctly. The car changes idle perfectly between cold start, warm & when AC is first selected. It's not always immediately that the car shuts down, sometimes it takes a few min, other time, quickly.

 

Could your compressor be going bad and locking up? And agree with taking the disconnect out.

 

Keep in mind its not necessarily the ecm or ac system ground that's the problem. It could easily be a ground splice for multiple circuits just as easily as it could be a ground connection. Look for something that the offending circuits have in common.

One thing is for sure, the relay clicking is a bigtime clue. If this is consistently repeatable then you have very good chances of figuring this out within a reasonable amount of time.

 

Load test the battery it should hold 10.8 V for 15 seconds @ 200 amp draw.
If it fails the load test recharge @ 30 amps for 10 minutes & monitor the voltage.
If the voltage exceeds 16V during the recharge the battery is sulfating = surface charge it needs to be replaced.

Frame grounds under the battery tray need to be clean & tight. If you have external star washers add them. Check frame to block ground.

Bat wire @ the alt stud needs inspection they tend to melt @ the eyelet connection under the insulation boot. If the bat wire insulation is black @ the eyelet connection peel the insulation back you will probably find corrosion on the wire = high resistance = fix it.
My 90 is happier with a second 10ga wire added to the alt bat terminal connected to the distribution block behind the battery.

As suggested the AC compressor may be locking up. Engine off, supply 12v to the compressor blue wire to engage the clutch. Turn the compressor with a socket etc to check for drag/smooth operation.

 

Thank you all for the input.

I still have a couple more thing to tackle:

Clear my 4 post lift so I can,
Replace missing ground strap between engine & G101 (under battery).
Clean & reseat:
G106 - above oil filter on side of block
G107 - bell housing behind oil filter

However, over last night & this AM I have:

Cleaned & reseated:
G101 - battery to frame (was NOT in place)
G109 - Right frame below heater box
S130 - Feed from G109, seemed fine but cleaned & soldered crimp joint
G115 - Left front frame below headlights (this was suspect on removal)
G116 - Right front frame below headlights
Jump start block - cleaned & reseat connections
Ohm individual lines where possible from power block to destinations- fine
Ohm power sources to ECM connections - fine
Ohm ground sources to ECM connections - fine

Removed battery disconnect from ground side of battery.

We are already 100% better, ran the car for 20 min, varying lights/AC off/on, individual & together. No stalls, no stuttering relays, nothing, seemed very stable.

 

It sounds like the engine and g101 were not grounded together properly and the battery to frame ground was not in palce either ??

I believe you have found your problem. You could prove it with 100% certainty if you have fast enough multimeter, but its hard to justify 400+ bucks for a function that you only need once every few years or so ..