IAC Issues, Scorp and XR, Maybe the "EEC-IV CHIEF" Knows How To Resolve?

[brokencase]

Maybe I missed something while reading this thread but Dimitri indicated a overall voltage drop in the system occurred that threw off the TPS reading and thus establishing new adaptive low setting by the EECIV.

What exactly caused the "system wide" voltage drop?

[thesameguy]

Commonly when the engine is coming down from high revs engine speed drops below idle, resulting in the alternator dropping out for a second. Could also be something like radiator fans coming on, or some other high-amp accessory... Which, you know, XRs generally don't have.

[DPDISXR4Ti]

Commonly when the engine is coming down from high revs engine speed drops below idle, resulting in the alternator dropping out for a second.

With that line of thinking, I wonder if a better, more modern alternator might be a solution? Upgrading to a modern alternator from a Nissan is on my to-do list already - just need to get it done. Here's the link on that project...
http://forum.merkurclub.net/forum/viewt ... 29&t=34066

[thesameguy]

I'm inclined the blame failing or weak voltage regulators in the ECM over the alternator, but who knows? I do have a fair degree of confidence in my electrical system - everything is new from the alternator to the battery cables - but that doesn't mean there isn't still an issue.

I haven't checked, but I suspect the 115a Bosch from a Saab 9000 is bolt in. Or at least an XR alternator could probably be gut-swapped with one... I think the XR uses a regular Bosch style alternator.

[Ed Lijewski]

Commonly when the engine is coming down from high revs engine speed drops below idle, resulting in the alternator dropping out for a second. Could also be something like radiator fans coming on, or some other high-amp accessory... Which, you know, XRs generally don't have.

Pre FI carburetors were equipped with dashpots that prevented rpms dipping below idle after accelerator linkage depression was released.

My understanding is dashpot effect is written into computer controls.

Indeed, that code seems to be foolproof on all engine systems, as of years ago.

YMMV

[thesameguy]

Yes, although that's really the source of the issue I think. With open throttle, the idle valve opens up. When the throttle snaps shut - indicated by a voltage returned by the TPS - the idle valve closes back up to return idle at a controlled (non-stalling) rate. The "modern" TPS/idle valve interaction eliminates the need for a dashpot. This was a common improvement made in '80s fuel injection tech.

If I'm understanding this correctly, in this failure scenario, the preset voltage isn't reflective of *true* closed throttle, so even though your foot's off the pedal, the idle valve remains open some "sympathetic" amount and you get high idle. The fix is ensuring the "closed throttle" voltage the TPS sends isn't changing or, I suppose, preventing the ECM from learning a new, incorrect, artificially low, mechanically unobtanable voltage. If you can't do this, then the ECM never senses a return to idle, and never fully closes the idle valve.... it just sits there waiting for you to take your foot off the pedal so it can give you that nice, gradual, return to idle speed.

IIRC the TPS on EEC is 0-5v, which means it's regulated power from the ECM... 10-14v into the ECM, 5v out to the TPS. Very common arrangement in automobiledom to prevent exactly what we experience. That's why I think it's the 5v voltage regulator that sucks.... because it should be able to turn 6-14v into 5v reliably. Seems like it can't.

If this is true, we could regulate voltage into the ECM to prevent this lazy VR from tripping, or we could replace the VR in the ECM, or we could supply the TPS regulated 5v externally. Probably any of those would work. Whatever device would be employed in that third scenario, like DC-DC converter, might have some unwanted effects on the ECM. I'm not an electrical engineer.

[DPDISXR4Ti]

IIRC the TPS on EEC is 0-5v, which means it's regulated power from the ECM... 10-14v into the ECM, 5v out to the TPS. Very common arrangement in automobiledom to prevent exactly what we experience. That's why I think it's the 5v voltage regulator that sucks.... because it should be able to turn 6-14v into 5v reliably. Seems like it can't.

When I get a chance I should take a look inside a later PCM (the type I've been running for years now), to see if I different VR was used. As I believe I mentioned somewhere in this thread, I haven't had a high-idle problem since upgrading to an early-90's PCM (currently one from a '94 2.3 Ranger).

[thesameguy]

Just from personal experience I can name a handful of '80s cars that have various issues caused by not-long-lasting VRs. Almost makes me want to buy a reman ECM and see if this is something replaced by big companies. I'm betting not, since it's more of a poor performance issue rather than a failure issue.

I'll bet with some minor research this would be easy to test... hook up an EEC-IV ECM on the bench, power it up, and then monitor TPS voltage... screw with the input voltage (that 10-14v range) and see what that TPS output voltage looks like. Maybe a good winter (indoor) project. I'd just need to source a spare EEC connector.

[brokencase]

Just from personal experience I can name a handful of '80s cars that have various issues caused by not-long-lasting VRs. Almost makes me want to buy a reman ECM and see if this is something replaced by big companies. I'm betting not, since it's more of a poor performance issue rather than a failure issue.

I'll bet with some minor research this would be easy to test... hook up an EEC-IV ECM on the bench, power it up, and then monitor TPS voltage... screw with the input voltage (that 10-14v range) and see what that TPS output voltage looks like. Maybe a good winter (indoor) project. I'd just need to source a spare EEC connector.

Ford does a pretty good job on the front end of the power input to make the unit robust in the harsh automotive electrical environment.
A good reference is Intel application note AP-125
https://ecee.colorado.edu/~mcclurel/iap125.pdf

However, with the above being stated there are two electrical components subject to wear and tear. One is the electrolytic capacitors drying out after 30 years (As I recall there are about 4-5 electrolytics in the EECIV), the other is the MOV (metal oxide varistor). The MOV can wear after many incidents of surges over the years. Ford uses a big Zener in parallel with an MOV. The two work synergistically to protect one another.

Your experience with what you call failed VR's in 80's vehicles is most likely attributed to dried out capacitors. But an important thing to keep in mind is that just about EVERY electrical module in the vehicle has an electrolytic on the front end. They all add up as a combined effect to help reduce spikes in the overall system. Over time the individual caps dry out and start to put a larger burden on those remaining.