Ricardo Vega

So I recently got myself a 1989 S4 that was running good until a day ago. Cranks over but doesn't start. I've been thru this already. Haven't had the chance to use volt meter on injectors to test if its either that or spark. But my 87 S4's injector ECU went out like 4 years ago, sent it in to have it checked out and had had new chip installed. Worked like new.

Right now I do have my engine out of my 87 and doing work on it and did remove ECU's out of car since I'm also going to get my wiring harnesses worked on. I'll test out my 89 a little later today but since I had the computer work decided to ask you guys about them.

So I was thinking, since I did need to move my 89 that just died out of the way to pull my 87 out of car port can I use EUC's between the two year differences?

RV
1987 S4 130,XXX Black/Plum
1989 S4 81,XXX Red/Black

PorKen

The '87 may not have diagnostic enabled (in programming - just needs a chip swap) but otherwise there's no noticeable difference between S4 brains, hardware or software.

Kevin in Atlanta

And if you really want to test the brain - take the 89's and put it in the 87 and see if it starts.

Ricardo Vega

Yeah, I noticed my 89 had the diagnostic on dash but I'm afraid of trying it out and finding out the hardway. But if I do what would be the consequences if they were not? Engine blows up?

RV
1987 S4 130,XXX Black/Plum
1989 S4 81,XXX Red/Black

SwayBar

No consequences.

Mrmerlin

so the computers are swappable BUT, before swapping the computers,

I would be swapping the known running relays and check the fuel pump fuse.

Unless you have some other reason to suspect the computers, a relay/or fuse is the most probable cause for a no start

dr bob

Ricardo--

If the LH controller in the '89 has not yet been rebuilt, it's a Good Idea to have it done now regardless if it has failed or not. Go ahead and plug in the one from your '87 and see if it solves the no-start issue. Follow Stan's excellent guidance on the critical relays too. But have your '89 LH rebuilt if it hasn't been already.

Ricardo Vega

Okay guys, thanks for the wealth of recommendations and I did forget about fuses/relays. And the reason I did is because it was none of those in the past car. But will check it out in the next few days. I'll keep you guys posted.

Can some one refresh me on how to check relays? Fuses are easy since they are see thru.

Thanks.

RV

dr bob

Ricardo--

The easiest way to check the three critical relays is to plug new ones in and see if the problem disappears. The fuel pump, LH and EZK relays in your cars are common Bosch 53B type, so are inexpensive. You can swap the horn relay, which is generally lowest duty, into each of the other positions as a troubleshooting step. The general guidance for the aging relays though, particularly the fuel pump relay, is to just replace them all with new.

Other Things: The fuel pump, injection, ignition, and cooling fans each has its own dedicated power feed wire from the battery positive post. These connections must be clean and tight for the engine to run.

The Group 48 battery in your car is designed to have a vent hose that carries corrosive fumes out of the battery well while it's charging, but that's often not installed or connected when replacing the battery. The fumes will eventually affect the connections. Everything in the battery well needs to be clean and tight. For me that's an annual ritual with a small brass detailer's brush, followed by a coating of Vaseline after everything is assembled and tight. There's a ground point in the spare wheel area above the battery well, forward but hidden by a plastic cover. Good Idea to clean and protect that too. Connections for fuel pump, shift, temp and start safety switches for the automatic gearbox, tank level are all in that area too. Good Idea to pull each connector apart for inspection; even if they aren't seriously damaged they are tin-plated "self-wiping" pin connections that benefit from the effort of the disconnect-reconnect.